For multipoint video conferencing over a wide-area network, a video bridge is often used to combine the coded video signals from multiple participants into a single video for display. The video combining can be done in the coded-domain by concatenating coded video bitstreams, or in the pixel-domain by transcoding. In this paper we compare the video quality using the two approaches for a multipoint video conference over a symmetrical wide-area network such as Integrated Service Digital Network (ISDN). We observe that in most multipoint video conferences usually only one or two persons are active at one time, while other participants are just listening with little motion. To achieve similar video quality for all the conference participants, the active persons require higher bit rates than the inactive persons. By doing video combining using the transcoding approach and dynamically allocating more bits to the active persons, we show that the achieved video quality can be much better than that can be achieved using the coded-domain video combining approach.
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