RealAudio
Encyclopedia
RealAudio is a proprietary audio format
developed by RealNetworks
and first released in April 1995. It uses a variety of audio codec
s, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music. It can also be used as a streaming
audio format, that is played at the same time as it is downloaded. In the past, many internet radio
stations used RealAudio to stream their programming over the internet in real time. In recent years, however, the format has become less common and has given way to more popular audio formats. RealAudio was heavily used by the BBC
websites until 2009, though due to its declining use, only BBC World Service
continued to use the format until March 2011
of .ra (for Real Audio). In 1997, RealNetworks also began offering a video format called RealVideo
. The combination of the audio and video formats was called RealMedia
and used the file extension .rm. However, the latest version of RealProducer, Real's flagship encoder, reverted to using .ra for audio-only files, and began using .rv for video files (with or without audio), and .rmvb for VBR video files. The .ram (Real Audio Metadata) and .smil (Synchronized Multimedia Integration Language
) file formats are sometimes encountered as links from web pages (see Streaming Audio section below).
project, to extend their media framework.
When RealAudio was introduced, RealNetworks disclosed no technical details about the audio format or how it was encoded, but it was soon noticed that some of the audio codecs used in RealAudio were identical to those used in cellular telephones and digital television
. As these formats had been described in detail in various technical papers and standards documents, it was possible to write software capable of playing RealAudio based on this information.
A variety of unofficial players now exist, including MPlayer
, and Real Alternative. However, Real Alternative does not decode the audio data by itself, but relies on the dynamically linked libraries (DLLs) from the official RealPlayer. Thus Real Alternative requires RealPlayer to be installed (or at least its DLLs) in order to function. Most other players are based on FFmpeg
, which has its own audio codec library. The audio codecs in ffmpeg were written based on the publicly available information about the formats, and do not use the RealPlayer or Helix software. It is also possible to obtain codecs which allow Windows Media Player
to play some versions of RealAudio.
Although RealNetworks has made the Helix player available as an open source project, they have kept some of the audio codecs proprietary, and the Helix player can not play all RealAudio files.
format, meaning that it can be played while it is downloaded. It is possible to stream RealAudio using HTTP. In this case, the RealAudio file is retrieved similarly to a normal web page, but playback begins as soon as the first part is received and continues while the rest of the file is downloaded. Using HTTP streaming works best with pre-recorded files so some alternative protocols have been developed which work better for live broadcasts.
The first version of RealAudio used a proprietary protocol called PNA or PNM to send streaming audio data. RealNetworks later switched to the IETF standardized Real Time Streaming Protocol
(RTSP) but they use RTSP only to manage the connection. The actual audio data is sent with their own proprietary RDT
protocol, which they initially kept secret. Recently, some specifications for the RDT protocol have been made public through the Helix Community project. By around 2004 the open-source MPlayer project developed a means of playing the RDT streams.
In many cases, web pages do not link
directly to a RealAudio file. Instead, they link to a .ram (Real Audio Metadata) or SMIL
file. This is a small text file
containing a link to the audio stream. When a user clicks on such a link, the user's web browser downloads the .ram or .smil file and launches the user's media player. The media player reads the PNM or RTSP URL
from the file and then plays the stream.
With RealPlayer SP, it is now possible to save an audio stream to a file. Other programs, including MPlayer
, RM Downloader, VLC media player
, StreamBox VCR, HiDownload and Real7ime Converter can also save streams to a file.
s. Each codec is identified by a four character code
. Below is a list of the codecs and the version in which each was introduced:
While the newest version of RealPlayer should be able to play any RealAudio file, other programs may not support all codecs.
