By Mark Oliver, Stretch March 10, 2008 -- dspdesignline.com
Codecs are used to compress video to reduce the bandwidth required to transport streams, or to reduce the storage space required to archive them. The price for this compression is increased computational requirements: The higher the compression ratio, the more computational power is required.
Fixing the tradeoff between bandwidth and computational requirements has the effect of defining both the minimum channel bandwidth required to carry the encoded stream and the minimum specification of the decoding device. In traditional video systems such as broadcast television, the minimum specification of a decoder (in this case a set-top box) is readily defined.
Today, however, video is used in increasingly diverse applications with a correspondingly diverse set of client devices—from computers viewing Internet video to portable digital assistants (PDAs) and even the humble cell phone. The video streams for these devices are necessarily different.
To be made more compatible with a specific viewing device and channel bandwidth, the video stream must be encoded many times with different settings. Each combination of settings must yield a stream that targets the bandwidth of the channel carrying the stream to the consumer as well as the decode capability of the viewing device. If the original uncompressed stream is unavailable, the encoded stream must first be decoded and then re-encoded with the new settings. This quickly becomes prohibitively expensive.
In an ideal scenario, the video would be encoded only once with a high efficiency codec. The resulting stream would, when decoded, yield the full resolution video. Furthermore, in this ideal scenario, if a lower resolution or bandwidth stream was needed to reach further into the network to target a lower performance device, a small portion of the encoded stream would be sent without any additional processing. This smaller stream would be easier to decode and yield lower resolution video. In this way, the encoded video stream could adapt itself to both the bandwidth of the channel it was required to travel through and to the capabilities of the target device. These are exactly the qualities of a scalable video codec.
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