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RealAudio[.ra]

RealAudio is a proprietary audio format developed by RealNetworks. 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. Many internet radio stations use RealAudio to stream their programming over the internet in real time.

The first version of RealAudio was released in 1995. As of 2006, the current version is RealAudio 10.

File extensions

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).

Streaming Audio

RealAudio was developed as a streaming media 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. 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. 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.

 
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