MP3
Encyclopedia
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patent
ed digital audio
encoding
format using a form of lossy data compression
. It is a common audio format for consumer audio storage, as well as a de facto standard
of digital audio compression for the transfer and playback of music on digital audio players.
MP3 is an audio-specific format that was designed by the Moving Picture Experts Group
(MPEG) as part of its MPEG-1
standard and later extended in MPEG-2
standard. The first MPEG subgroup – Audio group was formed by several teams of engineers at Fraunhofer IIS
, University of Hannover, AT&T-Bell Labs
, Thomson-Brandt
, CCETT
, and others. MPEG-1 Audio (MPEG-1 Part 3), which included MPEG-1 Audio Layer I, II and III was approved as a committee draft of ISO
/IEC
standard in 1991, finalised in 1992 and published in 1993 (ISO/IEC 11172-3:1993). Backwards compatible MPEG-2 Audio (MPEG-2 Part 3) with additional bit rates and sample rates was published in 1995 (ISO/IEC 13818-3:1995).
The use in MP3 of a lossy
compression algorithm
is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners. An MP3 file that is created using the setting of 128 kbit/s will result in a file that is about 1/11 the size than the CD
file created from the original audio source. An MP3 file can also be constructed at higher or lower bit rates, with higher or lower resulting quality.
The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory
resolution ability of most people. This method is commonly referred to as perceptual coding
. It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.
. In 1894, Alfred Marshall Mayer reported that a tone could be rendered inaudible by another tone of lower frequency. In 1959, Richard Ehmer described a complete set of auditory curves regarding this phenomenon. Ernst Terhardt et al. created an algorithm describing auditory masking with high accuracy. This work added to a variety of reports from authors dating back to Fletcher, and to the work that initially determined critical ratios and critical bandwidths.
The psychoacoustic masking
codec
was first proposed in 1979, apparently independently, by Manfred R. Schroeder
, et al. from AT&T-Bell Labs in Murray Hill, NJ, and M. A. Krasner both in the United States. Krasner was the first to publish and to produce hardware for speech (not usable as music bit compression), but the publication of his results as a relatively obscure Lincoln Laboratory
Technical Report did not immediately influence the mainstream of psychoacoustic codec development. Manfred Schroeder was already a well-known and revered figure in the worldwide community of acoustical and electrical engineers, but his paper was not much noticed, since it described negative results due to the particular nature of speech and the linear predictive coding
(LPC) gain present in speech. Both Krasner and Schroeder built upon the work performed by Eberhard F. Zwicker in the areas of tuning and masking of critical bands, that in turn built on the fundamental research in the area from Bell Labs
of Harvey Fletcher and his collaborators. A wide variety of (mostly perceptual) audio compression algorithms were reported in IEEE's refereed Journal on Selected Areas in Communications. That journal reported in February 1988 on a wide range of established, working audio bit compression technologies, some of them using auditory masking as part of their fundamental design, and several showing real-time hardware implementations.
The immediate predecessors of MP3 were "Optimum Coding in the Frequency Domain" (OCF), and Perceptual Transform Coding (PXFM). These two codecs, along with block-switching contributions from Thomson-Brandt, were merged into a codec called ASPEC, which was submitted to MPEG, and which won the quality competition, but that was mistakenly rejected as too complex to implement. The first practical implementation of an audio perceptual coder (OCF) in hardware (Krasner's hardware was too cumbersome and slow for practical use), was an implementation of a psychoacoustic transform coder based on Motorola 56000 DSP
chips.
As a doctoral student at Germany's University of Erlangen-Nuremberg, Karlheinz Brandenburg
began working on digital music compression in the early 1980s, focusing on how people perceive music. He completed his doctoral work in 1989. MP3 is directly descended from OCF and PXFM. MP3 represents the outcome of the collaboration of Karlheinz Brandenburg, working as a postdoc at AT&T-Bell Labs with James D. (JJ) Johnston of AT&T-Bell Labs, collaborating with the Fraunhofer Institut for Integrated Circuits, Erlangen, with relatively minor contributions from the MP2 branch of psychoacoustic sub-band coders. In 1990 Brandenburg became an assistant professor at Erlangen-Nuremberg. While there, he continued to work on music compression with scientists at the Fraunhofer Society
(in 1993 he joined the staff of the Fraunhofer Institute).
The song Tom's Diner
by Suzanne Vega
was the first song used by Karlheinz Brandenburg
to develop the MP3. Brandenburg adopted the song for testing purposes, listening to it again and again each time refining the scheme, making sure it did not adversely affect the subtlety of Vega's voice.
MPEG-1 Audio Layer 2 encoding began as the Digital Audio Broadcast (DAB) project managed by Egon Meier-Engelen of the Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt (later on called Deutsches Zentrum für Luft- und Raumfahrt, German Aerospace Center
) in Germany
. The European Community financed this project, commonly known as EU-147 (or Eureka 147), from 1987 to 1994 as a part of the EUREKA
research program. MUSICAM Audio Coding was developed as part of the Eureka 147 project and has been subject to the standardization process within the ISO/Moving Pictures Expert Group (MPEG).
(the Netherlands
), CCETT
(France
) and Institut für Rundfunktechnik
(Germany
) was chosen due to its simplicity and error robustness, as well as its low computational power associated with the encoding of high quality compressed audio. The Musicam format, based on sub-band coding
, was the basis of the MPEG Audio compression format (sampling rates, structure of frames, headers, number of samples per frame).
Much of its technology and ideas were incorporated into the definition of ISO MPEG Audio Layer I and Layer II and the filter bank alone into Layer III (MP3) format as part of the computationally inefficient hybrid filter bank. Under the chairmanship of Professor Musmann (University of Hannover) the editing of the standard was made under the responsibilities of Leon van de Kerkhof (Layer I) and Gerhard Stoll (Layer II).
ASPEC was the joint proposal of AT&T Bell Laboratories, Thomson Consumer Electronics, Fraunhofer Society and CNET
. It provided the highest coding efficiency.
A working group
consisting of Leon van de Kerkhof (The Netherlands), Gerhard Stoll (Germany), Leonardo Chiariglione
(Italy), Yves-François Dehery (France), Karlheinz Brandenburg (Germany) and James D. Johnston (USA) took ideas from ASPEC, integrated the filter bank
from Layer 2, added some of their own ideas and created MP3, which was designed to achieve the same quality at 128 kbit/s as MP2
at 192 kbit/s.
All algorithms for MPEG-1 Audio Layer I, II and III were approved in 1991 and finalized in 1992 as part of MPEG-1
, the first standard suite by MPEG, which resulted in the international standard ISO
/IEC
11172-3 (a.k.a. MPEG-1 Audio or MPEG-1 Part 3), published in 1993. Further work on MPEG audio was finalized in 1994 as part of the second suite of MPEG standards, MPEG-2
, more formally known as international standard ISO/IEC 13818-3 (a.k.a. MPEG-2 Part 3 or backwards compatible MPEG-2 Audio or MPEG-2 Audio BC), originally published in 1995. MPEG-2 Part 3 (ISO/IEC 13818-3) defined additional bit rates and sample rates for MPEG-1 Audio Layer I, II and III. The new sampling rates are exactly half that of those originally defined for MPEG-1 Audio. MPEG-2 Part 3 also enhanced MPEG-1's audio by allowing the coding of audio programs with more than two channels, up to 5.1 multichannel. There is also MPEG-2.5 audio, a proprietary unofficial extension developed by Fraunhofer IIS. It enables MP3 to work satisfactorily at very low bitrates and added lower sampling frequencies. MPEG-2.5 was not developed by MPEG and was never approved as an international standard.
Note: The ISO standard ISO/IEC 11172-3 (a.k.a. MPEG-1 Audio) defined three formats: the MPEG-1 Audio Layer I, Layer II and Layer III. The ISO standard ISO/IEC 13818-3 (a.k.a. MPEG-2 Audio) defined extended version of the MPEG-1 Audio – MPEG-2 Audio Layer I, Layer II and Layer III. MPEG-2 Audio (MPEG-2 Part 3) should not be confused with MPEG-2 AAC (MPEG-2 Part 7 – ISO/IEC 13818-7).
Compression efficiency of encoders is typically defined by the bit rate, because compression ratio depends on the bit depth and sampling rate
of the input signal. Nevertheless, compression ratios are often published. They may use the Compact Disc
(CD) parameters as references (44.1 kHz, 2 channels at 16 bits per channel or 2×16 bit), or sometimes the Digital Audio Tape
(DAT) SP parameters (48 kHz, 2×16 bit). Compression ratios with this latter reference are higher, which demonstrates the problem with use of the term compression ratio for lossy encoders.
Karlheinz Brandenburg used a CD recording of Suzanne Vega
's song "Tom's Diner
" to assess and refine the MP3 compression algorithm. This song was chosen because of its nearly monophonic
nature and wide spectral content, making it easier to hear imperfections in the compression format during playbacks. Some jokingly refer to Suzanne Vega as "The mother of MP3". Some more critical audio excerpts (glockenspiel
, triangle
, accordion
, etc.) were taken from the EBU
V3/SQAM reference compact disc and have been used by professional sound engineers to assess the subjective quality of the MPEG Audio formats. This particular track has an interesting property in that the two channels are almost, but not completely, the same, leading to a case where Binaural Masking Level Depression causes spatial unmasking of noise artifacts unless the encoder properly recognizes the situation and applies corrections similar to those detailed in the MPEG-2 AAC psychoacoustic model.
based) of compressed audio. Some other real time implementation of MPEG Audio encoders were available for the purpose of digital broadcasting (radio DAB, television DVB) towards consumer receivers and set top boxes.
