Multiple description coding

Multiple Description Coding (MDC) is a coding technique which fragments a single media stream into n independent sub streams (n >= 2) of equal importance, referred to as descriptions. The packets of each description are routed over multiple, (partially) disjoint pathes. 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 loss of quality.

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 required to decode the media stream, enhancement layers are applied to improve stream quality. However, each enhancement layer n + 1 depends on the 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.

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 codecs 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.

A general example of a MD coding system with two descriptions is given in the figure.

File:Mdc example 2descr.jpg

References

• V. K. Goyal, "Multiple Description Coding: Compression Meets the Network," IEEE Signal Processing Magazine, vol. 18, no. 5, pp. 74 -- 94, Sept. 2001.
• R. Puri and K. Ramchandran, “Multiple description source coding through forward error correction codes,” IEEE Proceedings Asilomar Conference on Signals, Systems, and Computers, Asilomar, CA, October 1999.