12.6. Low Bitrate Streaming

12.6. Low Bitrate Streaming

This scheme has been implemented with H.263 and non-standard RTP packetization and has given good results [20]. There is no technical reason why similarly good results could not be achievable with H.264.

In today's Internet streaming, some of the offered bitrates are relatively low in order to allow terminals with dial-up modems to access the content. In wired IP networks, relatively large packets, say 500 - 1500 bytes, are preferred to smaller and more frequently occurring packets in order to reduce network congestion. Moreover, use of large packets decreases the amount of RTP/UDP/IP header overhead. For low bitrate video, the use of large packets means that sometimes up to few pictures should be encapsulated in one packet.

However, the loss of a packet including many coded pictures would have drastic consequences for visual quality, as there is practically no way to conceal the loss of an entire picture other than repeating the previous one. One way to construct relatively large packets and maintain possibilities for successful loss concealment is to construct multi-time aggregation packets (MTAPs) that contain interleaved slices from several pictures. An MTAP should not contain spatially adjacent slices from the same picture or spatially overlapping slices from any picture. If a packet is lost, it is likely that a lost slice is surrounded by spatially adjacent slices of the same picture and spatially corresponding slices of the temporally previous and succeeding pictures. Consequently, concealment of the lost slice is likely to be relatively successful.