On July 7, 1994, the Fraunhofer Society
released the first software MP3 encoder called l3enc
. The filename extension
.mp3 was chosen by the Fraunhofer team on July 14, 1995 (previously, the files had been named .bit). With the first real-time software MP3 player Winplay3
(released September 9, 1995) many people were able to encode and play back MP3 files on their PCs. Because of the relatively small hard drives back in that time (~ 500–1000 MB
) lossy compression was essential to store non-instrument based (see tracker and MIDI
) music for
playback on computer.
. The popularity of MP3s began to rise rapidly with the advent of Nullsoft
's audio player Winamp
, released in 1997. In 1998, the first portable solid state digital audio player MPMan, developed by SaeHan Information Systems
which is headquartered in Seoul
, South Korea
, was released and the Rio PMP300
was sold afterwards, despite legal suppression efforts by the RIAA.
In November 1997, the website mp3.com
was offering thousands of MP3s created by independent artists for free. The small size of MP3 files enabled widespread peer-to-peer
file sharing
of music ripped
from CDs, which would have previously been nearly impossible. The first large peer-to-peer filesharing network, Napster
, was launched in 1999.
The ease of creating and sharing MP3s resulted in widespread copyright
infringement. Major record companies argue that this free sharing of music reduces sales, and call it "music piracy". They reacted by pursuing lawsuits against Napster
(which was eventually shut down and later sold) and against individual users who engaged in file sharing.
Despite the popularity of the MP3 format, online music retailers often use other proprietary formats that are encrypted or obfuscated in order to make it difficult to use purchased music files in ways not specifically authorized by the record companies. Attempting to control the use of files in this way is known as Digital Rights Management
. Record companies argue that this is necessary to prevent the files from being made available on peer-to-peer file sharing networks. This has other side effects, though, such as preventing users from playing back their purchased music on different types of devices. However, the audio content of these files can usually be converted into an unencrypted format. For instance, users are often allowed to burn files to audio CD
, which requires conversion to an unencrypted audio format.
Unauthorized MP3 file sharing continues on next-generation peer-to-peer networks. Some authorized services, such as Apple iTunes, Beatport
, Bleep
, Juno Records
, eMusic
, Zune Marketplace, Walmart.com
, Rhapsody
, the legal incarnation of Napster
, and Amazon.com
sell unrestricted music in the MP3 format.
standard does not include a precise specification for an MP3 encoder, but does provide example psychoacoustic models, rate loop, and the like in the non-normative part of the original standard. At present, these suggested implementations are quite dated. Implementers of the standard were supposed to devise their own algorithms suitable for removing parts of the information from the audio input. As a result, there are many different MP3 encoders available, each producing files of differing quality. Comparisons are widely available, so it is easy for a prospective user of an encoder to research the best choice. It must be kept in mind that an encoder that is proficient at encoding at higher bit rates (such as LAME
) is not necessarily as good at lower bit rates.
During encoding, 576 time-domain samples are taken and are transformed to 576 frequency-domain samples. If there is a transient, 192 samples are taken instead of 576. This is done to limit the temporal spread of quantization noise accompanying the transient. (See psychoacoustics
.)
s are "bitstream
compliant", which means that the decompressed output – that they produce from a given MP3 file – will be the same, within a specified degree of rounding
tolerance, as the output specified mathematically in the ISO/IEC high standard document (ISO/IEC 11172-3). Therefore, comparison of decoders is usually based on how computationally efficient they are (i.e., how much memory
or CPU time they use in the decoding process).
, which specifies how many kilobits the file may use per second of audio. The higher the bit rate, the larger the compressed file will be, and, generally, the closer it will sound to the original file.
With too low a bit rate, compression artifact
s (i.e. sounds that were not present in the original recording) may be audible in the reproduction. Some audio is hard to compress because of its randomness and sharp attacks. When this type of audio is compressed, artifacts such as ringing or pre-echo
are usually heard. A sample of applause compressed with a relatively low bit rate provides a good example of compression artifacts.
Besides the bit rate of an encoded piece of audio, the quality of MP3 files also depends on the quality of the encoder itself, and the difficulty of the signal being encoded. As the MP3 standard allows quite a bit of freedom with encoding algorithms, different encoders may feature quite different quality, even with identical bit rates. As an example, in a public listening test featuring two different MP3 encoders at about 128 kbit/s, one scored 3.66 on a 1–5 scale, while the other scored only 2.22.
Quality is dependent on the choice of encoder and encoding parameters.
The simplest type of MP3 file uses one bit rate for the entire file — this is known as Constant Bit Rate
(CBR) encoding. Using a constant bit rate makes encoding simpler and faster. However, it is also possible to create files where the bit rate changes throughout the file. These are known as Variable Bit Rate
(VBR) files. The idea behind this is that, in any piece of audio, some parts will be much easier to compress, such as silence or music containing only a few instruments, while others will be more difficult to compress. So, the overall quality of the file may be increased by using a lower bit rate for the less complex passages and a higher one for the more complex parts. With some encoders, it is possible to specify a given quality, and the encoder will vary the bit rate accordingly. Users who know a particular "quality setting" that is transparent
to their ears can use this value when encoding all of their music, and generally speaking not need to worry about performing personal listening tests on each piece of music to determine the correct bit rate.
Perceived quality can be influenced by listening environment (ambient noise), listener attention, and listener training and in most cases by listener audio equipment (such as sound cards, speakers and headphones).
A test given to new students by Stanford University
Music Professor Jonathan Berger showed that student preference for MP3 quality music has risen each year. Berger said the students seem to prefer the 'sizzle' sounds that MP3s bring to music.
A sample rate of 44.1 kHz is almost always used, because this is also used for CD audio
, the main source used for creating MP3 files. A greater variety of bit rates are used on the Internet. The rate of 128 kbit/s is commonly used, at a compression ratio of 11:1, offering adequate audio quality in a relatively small space. As Internet bandwidth
availability and hard drive sizes have increased, higher bit rates up to 320 kbit/s are widespread.
Uncompressed audio as stored on an audio-CD has a bit rate of 1,411.2 kbit/s,16 bit/sample × 44100 samples/second × 2 channels / 1000 bits/kilobit so the bitrates 128, 160 and 192 kbit/s represent compression ratios
of approximately 11:1, 9:1 and 7:1 respectively.
Non-standard bit rates up to 640 kbit/s can be achieved with the LAME
encoder and the freeformat option, although few MP3 players can play those files. According to the ISO standard, decoders are only required to be able to decode streams up to 320 kbit/s.
(VBR), accomplished via bitrate switching on a per-frame basis, but only layer III decoders must support it. VBR is used when the goal is to achieve a fixed level of quality. The final file size of a VBR encoding is less predictable than with constant bitrate
. Average bitrate
is VBR implemented as a compromise between the two – the bitrate is allowed to vary for more consistent quality, but is controlled to remain near an average value chosen by the user, for predictable file sizes. Although an MP3 decoder must support VBR to be standards compliant, historically some decoders have bugs with VBR decoding, particularly before VBR encoders became widespread.
Layer III audio can also use a "bit reservoir", a partially full frame's ability to hold part of the next frame's audio data, allowing temporary changes in effective bitrate, even in a constant bitrate stream.
An MP3 file is made up of multiple MP3 frames, which consist of a header and a data block. This sequence of frames is called an elementary stream
. Frames are not independent items ("byte reservoir") and therefore cannot be extracted on arbitrary frame boundaries. The MP3 Data blocks contain the (compressed) audio information in terms of frequencies and amplitudes. The diagram shows that the MP3 Header consists of a sync word, which is used to identify the beginning of a valid frame. This is followed by a bit indicating that this is the MPEG standard and two bits that indicate that layer 3 is used; hence MPEG-1 Audio Layer 3 or MP3. After this, the values will differ, depending on the MP3 file. ISO
/IEC
11172-3 defines the range of values for each section of the header along with the specification of the header. Most MP3 files today contain ID3
metadata
, which precedes or follows the MP3 frames; as noted in the diagram.
Newer audio compression formats such as Vorbis
, WMA Pro and AAC
are generally void of a number of these limitations.
In technical terms, some limitations include:
A "tag" in an audio file is a section of the file that contains metadata
such as the title, artist, album, track number or other information about the file's contents. The MP3 standards do not define tag formats for MP3 files, nor is there a standard container format
that would support metadata and obviate the need for tags.
However, several de facto standards for tag formats exist. As of 2010, the most widespread are ID3v1 and ID3v2
, and the more recently introduced APEv2
. These tags are normally embedded at the beginning or end of MP3 files, separate from the actual MP3 frame data. MP3 decoders normally either read info from the tags, or just treat them as ignorable, non-MP3 junk data.
Playing & editing software often contains tag editing functionality, but there are also tag editor
applications dedicated to the purpose.
