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