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RealVideo is a suite of proprietary video compression formats developed by RealNetworks – the specific format changes with the version. It was first released in 1997 and is at version 10.〔RealNetworks (Products - Codecs ), Retrieved on 2009-08-07〕 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 entire video in advance. Compression and decompression software for each version are called "codecs". ==Technology== The first version of RealVideo was announced in 1997 and was based on the H.263 format. At the time, RealNetworks issued a press release saying they had licensed Iterated Systems' ClearVideo technology and were including it as the RealVideo Fractal Codec.〔(RealVideo Fractal Codec ) Reference to RealPlayer Fractal Codec installation option〕〔Terran Interactive, Inc. (1998) (Codec Central - ClearVideo ), Retrieved on 2009-08-12〕 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 format. RealVideo codecs are identified by four-character codes. RV10 and RV20 are the H.263-based codecs. RV30 and RV40 are RealNetworks' proprietary codecs. 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 Real Data Transport (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. RealNetworks says that the RealVideo and RealAudio codecs are not available in source code under the RPSL license. Source code is available only under RCSL license for commercial porting to non-supported processors and operating systems. While RealNetworks owns most of the intellectual property for RealVideo and RealAudio, RealNetworks has licensed third party technology for certain aspects of those codecs. RealNetworks claims that it does not have the rights to license that technology under an open source license.〔Helix Community, (Helix Producer Category FAQ ), Retrieved on 2009-08-02〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「RealVideo」の詳細全文を読む スポンサード リンク
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