Aside from metadata pertaining to the audio content, tags may also be used for DRM
.
and other factors, it is sometimes desirable to adjust the playback volume of audio files such that a consistent average loudness
is perceived. This normalization
, while similar in purpose, is distinct from dynamic range compression.
ReplayGain is one standard for measuring and storing the loudness of an MP3 file in its metadata tag, enabling a ReplayGain-compliant player to automatically adjust the overall playback volume for each file. MP3Gain
may be used to reversibly modify files based on ReplayGain measurements so that adjusted playback can be achieved on players without ReplayGain capability.
s related to MP3 decoding or encoding. These claims have led to a number of legal threats and actions from a variety of sources, resulting in uncertainty about which patents must be licensed in order to create MP3 products without committing patent infringement in countries that allow software patents.
The various MP3-related patents expire on dates ranging from 2007 to 2017 in the U.S. The initial near-complete MPEG-1 standard (parts 1, 2 and 3) was publicly available on December 6, 1991 as ISO CD 11172. In the United States, patents cannot claim inventions that were already publicly disclosed more than a year prior to the filing date, but for patents filed prior to June 8, 1995, submarine patent
s made it possible to extend the effective lifetime of a patent through application extensions. Patents filed for anything disclosed in ISO CD 11172 a year or more after its publication are questionable; if only the known MP3 patents filed by December 1992 are considered, then MP3 decoding may be patent free in the US by September 2015 when expires which had a PCT filing in Oct 1992.
Technicolor (formerly called Thomson Consumer Electronics) claims to control MP3 licensing of the Layer 3 patents in many countries, including the United States
, Japan
, Canada
and EU countries. Technicolor has been actively enforcing these patents.
MP3 license revenues generated about €100 million for the Fraunhofer Society in 2005.
In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that a license was required to "distribute and/or sell decoders and/or encoders". The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and Thomson. To make, sell and/or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us."
However, there exist both free
and/or proprietary alternatives, with free formats such as Vorbis
, FLAC, and others. Microsoft
's usage of its own proprietary Windows Media
format allows it to avoid licensing issues associated with these patents by avoiding usage of the MP3 format entirely. Until the key patents expire, unlicensed encoders and players could be infringing
in countries where the patents are valid.
In spite of the patent restrictions, the perpetuation of the MP3 format continues. The reasons for this appear to be the network effect
s caused by:
Additionally, patent holders declined to enforce license fees on free
and open source
decoders, which allows many free MP3 decoders to develop.
Sisvel S.p.A. and its U.S. subsidiary Audio MPEG, Inc. previously sued Thomson for patent infringement on MP3 technology, but those disputes were resolved in November 2005 with Sisvel granting Thomson a license to their patents. Motorola also recently signed with Audio MPEG to license MP3-related patents.
In September 2006, German officials seized MP3 players from SanDisk
's booth at the IFA show in Berlin after an Italian patents firm won an injunction on behalf of Sisvel against SanDisk in a dispute over licensing rights. The injunction was later reversed by a Berlin judge, but that reversal was in turn blocked the same day by another judge from the same court, "bringing the Patent Wild West to Germany" in the words of one commentator.
In February 2007, Texas MP3 Technologies sued Apple, Samsung Electronics and Sandisk in eastern Texas federal court
, claiming infringement of a portable MP3 player patent that Texas MP3 said it had been assigned. Apple and Sandisk both settled the claims against them in January 2009. Samsung settled as well.
Alcatel-Lucent
has asserted several MP3 coding and compression patents, allegedly inherited from AT&T-Bell Labs, in litigation of its own. In November 2006 (prior to the companies' merger), Alcatel sued
Microsoft
for allegedly infringing seven patents. On February 23, 2007, a San Diego jury awarded Alcatel-Lucent
US $1.52 billion in damages for infringement of two of them. The court subsequently tossed the award, however, finding that one patent had not been infringed and that the other was not even owned by Alcatel-Lucent
; it was co-owned by AT&T
and Fraunhofer, who had licensed it to Microsoft
, the judge ruled. That defense judgment was upheld on appeal in 2008. See Alcatel-Lucent v. Microsoft
for more information.
s exist. Among these, mp3PRO
, AAC
, and MP2
are all members of the same technological family as MP3 and depend on roughly similar psychoacoustic models. The Fraunhofer Gesellschaft owns many of the basic patent
s underlying these codecs as well, with others held by Dolby Labs, Sony
, Thomson Consumer Electronics, and AT&T
. In addition, there is also the open source file format Vorbis
that has been available free of charge and without any known patent restrictions.
Software patent
Software patent does not have a universally accepted definition. One definition suggested by the Foundation for a Free Information Infrastructure is that a software patent is a "patent on any performance of a computer realised by means of a computer program".In 2005, the European Patent Office...
ed digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
encoding
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
format using a form of lossy data compression
Lossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...
. It is a common audio format for consumer audio storage, as well as a de facto standard
De facto standard
A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces...
of digital audio compression for the transfer and playback of music on digital audio players.
MP3 is an audio-specific format that was designed by the Moving Picture Experts Group
Moving Picture Experts Group
The Moving Picture Experts Group is a working group of experts that was formed by ISO and IEC to set standards for audio and video compression and transmission. It was established in 1988 by the initiative of Hiroshi Yasuda and Leonardo Chiariglione, who has been from the beginning the Chairman...
(MPEG) as part of its MPEG-1
MPEG-1
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting possible.Today, MPEG-1 has become...
standard and later extended in MPEG-2
MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...
standard. The first MPEG subgroup – Audio group was formed by several teams of engineers at Fraunhofer IIS
Fraunhofer Society
The Fraunhofer Society is a German research organization with 60 institutes spread throughout Germany, each focusing on different fields of applied science . It employs around 18,000, mainly scientists and engineers, with an annual research budget of about €1.65 billion...
, University of Hannover, AT&T-Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, Thomson-Brandt
Thomson SA
Technicolor SA , formerly Thomson SA and Thomson Multimedia, is a French international provider of solutions for the creation, management, post-production, delivery and access of video, for the Communication, Media and Entertainment industries. Technicolor’s headquarters are located in Issy les...
, CCETT
Centre commun d'études de télévision et télécommunications
CCETT or Centre commun d'études de télévision et télécommunications was a research centre created in Rennes in 1972 jointly by the Office de Radiodiffusion Télévision Française and Centre National...
, and others. MPEG-1 Audio (MPEG-1 Part 3), which included MPEG-1 Audio Layer I, II and III was approved as a committee draft of ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
/IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
standard in 1991, finalised in 1992 and published in 1993 (ISO/IEC 11172-3:1993). Backwards compatible MPEG-2 Audio (MPEG-2 Part 3) with additional bit rates and sample rates was published in 1995 (ISO/IEC 13818-3:1995).
The use in MP3 of a lossy
Lossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...
compression algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners. An MP3 file that is created using the setting of 128 kbit/s will result in a file that is about 1/11 the size than the CD
Red Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
file created from the original audio source. An MP3 file can also be constructed at higher or lower bit rates, with higher or lower resulting quality.
The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
resolution ability of most people. This method is commonly referred to as perceptual coding
Psychoacoustics
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...
. It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.
Development
The MP3 lossy audio data compression algorithm takes advantage of a perceptual limitation of human hearing called auditory maskingAuditory masking
Auditory masking occurs when the perception of one sound is affected by the presence of another sound.- Simultaneous masking :Simultaneous masking is when a sound is made inaudible by a "masker", a noise or unwanted sound of the same duration as the original sound.-Critical bandwidth:If two sounds...
. In 1894, Alfred Marshall Mayer reported that a tone could be rendered inaudible by another tone of lower frequency. In 1959, Richard Ehmer described a complete set of auditory curves regarding this phenomenon. Ernst Terhardt et al. created an algorithm describing auditory masking with high accuracy. This work added to a variety of reports from authors dating back to Fletcher, and to the work that initially determined critical ratios and critical bandwidths.
The psychoacoustic masking
Auditory masking
Auditory masking occurs when the perception of one sound is affected by the presence of another sound.- Simultaneous masking :Simultaneous masking is when a sound is made inaudible by a "masker", a noise or unwanted sound of the same duration as the original sound.-Critical bandwidth:If two sounds...
codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
was first proposed in 1979, apparently independently, by Manfred R. Schroeder
Manfred R. Schroeder
Manfred Robert Schröder was a German physicist, most known for his contributions to acoustics and computer graphics. He wrote three books and published over 150 articles in his field....
, et al. from AT&T-Bell Labs in Murray Hill, NJ, and M. A. Krasner both in the United States. Krasner was the first to publish and to produce hardware for speech (not usable as music bit compression), but the publication of his results as a relatively obscure Lincoln Laboratory
Lincoln Laboratory
MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense research and development center chartered to apply advanced technology to problems of national security. Research and development activities focus on long-term technology development as well as...
Technical Report did not immediately influence the mainstream of psychoacoustic codec development. Manfred Schroeder was already a well-known and revered figure in the worldwide community of acoustical and electrical engineers, but his paper was not much noticed, since it described negative results due to the particular nature of speech and the linear predictive coding
Linear predictive coding
Linear predictive coding is a tool used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech in compressed form, using the information of a linear predictive model...
