is a proprietary video format developed by RealNetworks.
It was first released in 1997 and as of 2006 is at version
10. RealVideo is supported on many platforms, including
Windows, Mac, Linux, Solaris, and several mobile phones.
is usually paired with RealAudio and packaged in a RealMedia
(.rm) container. RealMedia is suitable for use as a streaming
media format, that is one which is viewed while it is
being sent over the network. Streaming video can be used
to watch live television, since it does not require downloading
the video in advance.
first version of RealVideo was announced in 1997 and was
based on the H.263 codec. At the time, RealNetworks put
out a press release saying they had licensed Iterated
Systems' ClearVideo technology and were including it in
RealVideo. However, support for ClearVideo quietly disappeared
in the next version of RealVideo.
continued to use H.263 until RealVideo 8, when the company
switched to a proprietary video codec. RealVideo codecs
are identified by four character codes. RV10 and RV20
are the H.263-based codecs. RV30 and RV40 are RealNetworks'
proprietary formats. These identifiers have been the source
of some confusion, as people may assume that RV10 is RealVideo
version 10, when it is actually the first version of RealVideo.
RealVideo 10 uses RV40.
can be played from a RealMedia file or streamed over the
network using the Real Time Streaming Protocol (RTSP),
a standard protocol for streaming media developed by the
IETF. However, RealNetworks uses RTSP only to set up and
manage the connection. The actual video data is sent with
their own proprietary RDT protocol. This tactic has drawn
criticism because it made it difficult to use RealVideo
with other player and server software. However, the open
source MPlayer project has now developed software capable
of playing the RDT streams.
facilitate real-time streaming, RealVideo (and RealAudio)
normally uses constant bit rate encoding, so that the
same amount of data is sent over the network each second.
Recently, RealNetworks has introduced a variable bit rate
form called RealMedia Variable Bitrate (rmvb). This allows
for better video quality, however this format is less
suited for streaming because it is difficult to predict
how much network capacity a certain video stream will
need. Video with fast motion or rapidly changing scenes
will require a higher bit rate. If the bit rate of a video
stream increases significantly, it may exceed the speed
at which data can be transmitted over the network, leading
to an interruption in the video.