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.