(LPC) gain present in speech. Both Krasner and Schroeder built upon the work performed by Eberhard F. Zwicker in the areas of tuning and masking of critical bands, that in turn built on the fundamental research in the area from Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
of Harvey Fletcher and his collaborators. A wide variety of (mostly perceptual) audio compression algorithms were reported in IEEE's refereed Journal on Selected Areas in Communications. That journal reported in February 1988 on a wide range of established, working audio bit compression technologies, some of them using auditory masking as part of their fundamental design, and several showing real-time hardware implementations.
The immediate predecessors of MP3 were "Optimum Coding in the Frequency Domain" (OCF), and Perceptual Transform Coding (PXFM). These two codecs, along with block-switching contributions from Thomson-Brandt, were merged into a codec called ASPEC, which was submitted to MPEG, and which won the quality competition, but that was mistakenly rejected as too complex to implement. The first practical implementation of an audio perceptual coder (OCF) in hardware (Krasner's hardware was too cumbersome and slow for practical use), was an implementation of a psychoacoustic transform coder based on Motorola 56000 DSP
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
chips.
As a doctoral student at Germany's University of Erlangen-Nuremberg, Karlheinz Brandenburg
Karlheinz Brandenburg
Karlheinz Brandenburg is an audio engineer who has contributed to the audio compression format MPEG Audio Layer 3, more commonly known as MP3.- Biography :...
began working on digital music compression in the early 1980s, focusing on how people perceive music. He completed his doctoral work in 1989. MP3 is directly descended from OCF and PXFM. MP3 represents the outcome of the collaboration of Karlheinz Brandenburg, working as a postdoc at AT&T-Bell Labs with James D. (JJ) Johnston of AT&T-Bell Labs, collaborating with the Fraunhofer Institut for Integrated Circuits, Erlangen, with relatively minor contributions from the MP2 branch of psychoacoustic sub-band coders. In 1990 Brandenburg became an assistant professor at Erlangen-Nuremberg. While there, he continued to work on music compression with scientists at the Fraunhofer Society
Fraunhofer Society
The Fraunhofer Society is a German research organization with 60 institutes spread throughout Germany, each focusing on different fields of applied science . It employs around 18,000, mainly scientists and engineers, with an annual research budget of about €1.65 billion...
(in 1993 he joined the staff of the Fraunhofer Institute).
The song Tom's Diner
Tom's Diner
"Tom's Diner" is an a cappella pop song written in 1981 by American singer-songwriter Suzanne Vega. It was first released as a track on the January 1984 issue of Fast Folk Musical Magazine. When first featured on one of her own studio albums, it appeared as the first track of her Solitude Standing...
by Suzanne Vega
Suzanne Vega
Suzanne Nadine Vega is an American songwriter and singer known for her eclectic folk-inspired music.Two of Vega's songs reached the top 10 of various international chart listings: "Luka" and "Tom's Diner"...
was the first song used by Karlheinz Brandenburg
Karlheinz Brandenburg
Karlheinz Brandenburg is an audio engineer who has contributed to the audio compression format MPEG Audio Layer 3, more commonly known as MP3.- Biography :...
to develop the MP3. Brandenburg adopted the song for testing purposes, listening to it again and again each time refining the scheme, making sure it did not adversely affect the subtlety of Vega's voice.
MPEG-1 Audio Layer 2 encoding began as the Digital Audio Broadcast (DAB) project managed by Egon Meier-Engelen of the Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt (later on called Deutsches Zentrum für Luft- und Raumfahrt, German Aerospace Center
German Aerospace Center
The German Aerospace Center is the national centre for aerospace, energy and transportation research of the Federal Republic of Germany. It has multiple locations throughout Germany. Its headquarters are located in Cologne. It is engaged in a wide range of research and development projects in...
) in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. The European Community financed this project, commonly known as EU-147 (or Eureka 147), from 1987 to 1994 as a part of the EUREKA
EUREKA
EUREKA, often abbreviated as "E!" or "Σ!", is a pan-European research and development funding and coordination organization. EUREKA aims to coordinate efforts of governments, research institutes and commercial companies concerning innovation...
research program. MUSICAM Audio Coding was developed as part of the Eureka 147 project and has been subject to the standardization process within the ISO/Moving Pictures Expert Group (MPEG).
Standardization
In 1991, there were only two proposals available that could be completely assessed for an MPEG audio standard: Musicam (Masking pattern adapted Universal Subband Integrated Coding And Multiplexing) and ASPEC (Adaptive Spectral Perceptual Entropy Coding). The Musicam technique, as proposed by PhilipsPhilips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
(the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
), CCETT
Centre commun d'études de télévision et télécommunications
CCETT or Centre commun d'études de télévision et télécommunications was a research centre created in Rennes in 1972 jointly by the Office de Radiodiffusion Télévision Française and Centre National...
(France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
) and Institut für Rundfunktechnik
Institut für Rundfunktechnik
The Institut für Rundfunktechnik GmbH is the research centre of the German broadcasters , Austria's broadcaster and the Swiss public broadcaster . It is located in Munich and is responsible for the research and standardisation of broadcasting technology...
(Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
) was chosen due to its simplicity and error robustness, as well as its low computational power associated with the encoding of high quality compressed audio. The Musicam format, based on sub-band coding
Sub-band coding
Sub-band coding is any form of transform coding that breaks a signal into a number of different frequency bands and encodes each one independently. This decomposition is often the first step in data compression for audio and video signals....
, was the basis of the MPEG Audio compression format (sampling rates, structure of frames, headers, number of samples per frame).
Much of its technology and ideas were incorporated into the definition of ISO MPEG Audio Layer I and Layer II and the filter bank alone into Layer III (MP3) format as part of the computationally inefficient hybrid filter bank. Under the chairmanship of Professor Musmann (University of Hannover) the editing of the standard was made under the responsibilities of Leon van de Kerkhof (Layer I) and Gerhard Stoll (Layer II).
ASPEC was the joint proposal of AT&T Bell Laboratories, Thomson Consumer Electronics, Fraunhofer Society and CNET
Centre national d'études des télécommunications
CNET or Centre national d'études des télécommunications was a French national research centre in telecommunications....
. It provided the highest coding efficiency.
A working group
Working group
A working group is an interdisciplinary collaboration of researchers working on new research activities that would be difficult to develop under traditional funding mechanisms . The lifespan of the WG can last anywhere between a few months and several years...
consisting of Leon van de Kerkhof (The Netherlands), Gerhard Stoll (Germany), Leonardo Chiariglione
Leonardo Chiariglione
Leonardo Chiariglione is an Italianengineer. He has been at the forefront of a number of initiatives that have helped shape media technology and business as we know them today, in particular he is the chairman and co-founded the Moving Picture Experts Group together with Hiroshi Yasuda.-...
(Italy), Yves-François Dehery (France), Karlheinz Brandenburg (Germany) and James D. Johnston (USA) took ideas from ASPEC, integrated the filter bank
Filter bank
In signal processing, a filter bank is an array of band-pass filters that separates the input signal into multiple components, each one carrying a single frequency subband of the original signal. One application of a filter bank is a graphic equalizer, which can attenuate the components...
from Layer 2, added some of their own ideas and created MP3, which was designed to achieve the same quality at 128 kbit/s as MP2
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
at 192 kbit/s.
All algorithms for MPEG-1 Audio Layer I, II and III were approved in 1991 and finalized in 1992 as part of MPEG-1
MPEG-1
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting possible.Today, MPEG-1 has become...
, the first standard suite by MPEG, which resulted in the international standard ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
/IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
11172-3 (a.k.a. MPEG-1 Audio or MPEG-1 Part 3), published in 1993. Further work on MPEG audio was finalized in 1994 as part of the second suite of MPEG standards, MPEG-2
MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...
, more formally known as international standard ISO/IEC 13818-3 (a.k.a. MPEG-2 Part 3 or backwards compatible MPEG-2 Audio or MPEG-2 Audio BC), originally published in 1995. MPEG-2 Part 3 (ISO/IEC 13818-3) defined additional bit rates and sample rates for MPEG-1 Audio Layer I, II and III. The new sampling rates are exactly half that of those originally defined for MPEG-1 Audio. MPEG-2 Part 3 also enhanced MPEG-1's audio by allowing the coding of audio programs with more than two channels, up to 5.1 multichannel. There is also MPEG-2.5 audio, a proprietary unofficial extension developed by Fraunhofer IIS. It enables MP3 to work satisfactorily at very low bitrates and added lower sampling frequencies. MPEG-2.5 was not developed by MPEG and was never approved as an international standard.
| Version | First public release date (First edition) | Latest public release date (edition) | |
|---|---|---|---|
| MPEG-1 Audio Layer III | ISO/IEC 11172-3 (MPEG-1 Part 3) | 1993 | |
| MPEG-2 Audio Layer III | ISO/IEC 13818-3 (MPEG-2 Part 3) | 1995 | 1998 |
| MPEG-2.5 Audio Layer III | nonstandard, proprietary |
Note: The ISO standard ISO/IEC 11172-3 (a.k.a. MPEG-1 Audio) defined three formats: the MPEG-1 Audio Layer I, Layer II and Layer III. The ISO standard ISO/IEC 13818-3 (a.k.a. MPEG-2 Audio) defined extended version of the MPEG-1 Audio – MPEG-2 Audio Layer I, Layer II and Layer III. MPEG-2 Audio (MPEG-2 Part 3) should not be confused with MPEG-2 AAC (MPEG-2 Part 7 – ISO/IEC 13818-7).
