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, JetAudio 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.
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. The open-source MPlayer project eventually 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, it is not possible to save an audio stream to a file, but other programs, including MPlayer, RM Downloader, VLC media player, StreamBox VCR, HiDownload and Real7ime Converter can do so.
While the newest version of RealPlayer should be able to play any RealAudio file, other programs may not support all codecs.
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