Multiple description coding
Encyclopedia
Multiple description coding (MDC) is a coding technique that fragments a single media stream
into n substreams (n ≥ 2) referred to as descriptions. The 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 networks 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 correct received data.
Besides increased fault tolerance, MDC allows for rate-adaptive streaming: Content providers send all descriptions of a stream without paying attention to the download limitations of clients. Receivers that can't sustain the data rate only subscribe to a subset of these streams, thus freeing the content provider from sending additional streams at lower data rates.
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 Information Theory.
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
into n substreams (n ≥ 2) referred to as descriptions. The 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 networks such as the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
— 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
MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...
and MPEG-4
MPEG-4
MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...
. 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 correct received data.
Besides increased fault tolerance, MDC allows for rate-adaptive streaming: Content providers send all descriptions of a stream without paying attention to the download limitations of clients. Receivers that can't sustain the data rate only subscribe to a subset of these streams, thus freeing the content provider from sending additional streams at lower data rates.
The vast majority of state-of-the art codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
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 Information Theory.