Compression efficiency of encoders is typically defined by the bit rate, because compression ratio depends on the bit depth and sampling rate
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
of the input signal. Nevertheless, compression ratios are often published. They may use the Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
(CD) parameters as references (44.1 kHz, 2 channels at 16 bits per channel or 2×16 bit), or sometimes the Digital Audio Tape
Digital Audio Tape
Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...
(DAT) SP parameters (48 kHz, 2×16 bit). Compression ratios with this latter reference are higher, which demonstrates the problem with use of the term compression ratio for lossy encoders.
Karlheinz Brandenburg used a CD recording of Suzanne Vega
Suzanne Vega
Suzanne Nadine Vega is an American songwriter and singer known for her eclectic folk-inspired music.Two of Vega's songs reached the top 10 of various international chart listings: "Luka" and "Tom's Diner"...
's song "Tom's Diner
Tom's Diner
"Tom's Diner" is an a cappella pop song written in 1981 by American singer-songwriter Suzanne Vega. It was first released as a track on the January 1984 issue of Fast Folk Musical Magazine. When first featured on one of her own studio albums, it appeared as the first track of her Solitude Standing...
" to assess and refine the MP3 compression algorithm. This song was chosen because of its nearly monophonic
Monaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
nature and wide spectral content, making it easier to hear imperfections in the compression format during playbacks. Some jokingly refer to Suzanne Vega as "The mother of MP3". Some more critical audio excerpts (glockenspiel
Glockenspiel
A glockenspiel is a percussion instrument composed of a set of tuned keys arranged in the fashion of the keyboard of a piano. In this way, it is similar to the xylophone; however, the xylophone's bars are made of wood, while the glockenspiel's are metal plates or tubes, and making it a metallophone...
, triangle
Triangle (instrument)
The triangle is an idiophone type of musical instrument in the percussion family. It is a bar of metal, usually steel but sometimes other metals like beryllium copper, bent into a triangle shape. The instrument is usually held by a loop of some form of thread or wire at the top curve...
, accordion
Accordion
The accordion is a box-shaped musical instrument of the bellows-driven free-reed aerophone family, sometimes referred to as a squeezebox. A person who plays the accordion is called an accordionist....
, etc.) were taken from the EBU
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...
V3/SQAM reference compact disc and have been used by professional sound engineers to assess the subjective quality of the MPEG Audio formats. This particular track has an interesting property in that the two channels are almost, but not completely, the same, leading to a case where Binaural Masking Level Depression causes spatial unmasking of noise artifacts unless the encoder properly recognizes the situation and applies corrections similar to those detailed in the MPEG-2 AAC psychoacoustic model.
Going public
A reference simulation software implementation, written in the C language and later known as ISO 11172-5, was developed (in 1991–1996) by the members of the ISO MPEG Audio committee in order to produce bit compliant MPEG Audio files (Layer 1, Layer 2, Layer 3). It was approved as a committee draft of ISO/IEC technical report in March 1994 and printed as document CD 11172-5 in April 1994. It was approved as a draft technical report (DTR/DIS) in November 1994, finalized in 1996 and published as international standard ISO/IEC TR 11172-5:1998 in 1998. The reference software in C language was later published as a freely available ISO standard. Working in non-real time on a number of operating systems, it was able to demonstrate the first real time hardware decoding (DSPDigital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
based) of compressed audio. Some other real time implementation of MPEG Audio encoders were available for the purpose of digital broadcasting (radio DAB, television DVB) towards consumer receivers and set top boxes.
On July 7, 1994, the Fraunhofer Society
Fraunhofer Society
The Fraunhofer Society is a German research organization with 60 institutes spread throughout Germany, each focusing on different fields of applied science . It employs around 18,000, mainly scientists and engineers, with an annual research budget of about €1.65 billion...
released the first software MP3 encoder called l3enc
L3enc
Fraunhofer l3enc was the first public software able to encode PCM files to the MP3 format. The first public version was released in July 1994. This commandline tool was shareware and limited to 112 kbit/s. It was available for MS DOS, Linux, Solaris, SunOS, NeXTstep and IRIX...
. The filename extension
Filename extension
A filename extension is a suffix to the name of a computer file applied to indicate the encoding of its contents or usage....
.mp3 was chosen by the Fraunhofer team on July 14, 1995 (previously, the files had been named .bit). With the first real-time software MP3 player Winplay3
WinPlay3
WinPlay3 was the first real-time MP3 audio player for PCs running Windows, both 16-bit and 32-bit . Prior to this, audio compressed with MP3 had to be decompressed prior to listening. It was released by Fraunhofer IIS , creators of the MP3 format, on September 9, 1995. The latest version was...
(released September 9, 1995) many people were able to encode and play back MP3 files on their PCs. Because of the relatively small hard drives back in that time (~ 500–1000 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
) lossy compression was essential to store non-instrument based (see tracker and MIDI
Musical Instrument Digital Interface
MIDI is an industry-standard protocol, first defined in 1982 by Gordon Hall, that enables electronic musical instruments , computers and other electronic equipment to communicate and synchronize with each other...
) music for
playback on computer.
Internet
From the second half of 1994 through the late 1990s, MP3 files began to spread on the InternetInternet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
. The popularity of MP3s began to rise rapidly with the advent of Nullsoft
Nullsoft
Nullsoft, Inc. is a software house founded in Sedona, Arizona in 1997 by Justin Frankel. Its most known products include the Winamp media player and the SHOUTcast MP3 streaming media server. In recent years, their open source installer system, NSIS, has also risen in popularity as a widely used...
's audio player Winamp
Winamp
Winamp is a media player for Windows-based PCs and Android devices, written by Nullsoft, now a subsidiary of AOL. It is proprietary freeware/shareware, multi-format, extensible with plug-ins and skins, and is noted for its graphical sound visualization, playlist, and media library features.Winamp...
, released in 1997. In 1998, the first portable solid state digital audio player MPMan, developed by SaeHan Information Systems
SaeHan Information Systems
SaeHan Information Systems is a South Korean information systems company based in Yeouido, Seoul. The company started out as a small subsidiary of Jaeil Textiles in January, 1973. SaeHan Information Systems is credited with the development of the world's first MP3 player....
which is headquartered in Seoul
Seoul
Seoul , officially the Seoul Special City, is the capital and largest metropolis of South Korea. A megacity with a population of over 10 million, it is the largest city proper in the OECD developed world...
, South Korea
South Korea
The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...
, was released and the Rio PMP300
Rio PMP300
The Rio PMP300 was a portable consumer MP3 digital audio player , and was produced by Diamond Multimedia. It was introduced September 15, 1998, and it shipped later that year.-Features:...
was sold afterwards, despite legal suppression efforts by the RIAA.
In November 1997, the website mp3.com
MP3.com
MP3.com is a web site operated by CNET Networks providing information about digital music and artists, songs, services, community, and technologies. It is probably better known for its original incarnation, as a legal, free music-sharing service, popular with independent musicians for promoting...
was offering thousands of MP3s created by independent artists for free. The small size of MP3 files enabled widespread peer-to-peer
Peer-to-peer
Peer-to-peer computing or networking is a distributed application architecture that partitions tasks or workloads among peers. Peers are equally privileged, equipotent participants in the application...
file sharing
File sharing
File sharing is the practice of distributing or providing access to digitally stored information, such as computer programs, multimedia , documents, or electronic books. It may be implemented through a variety of ways...
of music ripped
Ripping
Ripping is the process of copying audio or video content to a hard disk, typically from removable media. The word is used to refer to all forms of media. Despite the name, neither the media nor the data is damaged after extraction....
from CDs, which would have previously been nearly impossible. The first large peer-to-peer filesharing network, Napster
Napster
Napster is an online music store and a Best Buy company. It was originally founded as a pioneering peer-to-peer file sharing Internet service that emphasized sharing audio files that were typically digitally encoded music as MP3 format files...
, was launched in 1999.
The ease of creating and sharing MP3s resulted in widespread copyright
Copyright
Copyright is a legal concept, enacted by most governments, giving the creator of an original work exclusive rights to it, usually for a limited time...
infringement. Major record companies argue that this free sharing of music reduces sales, and call it "music piracy". They reacted by pursuing lawsuits against Napster
Napster
Napster is an online music store and a Best Buy company. It was originally founded as a pioneering peer-to-peer file sharing Internet service that emphasized sharing audio files that were typically digitally encoded music as MP3 format files...
(which was eventually shut down and later sold) and against individual users who engaged in file sharing.
Despite the popularity of the MP3 format, online music retailers often use other proprietary formats that are encrypted or obfuscated in order to make it difficult to use purchased music files in ways not specifically authorized by the record companies. Attempting to control the use of files in this way is known as Digital Rights Management
Digital rights management
Digital rights management is a class of access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. DRM is any technology that inhibits uses of digital content that...
. Record companies argue that this is necessary to prevent the files from being made available on peer-to-peer file sharing networks. This has other side effects, though, such as preventing users from playing back their purchased music on different types of devices. However, the audio content of these files can usually be converted into an unencrypted format. For instance, users are often allowed to burn files to audio CD
Red Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
, which requires conversion to an unencrypted audio format.
