RealAudio is a proprietary audio format developed by RealNetworks and first released in 1995. It uses a variety of audio codecs, 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. When this format is available, it is generally one of multiple formats available and consequently having software to play this format is almost never necessary. RealAudio was heavily used by the BBC websites until 2009, though due to its declining use, only BBC World Service is still available in this format.
RealAudio files were originally identified by a filename extension 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).
The official player for RealMedia content is RealNetworks' RealPlayer SP, currently at version 12, 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 source 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.
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