viewer to select the correct streaming file. First,
viewer to select the correct streaming file. First, this is an unnecessary complication for the viewer, and second it does not take into account network congestion. The leading codec suppliers developed multiple bit rate codecs to allow several streams of different bit rates to be bundled into one file. The streaming server then negotiates with the end-user s media player to determine the optimum rate to use for the stream delivery. No user interaction is necessary. A typical example is SureStream from RealNetworks. With SureStream technology, up to eight different bit rates can be encoded. The Helix Server automatically selects the best bit rate to serve for the network conditions. The Microsoft Windows Media Intelligent Streaming is a similar feature. All the viewer sees during network congestion is a gradual lowering of quality, rather than freezes typical of a fixed rate system. Variable bit rate (VBR) Streaming codecs by default use constant bit-rate (CBR) encoding. DVDs and digital satellite television both use variable bit-rate encoding. The DVD has a maximum file size; so, to give the maximum playing time, a low bit rate is indicated. The MPEG-2 compression used differs from an analog recording like VHS. The VHS tape has a fixed video quality, whereas at a fixed bit rate, the MPEG video will have a variable quality, dependent on the entropy of the original content. To avoid visible MPEG artifacts, the DVD encoding rate increases when there is rapid subject movement. During scenes with motion the bit rate increases from a typical value of 3 Mbit/s up to a maximum of 8 Mbit/s. The end result is a constant video quality for the viewer. The bit rate is a trade-off between artifacts and the file size. To achieve the optimum encode within the limit of the maximum file size of the DVD, two passes are used. The first analyzes the material, the second encodes using the parameters from the first. The process can be automatic, or for the best quality transfers the compressionist will adjust the encode rates manually on a scene-by-scene basis. Similar variable bit rates can be used for multiplexed digital satellite television channels. The technique of statistical multiplexing allows channels to aggregate bit rates up to the total that a transponder can carry. If one channel needs more bits, it can borrow from the other channels. This technique is more successful the more channels there are in a multiplex. Statistical multiplexing can be used to add additional channels to a transponder, or to give better quality output when compared with an equal number of fixed bit rate channels. Video encoding 171
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