Unauthorized MP3 file sharing continues on next-generation peer-to-peer networks. Some authorized services, such as Apple iTunes, Beatport
Beatport
Beatport is an online music store specializing in electronic dance music and culture. Beatport is a privately held company owned and operated by Beatport LLC and based in Denver, Colorado.-History:...
, Bleep
Bleep.com
Bleep is an online music store focusing on the independent music sector. Created by Warp Records and launched in January 2004, Bleep was one of the UK's first legal music download businesses and the only one to originate from within the music industry...
, Juno Records
Juno Records
Juno Records is a UK-based online dance music retail store, selling vinyl records, CDs, music downloads and music accessories, founded by Richard Atherton and Sharon Boyd. The website was created in 1996 as an information-only site called The Dance Music Resource Pages, listing new dance music...
, eMusic
EMusic
eMusic is an online music and audiobook store that operates by subscription. It is headquartered in New York City with an office in London and owned by Dimensional Associates. As of September 2008 eMusic has over 400,000 subscribers....
, Zune Marketplace, Walmart.com
Wal-Mart
Wal-Mart Stores, Inc. , branded as Walmart since 2008 and Wal-Mart before then, is an American public multinational corporation that runs chains of large discount department stores and warehouse stores. The company is the world's 18th largest public corporation, according to the Forbes Global 2000...
, Rhapsody
Rhapsody (online music service)
Rhapsody is an online music store subscription service, launched in December 2001, and available in the United States only. On April 6, 2010, Rhapsody officially declared its independence from RealNetworks. Downloaded files come with restrictions on their use, enforced by Helix, Rhapsody's version...
, the legal incarnation of Napster
Napster (pay service)
Napster is an online music store and a Best Buy company. It was originally founded as a file sharing service. For more information about its founding mission as a free file sharing service, see Napster.-History:...
, and Amazon.com
Amazon.com
Amazon.com, Inc. is a multinational electronic commerce company headquartered in Seattle, Washington, United States. It is the world's largest online retailer. Amazon has separate websites for the following countries: United States, Canada, United Kingdom, Germany, France, Italy, Spain, Japan, and...
sell unrestricted music in the MP3 format.
Encoding audio
The MPEG-1MPEG-1
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting possible.Today, MPEG-1 has become...
standard does not include a precise specification for an MP3 encoder, but does provide example psychoacoustic models, rate loop, and the like in the non-normative part of the original standard. At present, these suggested implementations are quite dated. Implementers of the standard were supposed to devise their own algorithms suitable for removing parts of the information from the audio input. As a result, there are many different MP3 encoders available, each producing files of differing quality. Comparisons are widely available, so it is easy for a prospective user of an encoder to research the best choice. It must be kept in mind that an encoder that is proficient at encoding at higher bit rates (such as LAME
LAME
LAME is a free software codec used to encode/compress audio into the lossy MP3 file format.-History:The name LAME is a recursive acronym for "LAME Ain't an MP3 Encoder". Around mid-1998, Mike Cheng created LAME 1.0 as a set of modifications against the "8Hz-MP3" encoder source code...
) is not necessarily as good at lower bit rates.
During encoding, 576 time-domain samples are taken and are transformed to 576 frequency-domain samples. If there is a transient, 192 samples are taken instead of 576. This is done to limit the temporal spread of quantization noise accompanying the transient. (See psychoacoustics
Psychoacoustics
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...
.)
Decoding audio
Decoding, on the other hand, is carefully defined in the standard. Most decoderDecoder
A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode...
s are "bitstream
Elementary stream
An elementary stream as defined by MPEG communication protocol is usually the output of an audio or video encoder. ES contains only one kind of data, e.g. audio, video or closed caption. An elementary stream is often referred to as "elementary", "data", "audio", or "video" bitstreams or streams...
compliant", which means that the decompressed output – that they produce from a given MP3 file – will be the same, within a specified degree of rounding
Rounding
Rounding a numerical value means replacing it by another value that is approximately equal but has a shorter, simpler, or more explicit representation; for example, replacing $23.4476 with $23.45, or the fraction 312/937 with 1/3, or the expression √2 with 1.414.Rounding is often done on purpose to...
tolerance, as the output specified mathematically in the ISO/IEC high standard document (ISO/IEC 11172-3). Therefore, comparison of decoders is usually based on how computationally efficient they are (i.e., how much memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
or CPU time they use in the decoding process).
Audio quality
When performing lossy audio encoding, such as creating an MP3 file, there is a trade-off between the amount of space used and the sound quality of the result. Typically, the creator is allowed to set a bit rateBit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
, which specifies how many kilobits the file may use per second of audio. The higher the bit rate, the larger the compressed file will be, and, generally, the closer it will sound to the original file.
With too low a bit rate, compression artifact
Compression artifact
A compression artifact is a noticeable distortion of media caused by the application of lossy data compression....
s (i.e. sounds that were not present in the original recording) may be audible in the reproduction. Some audio is hard to compress because of its randomness and sharp attacks. When this type of audio is compressed, artifacts such as ringing or pre-echo
Pre-echo
Pre-echo is a digital audio compression artifact where a sound is heard before it occurs . It is most noticeable in impulsive sounds from percussion instruments such as castanets or cymbals....
are usually heard. A sample of applause compressed with a relatively low bit rate provides a good example of compression artifacts.
Besides the bit rate of an encoded piece of audio, the quality of MP3 files also depends on the quality of the encoder itself, and the difficulty of the signal being encoded. As the MP3 standard allows quite a bit of freedom with encoding algorithms, different encoders may feature quite different quality, even with identical bit rates. As an example, in a public listening test featuring two different MP3 encoders at about 128 kbit/s, one scored 3.66 on a 1–5 scale, while the other scored only 2.22.
Quality is dependent on the choice of encoder and encoding parameters.
The simplest type of MP3 file uses one bit rate for the entire file — this is known as Constant Bit Rate
Constant bitrate
Constant bitrate is a term used in telecommunications, relating to the quality of service. Compare with variable bitrate.When referring to codecs, constant bit rate encoding means that the rate at which a codec's output data should be consumed is constant...
(CBR) encoding. Using a constant bit rate makes encoding simpler and faster. However, it is also possible to create files where the bit rate changes throughout the file. These are known as Variable Bit Rate
Variable bitrate
Variable bitrate is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate , VBR files vary the amount of output data per time segment...
(VBR) files. The idea behind this is that, in any piece of audio, some parts will be much easier to compress, such as silence or music containing only a few instruments, while others will be more difficult to compress. So, the overall quality of the file may be increased by using a lower bit rate for the less complex passages and a higher one for the more complex parts. With some encoders, it is possible to specify a given quality, and the encoder will vary the bit rate accordingly. Users who know a particular "quality setting" that is transparent
Transparency (data compression)
In data compression or psychoacoustics, transparency is the ideal result of lossy data compression. If a lossy compressed result is perceptually indistinguishable from the uncompressed input, then the compression can be declared to be transparent...
to their ears can use this value when encoding all of their music, and generally speaking not need to worry about performing personal listening tests on each piece of music to determine the correct bit rate.
Perceived quality can be influenced by listening environment (ambient noise), listener attention, and listener training and in most cases by listener audio equipment (such as sound cards, speakers and headphones).
A test given to new students by Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
Music Professor Jonathan Berger showed that student preference for MP3 quality music has risen each year. Berger said the students seem to prefer the 'sizzle' sounds that MP3s bring to music.
Bit rate
Several bit rates are specified in the MPEG-1 Audio Layer III standard: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kbit/s, and the available sampling frequencies are 32, 44.1 and 48 kHz. Additional extensions were defined in MPEG-2 Audio Layer III: bit rates 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160 kbit/s and sampling frequencies 16, 22.05 and 24 kHz.A sample rate of 44.1 kHz is almost always used, because this is also used for CD audio
Red Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
, the main source used for creating MP3 files. A greater variety of bit rates are used on the Internet. The rate of 128 kbit/s is commonly used, at a compression ratio of 11:1, offering adequate audio quality in a relatively small space. As Internet bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
availability and hard drive sizes have increased, higher bit rates up to 320 kbit/s are widespread.
Uncompressed audio as stored on an audio-CD has a bit rate of 1,411.2 kbit/s,16 bit/sample × 44100 samples/second × 2 channels / 1000 bits/kilobit so the bitrates 128, 160 and 192 kbit/s represent compression ratios
Data compression ratio
Data compression ratio, also known as compression power, is a computer-science term used to quantify the reduction in data-representation size produced by a data compression algorithm...
of approximately 11:1, 9:1 and 7:1 respectively.
