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'''Multiple description coding (MDC)''' in [[computing]] is a [[Channel coding|coding]] technique that fragments a single [[Streaming media|media stream]] into ''n'' substreams (''n'' ≥ 2) referred to as descriptions. The [[Packet (information technology)|packets]] of each description are routed over multiple, (partially) disjoint paths. In order to decode the media stream, any description can be used, however, the quality improves with the number of descriptions received in parallel. The idea of MDC is to provide error resilience to media streams. Since an arbitrary subset of descriptions can be used to decode the original stream, [[network congestion]] or packet loss — which are common in [[best-effort network]]s such as the [[Internet]] — will not interrupt the stream but only cause a (temporary) loss of quality. The quality of a stream can be expected to be roughly proportional to data rate sustained by the receiver.▼
▲'''Multiple description coding (MDC)''' is a [[Channel coding|coding]] technique that fragments a single [[Streaming media|media stream]] into ''n'' substreams (''n'' ≥ 2) referred to as descriptions. The [[Packet (information technology)|packets]] of each description are routed over multiple, (partially) disjoint paths. In order to decode the media stream, any description can be used, however, the quality improves with the number of descriptions received in parallel. The idea of MDC is to provide error resilience to media streams. Since an arbitrary subset of descriptions can be used to decode the original stream, [[network congestion]] or packet loss — which are common in [[best-effort network]]s such as the [[Internet]] — will not interrupt the stream but only cause a (temporary) loss of quality. The quality of a stream can be expected to be roughly proportional to data rate sustained by the receiver.
MDC is a form of data partitioning, thus comparable to layered coding as it is used in [[MPEG-2]] and [[MPEG-4]]. Yet, in contrast to MDC, layered coding mechanisms generate a base layer and n enhancement layers. The base layer is necessary for the media stream to be decoded, enhancement layers are applied to improve stream quality. However, the first enhancement layer depends on the base layer and each enhancement layer ''n'' + 1 depends on its subordinate layer ''n'', thus can only be applied if ''n'' was already applied. Hence, media streams using the layered approach are interrupted whenever the base layer is missing and, as a consequence, the data of the respective enhancement layers is rendered useless. The same applies for missing enhancement layers. In general, this implies that in lossy networks the quality of a media stream is not proportional to the amount of correctly received data.
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The vast majority of state-of-the art [[codec]]s uses single description (SD) video coding. This approach does not partition any data at all. Despite the aforementioned advantages of MDC, SD codecs are still predominant. The reasons are probably the comparingly high complexity of codec development, the loss of some compression efficiency as well as the caused transmission overhead.
Though MDC has its practical roots in media communication, it is widely researched in the area of
A related technology is [[layered coding]], which also produces multiple compressed streams, but with a hierarchy between these streams.
==References==
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