Audio format
An audio format is a medium for storing sound and music. The term is applied to both the physical recording media and the recording formats of the audio content – in computer science it is often limited to the audio file format, but its wider use usually refers to the physical method used to...
developed by RealNetworks
RealNetworks
RealNetworks, Inc. is a provider of Internet media delivery software and services based in Downtown Seattle, Washington, United States. The company is the creator of RealAudio, a compressed audio format; RealVideo, a compressed video format; RealPlayer, a media player; RealDownloader, a download...
and first released in April 1995. It uses a variety of audio codec
Audio codec
All codecs are devices or computer programs capable of coding or decoding a digital data stream or signal.The term audio codec has two meanings depending on the context:...
s, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music. It can also be used as a streaming
Streaming media
Streaming 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...
audio format, that is played at the same time as it is downloaded. In the past, many internet radio
Internet radio
Internet radio is an audio service transmitted via the Internet...
stations used RealAudio to stream their programming over the internet in real time. In recent years, however, the format has become less common and has given way to more popular audio formats. RealAudio was heavily used by the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
websites until 2009, though due to its declining use, only BBC World Service
BBC World Service
The BBC World Service is the world's largest international broadcaster, broadcasting in 27 languages to many parts of the world via analogue and digital shortwave, internet streaming and podcasting, satellite, FM and MW relays...
continued to use the format until March 2011
File extensions
RealAudio files were originally identified by a filename extensionFilename extension
A filename extension is a suffix to the name of a computer file applied to indicate the encoding of its contents or usage....
of .ra (for Real Audio). In 1997, RealNetworks also began offering a video format called RealVideo
RealVideo
RealVideo is a suite of proprietary video compression formats developed by RealNetworks – the specific format changes with the version. It was first released in 1997 and is at version 10. RealVideo is supported on many platforms, including Windows, Mac, Linux, Solaris, and several mobile...
. The combination of the audio and video formats was called RealMedia
RealMedia
RealMedia is a proprietary multimedia container format created by RealNetworks. Its extension is ".rm". It is typically used in conjunction with RealVideo and RealAudio and is used for streaming content over the Internet....
and used the file extension .rm. However, the latest version of RealProducer, Real's flagship encoder, reverted to using .ra for audio-only files, and began using .rv for video files (with or without audio), and .rmvb for VBR video files. The .ram (Real Audio Metadata) and .smil (Synchronized Multimedia Integration Language
Synchronized Multimedia Integration Language
SMIL , the Synchronized Multimedia Integration Language, is a W3C recommended XML markup language for describing multimedia presentations. It defines markup for timing, layout, animations, visual transitions, and media embedding, among other things...
) file formats are sometimes encountered as links from web pages (see Streaming Audio section below).
Players
The official player for RealMedia content is RealNetworks' RealPlayer SP, currently at version 14, and is available for various platforms in binary form. Several features of this program have proven controversial (most recently, RP11's ability to record unprotected streaming media from web sites), and many alternative players have been developed. RealNetworks initially tried to discourage development of alternative players by keeping their audio format secret. However, in recent years, RealNetworks has made efforts to be somewhat more open, and has founded the Helix Community, a collaborative open sourceOpen 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...
project, to extend their media framework.
When RealAudio was introduced, RealNetworks disclosed no technical details about the audio format or how it was encoded, but it was soon noticed that some of the audio codecs used in RealAudio were identical to those used in cellular telephones and digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
. As these formats had been described in detail in various technical papers and standards documents, it was possible to write software capable of playing RealAudio based on this information.
A variety of unofficial players now exist, including MPlayer
MPlayer
MPlayer is a free and open source media player. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. Versions for OS/2, Syllable, AmigaOS and MorphOS are also available. The Windows version works, with some minor...
, and Real Alternative. However, Real Alternative does not decode the audio data by itself, but relies on the dynamically linked libraries (DLLs) from the official RealPlayer. Thus Real Alternative requires RealPlayer to be installed (or at least its DLLs) in order to function. Most other players are based on FFmpeg
FFmpeg
FFmpeg is a free software project that produces libraries and programs for handling multimedia data. The most notable parts of FFmpeg are libavcodec, an audio/video codec library used by several other projects, libavformat, an audio/video container mux and demux library, and the ffmpeg command line...
