<title>Low-delay center-bridged and distributed combining schemes for multipoint videoconferencing</title>

Abstract

Multipoint videoconferencing is a natural evolution of two-point videoconferencing and can increase its value to users. Currently, ITU-T's SG8 and SG15 are working on multipoint control related issues; ANSI's T1A1.5 is also working in this area. The video coding and related communication standards of the upcoming MPEG4 and H.26P will all include multipoint communication capability. This paper investigates transport structures and the associated combining schemes that can be used to support multipoint videoconferencing. Since low delay is a major issue for multipoint interactive cornmunications, a distributed structure which renders the lowest delay with many other system advantages is especially investigated.We first analyze the insertion delay of a. QCIF combiner bridge [1, 2] which have been proposing for multipoint videoconferencing. Partial input-output pipelining has been used to reduce the delay. To reduce the insertion delay caused by unevenly distributed inputs, a parallel parsing scheme is proposed. This parallel parsing scheme allows low complexity inputs not to be held up by high complexity inputs and can reduce insertion delay significantly. An efficient delay-reduction algorithm using intraslice coding was also cited in a previous proposal [2]. As a comparison, we describe low delay pel-dornain transcoding schemes which have similar delay performance with coded-domain combining hut have much higher complexities. We also describe a recent proposal [5] which eliminates most of the insertion delay but which requires major changes to existing standards and encoder and decoder implementations.The performance of a distributed transport structure in providing inultipoint video services is then investigated. Using the fact that. bandwidth usage is an important factor in estimating network complexity, it is shown that a distributed transport. structure saves 40 to 62.5% bandwidth for a 4-point conferencing and also renders the shortest delay when compared with other center-bridged structures such as [1, 2] and [5].We then discuss multipoint control and decoder implementations using the distributed transport structure. It is shown that. a. parallel decoding structure can be used to combine multiple inputs to realize a short dela.y which is identical to the dela.y of a two-point conference. The complexity of this special decoder, though, is la.rger than a regula.r decoder. A ca.scaded combiner-decoder at the user site is then shown to make a. good compromise between implementation complexity, bandwidth efficiency and delay performance. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx SPIE Vol. 2308 / 853 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspxNote that some links may be local and is free of charge. For example, in a three-way voice call, the chair initiates the call to all other participa.nt.s an(l the local link between the chai...