Joint (audio engineering)
Encyclopedia
In audio engineering
, joint refers to a joining of several channels of similar information in order to obtain higher quality, a smaller file size, or both.
has allowed for the transfer of relatively low bit rate
, acceptable-quality audio with modest Internet access speeds. Joint stereo refers to any number of encoding techniques used for this purpose. Two forms are described here, both of which are implemented in various ways with different codec
s, such as MP3
, AAC
and Ogg Vorbis.
. Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies. By exploiting this 'limitation', intensity stereo coding can reduce the data rate of an audio stream with little or no perceived change in apparent quality.
More specifically, the dominance of inter-aural time differences (ITD) for sound localization by humans is only present for lower frequencies. That leaves inter-aural amplitude differences (IAD) as the dominant location indicator for higher frequencies. The idea of intensity stereo coding is to merge the upper spectrum into just one channel (thus reducing overall differences between channels) and to transmit a little side information about how to pan
certain frequency regions to recover the IAD cues.
This type of coding does not perfectly reconstruct the original audio because of the loss of information which results in the simplification of the stereo image and can produce perceptible compression artifacts. However, for very low bitrates this type of coding usually yields a gain in perceived quality of the audio. It is supported by many audio compression formats (including MP3
, AAC
and Vorbis
) but not always by every encoder
.
. The side channel is the difference of the left and right channels, or 
. Unlike intensity stereo coding, M/S coding is a special case of transform coding
, and retains the audio perfectly without introducing artifacts. Lossless codecs
such as FLAC
or Monkey's Audio
use M/S stereo coding because of this characteristic.
To reconstruct the original signal, the channels are either added
or subtracted 
.
This form of coding is also sometimes known as matrix stereo and is used in many different forms of audio processing and recording equipment. It is not limited to digital systems and can even be created with passive audio transformer
s or analog amplifier
s. One example of the use of M/S stereo is in FM
stereo broadcasting, where
modulates
the carrier wave
and
modulates a subcarrier
. Another example of M/S stereo is the stereophonic microgroove record. Lateral motions of a stylus represent the sum of two channels and the vertical motion represents the difference between the channels.
technique used in audio data compression to reduce the data rate
.
The idea is to merge a given frequency range of multiple sound channels together so that the resulting encoding will preserve the sound information of that range not as a bundle of separate channels but as one homogeneous data stream. This will destroy the original channel separation permanently, as the information cannot be accurately reconstructed, but will greatly lessen the amount of required storage space. Only some forms of joint stereo use the joint frequency encoding technique, such as intensity stereo coding.
encoder, this is known as forced joint stereo.
Ogg Vorbis stereo files can employ either LR stereo or joint stereo. When using joint stereo, both MS stereo and intensity stereo methods may be used. As opposed to MP3 where MS stereo (when used) is applied before quantization, an Ogg Vorbis encoder applies MS stereo to samples in the frequency domain after quantization, making application of MS stereo a lossless step. After this step, any frequency area can be converted to intensity stereo by removing the corresponding part of the MS signal's S part. Ogg Vorbis' floor function will take care of the required left-right panning.
Audio engineering
An audio engineer, also called audio technician, audio technologist or sound technician, is a specialist in a skilled trade that deals with the use of machinery and equipment for the recording, mixing and reproduction of sounds. The field draws on many artistic and vocational areas, including...
, joint refers to a joining of several channels of similar information in order to obtain higher quality, a smaller file size, or both.
Joint stereo
The term joint stereo has become prominent as 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...
has allowed for the transfer of relatively low bit rate
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
, acceptable-quality audio with modest Internet access speeds. Joint stereo refers to any number of encoding techniques used for this purpose. Two forms are described here, both of which are implemented in various ways with different 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"...
s, such as MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
, 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 Ogg Vorbis.
Intensity stereo coding
This form of joint stereo uses a technique known as joint frequency encoding, which functions on the principle of sound localizationSound localization
Sound localization refers to a listener's ability to identify the location or origin of a detected sound in direction and distance. It may also refer to the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space .The sound localization mechanisms of the...
. Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies. By exploiting this 'limitation', intensity stereo coding can reduce the data rate of an audio stream with little or no perceived change in apparent quality.