Non-standard bit rates up to 640 kbit/s can be achieved with the LAME
LAME
LAME is a free software codec used to encode/compress audio into the lossy MP3 file format.-History:The name LAME is a recursive acronym for "LAME Ain't an MP3 Encoder". Around mid-1998, Mike Cheng created LAME 1.0 as a set of modifications against the "8Hz-MP3" encoder source code...
encoder and the freeformat option, although few MP3 players can play those files. According to the ISO standard, decoders are only required to be able to decode streams up to 320 kbit/s.
| MPEG-1 Audio Layer III |
MPEG-2 Audio Layer III |
nonstandard proprietary MPEG-2.5 Audio Layer III |
|---|---|---|
| - | 8 | 8 |
| - | 16 | 16 |
| - | 24 | 24 |
| 32 | 32 | 32 |
| 40 | 40 | 40 |
| 48 | 48 | 48 |
| 56 | 56 | 56 |
| 64 | 64 | 64 |
| 80 | 80 | 80 |
| 96 | 96 | 96 |
| 112 | 112 | 112 |
| 128 | 128 | 128 |
| - | 144 | 144 |
| 160 | 160 | 160 |
| 192 | - | - |
| 224 | - | - |
| 256 | - | - |
| 320 | - | - |
| MPEG-1 Audio Layer III |
MPEG-2 Audio Layer III |
nonstandard proprietary MPEG-2.5 Audio Layer III |
|---|---|---|
| - | - | 8000 Hz |
| - | - | 11025 Hz |
| - | - | 12000 Hz |
| - | 16000 Hz | - |
| - | 22050 Hz | - |
| - | 24000 Hz | - |
| 32000 Hz | - | - |
| 44100 Hz | - | - |
| 48000 Hz | - | - |
VBR
MPEG audio may use variable bitrateVariable bitrate
Variable bitrate is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate , VBR files vary the amount of output data per time segment...
(VBR), accomplished via bitrate switching on a per-frame basis, but only layer III decoders must support it. VBR is used when the goal is to achieve a fixed level of quality. The final file size of a VBR encoding is less predictable than with constant bitrate
Constant bitrate
Constant bitrate is a term used in telecommunications, relating to the quality of service. Compare with variable bitrate.When referring to codecs, constant bit rate encoding means that the rate at which a codec's output data should be consumed is constant...
. Average bitrate
Average bitrate
Average bitrate refers to the average amount of data transferredper unit of time, usually measured per second. This is commonly referred to for digital music or video. An MP3 file, for example, that has an average bit rate of 128 kbit/s transfers, on average, 128,000 bits every second...
is VBR implemented as a compromise between the two – the bitrate is allowed to vary for more consistent quality, but is controlled to remain near an average value chosen by the user, for predictable file sizes. Although an MP3 decoder must support VBR to be standards compliant, historically some decoders have bugs with VBR decoding, particularly before VBR encoders became widespread.
Layer III audio can also use a "bit reservoir", a partially full frame's ability to hold part of the next frame's audio data, allowing temporary changes in effective bitrate, even in a constant bitrate stream.
File structure
Elementary stream
An elementary stream as defined by MPEG communication protocol is usually the output of an audio or video encoder. ES contains only one kind of data, e.g. audio, video or closed caption. An elementary stream is often referred to as "elementary", "data", "audio", or "video" bitstreams or streams...
. Frames are not independent items ("byte reservoir") and therefore cannot be extracted on arbitrary frame boundaries. The MP3 Data blocks contain the (compressed) audio information in terms of frequencies and amplitudes. The diagram shows that the MP3 Header consists of a sync word, which is used to identify the beginning of a valid frame. This is followed by a bit indicating that this is the MPEG standard and two bits that indicate that layer 3 is used; hence MPEG-1 Audio Layer 3 or MP3. After this, the values will differ, depending on the MP3 file. ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
/IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
11172-3 defines the range of values for each section of the header along with the specification of the header. Most MP3 files today contain ID3
ID3
ID3 is a metadata container most often used in conjunction with the MP3 audio file format. It allows information such as the title, artist, album, track number, and other information about the file to be stored in the file itself....
metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
, which precedes or follows the MP3 frames; as noted in the diagram.
Design limitations
There are several limitations inherent to the MP3 format that cannot be overcome by any MP3 encoder.Newer audio compression formats such as Vorbis
Vorbis
Vorbis is a free software / open source project headed by the Xiph.Org Foundation . The project produces an audio format specification and software implementation for lossy audio compression...
, WMA Pro and AAC
Advanced Audio Coding
Advanced Audio Coding is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates....
are generally void of a number of these limitations.
In technical terms, some limitations include:
- Time resolution can be too low for highly transient signals and may cause smearing of percussive sounds.
- Due to the tree structure of the filter bank, pre-echo problems are made worse, as the combined impulse response of the two filter banks does not, and cannot, provide an optimum solution in time/frequency resolution.
- The combining of the two filter banks' outputs creates aliasing problems that must be handled partially by the "aliasing compensation" stage; however, that creates excess energy to be coded in the frequency domain, thereby decreasing coding efficiency.
- Frequency resolution is limited by the small long block window size, which decreases coding efficiency.
- There is no scale factor band for frequencies above 15.5/15.8 kHz.
- Joint stereo is done only on a frame-to-frame basis.
- Internal handling of the bit reservoir increases encoding delay.
- EncoderEncoderAn encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
/decoderDecoderA decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode...
overall delay is not defined, which means there is no official provision for gapless playbackGapless playbackGapless playback is the uninterrupted playback of consecutive audio tracks without intervening silence or clicks at the point of the track change. Gapless playback is common with compact discs, gramophone records, or tapes, but is not always available with other formats that employ compressed...
. However, some encoders such as LAMELAMELAME is a free software codec used to encode/compress audio into the lossy MP3 file format.-History:The name LAME is a recursive acronym for "LAME Ain't an MP3 Encoder". Around mid-1998, Mike Cheng created LAME 1.0 as a set of modifications against the "8Hz-MP3" encoder source code...
can attach additional metadata that will allow players that can handle it to deliver seamless playback. - The data stream can contain an optional checksum, but the checksum only protects the header data, not the audio data.
ID3 and other tags
- Main articles: ID3ID3ID3 is a metadata container most often used in conjunction with the MP3 audio file format. It allows information such as the title, artist, album, track number, and other information about the file to be stored in the file itself....
and APEv2 tagAPEv2 tagAn APE tag is a tag used to add metadata, such as the title, artist, or track number, to digital audio files.- APEv1 :The APEv1 tag was designed for the Monkey's Audio format....
A "tag" in an audio file is a section of the file that contains metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
such as the title, artist, album, track number or other information about the file's contents. The MP3 standards do not define tag formats for MP3 files, nor is there a standard container format
Container format
A container or wrapper format is a meta-file format whose specification describes how different data elements and metadata coexist in a computer file....
that would support metadata and obviate the need for tags.
However, several de facto standards for tag formats exist. As of 2010, the most widespread are ID3v1 and ID3v2
ID3
ID3 is a metadata container most often used in conjunction with the MP3 audio file format. It allows information such as the title, artist, album, track number, and other information about the file to be stored in the file itself....
, and the more recently introduced APEv2
APEv2 tag
An APE tag is a tag used to add metadata, such as the title, artist, or track number, to digital audio files.- APEv1 :The APEv1 tag was designed for the Monkey's Audio format....
. These tags are normally embedded at the beginning or end of MP3 files, separate from the actual MP3 frame data. MP3 decoders normally either read info from the tags, or just treat them as ignorable, non-MP3 junk data.
Playing & editing software often contains tag editing functionality, but there are also tag editor
Tag editor
A tag editor is a piece of software that supports editing metadata of multimedia file formats, rather than the actual file content...
applications dedicated to the purpose.
Aside from metadata pertaining to the audio content, tags may also be used for DRM
Digital rights management
Digital rights management is a class of access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. DRM is any technology that inhibits uses of digital content that...
.
Volume normalization
Since volume levels of different audio sources can vary greatly, due to the loudness warLoudness war
The loudness war or loudness race is a pejorative term for the apparent competition to digitally master and release recordings with increasing loudness.The phenomenon was first reported with respect to mastering practices for 7" singles...
and other factors, it is sometimes desirable to adjust the playback volume of audio files such that a consistent average loudness
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...
is perceived. This normalization
Audio normalization
Audio normalization is the application of a constant amount of gain to an audio recording in order to bring the average or peak amplitude to a target level ....
, while similar in purpose, is distinct from dynamic range compression.
ReplayGain is one standard for measuring and storing the loudness of an MP3 file in its metadata tag, enabling a ReplayGain-compliant player to automatically adjust the overall playback volume for each file. MP3Gain
MP3Gain
MP3Gain is an audio normalization software tool. The tool is available on multiple platforms and is free software. It analyzes the MP3 and reversibly changes its volume. The volume can be adjusted for single files or as album where all files would have the same perceived loudness...
may be used to reversibly modify files based on ReplayGain measurements so that adjusted playback can be achieved on players without ReplayGain capability.
Licensing and patent issues
Many organizations have claimed ownership of patentPatent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
s related to MP3 decoding or encoding. These claims have led to a number of legal threats and actions from a variety of sources, resulting in uncertainty about which patents must be licensed in order to create MP3 products without committing patent infringement in countries that allow software patents.
The various MP3-related patents expire on dates ranging from 2007 to 2017 in the U.S. The initial near-complete MPEG-1 standard (parts 1, 2 and 3) was publicly available on December 6, 1991 as ISO CD 11172. In the United States, patents cannot claim inventions that were already publicly disclosed more than a year prior to the filing date, but for patents filed prior to June 8, 1995, submarine patent
Submarine patent
A submarine patent is a patent whose issuance and publication are intentionally delayed by the applicant for a long time, such as several years. This strategy requires a patent system where patent applications are not published. In the United States, patent applications filed before November 2000...
s made it possible to extend the effective lifetime of a patent through application extensions. Patents filed for anything disclosed in ISO CD 11172 a year or more after its publication are questionable; if only the known MP3 patents filed by December 1992 are considered, then MP3 decoding may be patent free in the US by September 2015 when expires which had a PCT filing in Oct 1992.