, which has its own audio codec library. The audio codecs in ffmpeg were written based on the publicly available information about the formats, and do not use the RealPlayer or Helix software. It is also possible to obtain codecs which allow Windows Media Player
Windows Media Player
Windows Media Player is a media player and media library application developed by Microsoft that is used for playing audio, video and viewing images on personal computers running the Microsoft Windows operating system, as well as on Pocket PC and Windows Mobile-based devices...
to play some versions of RealAudio.
Although RealNetworks has made the Helix player available as an open source project, they have kept some of the audio codecs proprietary, and the Helix player can not play all RealAudio files.
Streaming audio
RealAudio was developed as a streaming mediaStreaming media
Streaming 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...
format, meaning that it can be played while it is downloaded. It is possible to stream RealAudio using HTTP. In this case, the RealAudio file is retrieved similarly to a normal web page, but playback begins as soon as the first part is received and continues while the rest of the file is downloaded. Using HTTP streaming works best with pre-recorded files so some alternative protocols have been developed which work better for live broadcasts.
The first version of RealAudio used a proprietary protocol called PNA or PNM to send streaming audio data. RealNetworks later switched to the IETF standardized Real Time Streaming Protocol
Real Time Streaming Protocol
The Real Time Streaming Protocol is a network control protocol designed for use in entertainment and communications systems to control streaming media servers. The protocol is used for establishing and controlling media sessions between end points...
(RTSP) but they use RTSP only to manage the connection. The actual audio data is sent with their own proprietary RDT
RDT stream transport
Real Data Transport is a proprietary transport protocol for the actual audio/video data, developed by RealNetworks in the 1990s.Commonly used in companion with a control protocol for streaming media like the IETF's based RTSP protocol....
protocol, which they initially kept secret. Recently, some specifications for the RDT protocol have been made public through the Helix Community project. By around 2004 the open-source MPlayer project developed a means of playing the RDT streams.
In many cases, web pages do not link
Hyperlink
In computing, a hyperlink is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks...
directly to a RealAudio file. Instead, they link to a .ram (Real Audio Metadata) or SMIL
Synchronized Multimedia Integration Language
SMIL , the Synchronized Multimedia Integration Language, is a W3C recommended XML markup language for describing multimedia presentations. It defines markup for timing, layout, animations, visual transitions, and media embedding, among other things...
file. This is a small text file
Text file
A text file is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists within a computer file system...
containing a link to the audio stream. When a user clicks on such a link, the user's web browser downloads the .ram or .smil file and launches the user's media player. The media player reads the PNM or RTSP URL
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
from the file and then plays the stream.
With RealPlayer SP, it is now possible to save an audio stream to a file. Other programs, including MPlayer
MPlayer
MPlayer is a free and open source media player. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. Versions for OS/2, Syllable, AmigaOS and MorphOS are also available. The Windows version works, with some minor...
, RM Downloader, VLC media player
VLC media player
VLC media player is a free and open source media player and multimedia framework written by the VideoLAN project.VLC is a portable multimedia player, encoder, and streamer supporting many audio and video codecs and file formats as well as DVDs, VCDs, and various streaming protocols. It is able to...
, StreamBox VCR, HiDownload and Real7ime Converter can also save streams to a file.
Codecs
RealAudio files are compressed using several different audio codecAudio codec
All codecs are devices or computer programs capable of coding or decoding a digital data stream or signal.The term audio codec has two meanings depending on the context:...
s. Each codec is identified by a four character code
FourCC
A FourCC is a sequence of four bytes used to uniquely identify data formats.The concept originated in the OSType scheme used in the Macintosh system software and was adopted for the Amiga/Electronic Arts Interchange File Format and derivatives...
. Below is a list of the codecs and the version in which each was introduced:
- lpcJ, 14_4: IS-54 VSELP (RealAudio 1)
- 28_8: G.728G.728G.728 is an ITU-T standard for speech coding operating at 16 kbit/s. It is officially described as Coding of speech at 16 kbit/s using low-delay code excited linear prediction....
