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RealVideo[.rm, .rmvb]

RealVideo 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.

RealVideo 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.

Technology

The 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.

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.

RealVideo 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.

To 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.

 
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