More specifically, the dominance of inter-aural time differences (ITD) for sound localization by humans is only present for lower frequencies. That leaves inter-aural amplitude differences (IAD) as the dominant location indicator for higher frequencies. The idea of intensity stereo coding is to merge the upper spectrum into just one channel (thus reducing overall differences between channels) and to transmit a little side information about how to pan
Panning (audio)
Panning is the spread of a sound signal into a new stereo or multi-channel sound field. A typical physical recording console pan control is a knob with a pointer which can be placed from the 8 o'clock dial position fully left to the 4 o'clock position fully right...
certain frequency regions to recover the IAD cues.
This type of coding does not perfectly reconstruct the original audio because of the loss of information which results in the simplification of the stereo image and can produce perceptible compression artifacts. However, for very low bitrates this type of coding usually yields a gain in perceived quality of the audio. It is supported by many audio compression formats (including MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
, 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 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...
) but not always by every encoder
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:...
.
M/S stereo coding
M/S stereo coding transforms the left and right channels into a mid channel and a side channel. The mid channel is the sum of the left and right channels, or. The side channel is the difference of the left and right channels, or . Unlike intensity stereo coding, M/S coding is a special case of transform codingTransform coding
Transform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically lossy, resulting in a lower quality copy of the original input....
, and retains the audio perfectly without introducing artifacts. Lossless codecs
Lossless data compression
Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...
such as FLAC
FLAC
FLAC is a codec which allows digital audio to be losslessly compressed such that file size is reduced without any information being lost...
or Monkey's Audio
Monkey's Audio
Monkey's Audio is a file format for audio data compression. Being a lossless format, Monkey's Audio does not discard data during the process of encoding, unlike lossy compression methods such as AAC, MP3, Vorbis and Musepack....
use M/S stereo coding because of this characteristic.
To reconstruct the original signal, the channels are either added
or subtracted .This form of coding is also sometimes known as matrix stereo and is used in many different forms of audio processing and recording equipment. It is not limited to digital systems and can even be created with passive audio transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s or analog amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s. One example of the use of M/S stereo is in FM
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
stereo broadcasting, where
modulatesModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
the carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
and
modulates a subcarrierSubcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
. Another example of M/S stereo is the stereophonic microgroove record. Lateral motions of a stylus represent the sum of two channels and the vertical motion represents the difference between the channels.
Joint frequency encoding
Joint frequency encoding is an encodingEncoder
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:...
technique used in audio data compression to reduce the data rate
Data rate
Data rate can refer to:* Bit rate, or data transfer rate* Data signaling rate* Data rate units-See also:* Baud rate* Channel capacity* Throughput* Bandwidth everything in this page is falsified...
.
The idea is to merge a given frequency range of multiple sound channels together so that the resulting encoding will preserve the sound information of that range not as a bundle of separate channels but as one homogeneous data stream. This will destroy the original channel separation permanently, as the information cannot be accurately reconstructed, but will greatly lessen the amount of required storage space. Only some forms of joint stereo use the joint frequency encoding technique, such as intensity stereo coding.
Implementations
When used within the MP3 compression process, joint stereo normally employs multiple techniques and can switch between them for each MPEG frame, including simple L/R stereo. With some encoding software it is possible to force the use of joint stereo without ever using L/R stereo. Within the LAMELAME
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, this is known as forced joint stereo.
Ogg Vorbis stereo files can employ either LR stereo or joint stereo. When using joint stereo, both MS stereo and intensity stereo methods may be used. As opposed to MP3 where MS stereo (when used) is applied before quantization, an Ogg Vorbis encoder applies MS stereo to samples in the frequency domain after quantization, making application of MS stereo a lossless step. After this step, any frequency area can be converted to intensity stereo by removing the corresponding part of the MS signal's S part. Ogg Vorbis' floor function will take care of the required left-right panning.
External links
- Jürgen Herre, Fraunhofer IIS. From Joint Stereo to Spatial Audio Coding - Recent Progress and Standardization. October 2004, Paper 157, DAFx'04 7th International Conference of Digital Audio Effects.