LD-CELP (RealAudio 2) - dnet: DolbyDolby LaboratoriesDolby Laboratories, Inc. , often shortened to Dolby Labs, is an American company specializing in audio noise reduction and audio encoding/compression.-History:...
AC3 (RealAudio 3) - sipr: Sipro Lab Telecom ACELP-NETACELPAlgebraic code-excited linear prediction is a patented speech coding algorithm by VoiceAge Corporation in which a limited set of pulses is distributed as excitation to linear prediction filter....
(RealAudio 4/5) - cook: G2/Cook CodecCook CodecThe cook codec is a lossy audio compression codec developed by RealNetworks. It is also known as Cooker, Gecko, RealAudio G2, and RealAudio 8 low bitrate ....
(RealAudio 6) - atrc: Sony ATRAC3ATRACAdaptive Transform Acoustic Coding is a family of proprietary audio compression algorithms developed by Sony. MiniDisc was the first commercial product to incorporate ATRAC in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time as CD while storing audio...
(RealAudio 8) - raac: MPEG-4MPEG-4MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...
LC-AACAdvanced Audio CodingAdvanced 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....
(RealAudio 9) - racp: MPEG-4MPEG-4MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...
HE-AACHE-AACHigh-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
(RealAudio 10) - ralf: RealAudio Lossless Format (RealAudio 10)
While the newest version of RealPlayer should be able to play any RealAudio file, other programs may not support all codecs.
See also
- IcecastIcecastIcecast is a streaming media project released as free software maintained by the Xiph.org Foundation. It also refers specifically to the server program which is part of the project. Icecast was created in December 1998/January 1999 by Jack Moffitt and Barath Raghavan to provide an open source...
– a free and open source streaming server for internet radio stations, supports formats AACAdvanced Audio CodingAdvanced 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....
, MP3MP3MPEG-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...
, OggOggOgg is a free, open container format maintained by the Xiph.Org Foundation. The creators of the Ogg format state that it is unrestricted by software patents and is designed to provide for efficient streaming and manipulation of high quality digital multimedia.The Ogg container format can multiplex...
VorbisVorbisVorbis 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...
, TheoraTheoraTheora is a free lossy video compression format. It is developed by the Xiph.Org Foundation and distributed without licensing fees alongside their other free and open media projects, including the Vorbis audio format and the Ogg container....
, and NSVNullsoft Streaming VideoNullsoft Streaming Video is a media container designed for streaming video content over the internet. NSV was developed by Nullsoft, the makers of Winamp.... - SHOUTcastSHOUTcastSHOUTcast is cross-platform proprietary software for streaming media over the Internet. The software, developed by Nullsoft , allows digital audio content, primarily in MP3 or HE-AAC format, to be broadcast to and from media player software, enabling the creation of Internet radio "stations"...
– a freeware server for internet radio stations, supports AACAdvanced Audio CodingAdvanced 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....
, MP3MP3MPEG-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...
, and NSVNullsoft Streaming VideoNullsoft Streaming Video is a media container designed for streaming video content over the internet. NSV was developed by Nullsoft, the makers of Winamp.... - Windows Media AudioWindows Media AudioWindows Media Audio is an audio data compression technology developed by Microsoft. The name can be used to refer to its audio file format or its audio codecs. It is a proprietary technology that forms part of the Windows Media framework. WMA consists of four distinct codecs...
– Microsoft'sMicrosoftMicrosoft 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...
media formats - 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....
External links
- RealPlayer – Official media player for RealAudio
- Helix Player is an open source media player for Linux, Solaris, and Symbian
- JetAudio, A free player that plays RealAudio streams and files, without the use of any RealAudio program but with the need to install the "FLV player" program
- A more technical description of the RealMedia format and codecs
- How to download Real Audio streams and convert Real Audio to MP3
- EBU subjective listening tests on low-bitrate audio codecs
- Unofficial Real7ime Converter information and download website
- Filter for playing RealNetworks audio/video files using Windows Media Player