Technicolor (formerly called Thomson Consumer Electronics) claims to control MP3 licensing of the Layer 3 patents in many countries, including the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and EU countries. Technicolor has been actively enforcing these patents.
MP3 license revenues generated about €100 million for the Fraunhofer Society in 2005.
In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that a license was required to "distribute and/or sell decoders and/or encoders". The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and Thomson. To make, sell and/or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us."
However, there exist both free
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
and/or proprietary alternatives, with free formats such as Vorbis
Vorbis
Vorbis is a free software / open source project headed by the Xiph.Org Foundation . The project produces an audio format specification and software implementation for lossy audio compression...
, FLAC, and others. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
's usage of its own proprietary Windows Media
Windows Media
Windows Media is a multimedia framework for media creation and distribution for Microsoft Windows. It consists of a software development kit with several application programming interfaces and a number of prebuilt technologies, and is the replacement of NetShow technologies.The Windows Media SDK...
format allows it to avoid licensing issues associated with these patents by avoiding usage of the MP3 format entirely. Until the key patents expire, unlicensed encoders and players could be infringing
Patent infringement
Patent infringement is the commission of a prohibited act with respect to a patented invention without permission from the patent holder. Permission may typically be granted in the form of a license. The definition of patent infringement may vary by jurisdiction, but it typically includes using or...
in countries where the patents are valid.
In spite of the patent restrictions, the perpetuation of the MP3 format continues. The reasons for this appear to be the network effect
Network effect
In economics and business, a network effect is the effect that one user of a good or service has on the value of that product to other people. When network effect is present, the value of a product or service is dependent on the number of others using it.The classic example is the telephone...
s caused by:
- familiarity with the format
- the large quantity of music now available in the MP3 format
- the wide variety of existing software and hardware that takes advantage of the file format and does not support the alternatives
- the lack of DRMDigital rights managementDigital rights management is a class of access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. DRM is any technology that inhibits uses of digital content that...
restrictions, which makes MP3 files easy to edit, copy and play in different portable digital players (SamsungSamsungThe Samsung Group is a South Korean multinational conglomerate corporation headquartered in Samsung Town, Seoul, South Korea...
, Apple, Creative, etc.) - the majority of home users not knowing or not caring about the patents' existence and often not considering such legal issues when choosing their music format for personal use
Additionally, patent holders declined to enforce license fees on free
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
and open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
decoders, which allows many free MP3 decoders to develop.
Sisvel S.p.A. and its U.S. subsidiary Audio MPEG, Inc. previously sued Thomson for patent infringement on MP3 technology, but those disputes were resolved in November 2005 with Sisvel granting Thomson a license to their patents. Motorola also recently signed with Audio MPEG to license MP3-related patents.
In September 2006, German officials seized MP3 players from SanDisk
SanDisk
SanDisk Corporation is an American multinational corporation that designs, develops and manufactures data storage solutions in a range of form factors using the flash memory, controller and firmware technologies. It was founded in 1988 by Dr. Eli Harari and Sanjay Mehrotra, non-volatile memory...
's booth at the IFA show in Berlin after an Italian patents firm won an injunction on behalf of Sisvel against SanDisk in a dispute over licensing rights. The injunction was later reversed by a Berlin judge, but that reversal was in turn blocked the same day by another judge from the same court, "bringing the Patent Wild West to Germany" in the words of one commentator.
In February 2007, Texas MP3 Technologies sued Apple, Samsung Electronics and Sandisk in eastern Texas federal court
United States District Court for the Eastern District of Texas
The United States District Court for the Eastern District of Texas is the Federal district court with jurisdiction over the eastern part of Texas and is a part of the Fifth Circuit. The court's headquarters are in Tyler, Texas and has five subdivision offices in Beaumont, Lufkin, Marshall,...
, claiming infringement of a portable MP3 player patent that Texas MP3 said it had been assigned. Apple and Sandisk both settled the claims against them in January 2009. Samsung settled as well.
Alcatel-Lucent
Alcatel-Lucent
Alcatel-Lucent is a global telecommunications corporation, headquartered in the 7th arrondissement of Paris, France. It provides telecommunications solutions to service providers, enterprises, and governments around the world, enabling these customers to deliver voice, data, and video services...
has asserted several MP3 coding and compression patents, allegedly inherited from AT&T-Bell Labs, in litigation of its own. In November 2006 (prior to the companies' merger), Alcatel sued
Alcatel-Lucent v. Microsoft
Lucent Technologies Inc. v. Gateway Inc. 470 F.Supp.2d 1180 is a patent case between Alcatel-Lucent and Microsoft litigated in the United States District Court for the Southern District of California and appealed to the United States Court of Appeals for the Federal Circuit. The litigation money...
Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
for allegedly infringing seven patents. On February 23, 2007, a San Diego jury awarded Alcatel-Lucent
Alcatel-Lucent
Alcatel-Lucent is a global telecommunications corporation, headquartered in the 7th arrondissement of Paris, France. It provides telecommunications solutions to service providers, enterprises, and governments around the world, enabling these customers to deliver voice, data, and video services...
US $1.52 billion in damages for infringement of two of them. The court subsequently tossed the award, however, finding that one patent had not been infringed and that the other was not even owned by Alcatel-Lucent
Alcatel-Lucent
Alcatel-Lucent is a global telecommunications corporation, headquartered in the 7th arrondissement of Paris, France. It provides telecommunications solutions to service providers, enterprises, and governments around the world, enabling these customers to deliver voice, data, and video services...
; it was co-owned by AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
and Fraunhofer, who had licensed it to Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
, the judge ruled. That defense judgment was upheld on appeal in 2008. See Alcatel-Lucent v. Microsoft
Alcatel-Lucent v. Microsoft
Lucent Technologies Inc. v. Gateway Inc. 470 F.Supp.2d 1180 is a patent case between Alcatel-Lucent and Microsoft litigated in the United States District Court for the Southern District of California and appealed to the United States Court of Appeals for the Federal Circuit. The litigation money...
for more information.
Alternative technologies
Many other lossy and lossless audio codecCodec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
s exist. Among these, mp3PRO
Mp3PRO
mp3PRO is an audio compression algorithm that combines the MP3 audio format with spectral band replication compression methods. It claims to achieve transparency at lower bitrates than MP3, resulting in a file nearly half the size of standard MP3...
, AAC
Advanced Audio Coding
Advanced Audio Coding is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates....
, and MP2
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
are all members of the same technological family as MP3 and depend on roughly similar psychoacoustic models. The Fraunhofer Gesellschaft owns many of the basic patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
s underlying these codecs as well, with others held by Dolby Labs, Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
, Thomson Consumer Electronics, and AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
. In addition, there is also the open source file format Vorbis
Vorbis
Vorbis is a free software / open source project headed by the Xiph.Org Foundation . The project produces an audio format specification and software implementation for lossy audio compression...
that has been available free of charge and without any known patent restrictions.
See also
- Audio compression (data)
- Comparison of audio codecsComparison of audio codecsThe following tables compare general and technical information for a variety of audio formats and audio compression formats. For listening tests comparing the perceived audio quality of audio formats and codecs, see the article Codec listening test....
- Copyright infringementCopyright infringementCopyright infringement is the unauthorized or prohibited use of works under copyright, infringing the copyright holder's exclusive rights, such as the right to reproduce or perform the copyrighted work, or to make derivative works.- "Piracy" :...
- Digital audio player
- DJ digital controllerDJ digital controllerDJ digital controllers are MIDI controllers or USB-to-analog devices used for controlling computer based DJ software, installed on a PC or laptop.-Operation:The DJ digital controllers aim to emulate the traditional mixer/turntable/CD turntable set up...
- PodcastPodcastA podcast is a series of digital media files that are released episodically and often downloaded through web syndication...
- LRC (file format)LRC (file format)LRC is a computer file format that synchronizes song lyrics with an audio file, such as MP3, Vorbis or MIDI. When an audio file is played with certain music players on a computer or on modern digital audio players, the song lyrics are displayed. The lyrics file generally has the same name as the...
- Media player
- MP3 blogMP3 blogAn MP3 blog is a type of blog in which the creator makes music files, normally in the MP3 format, available for download. They are also known as "musicblogs" or "audioblogs". MP3 blogs have become increasingly popular since 2003...
- MP3 SurroundMP3 SurroundMP3 Surround is an extension of MP3 for multi-channel audio support including 5.1 surround sound. It was developed by Fraunhofer IIS in collaboration with Thomson and Agere Systems, and released in December 2004....
- MP3HDMp3HDMPEG-1 Audio Layer III HD more commonly known and advertised by its abbreviation mp3HD is an audio compression codec developed by Technicolor formerly known as Thomson...
- Streaming mediaStreaming mediaStreaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
- MPEG
External links
- MP3-history.com, The Story of MP3 — How MP3 was invented, by Fraunhofer IIS
- MPEG.chiariglione.org, MPEG Official Web site
- Wiki.HydrogenAudio.org, MP3
- RFC 3119, A More Loss-Tolerant RTP Payload Format for MP3 Audio
- RFC 3003, The audio/mpeg Media Type