Eliminating periodical key frame in online lecture broadcasting by Peer-Assisted Reference Frame Synchronization

In traditional video broadcast systems, the encoders have to insert periodical I frames (a.k.a. key frame) in order to allow newly arrived receivers to be able to start the decoding process in a timely manner. However, such insertion of periodical I frames will result a loss of encoding efficiency as these frames could actually have been encoded into more bit-rate efficient P/B frames instead. This issue is even more relevant for video sequences with very slow scene changes like that in an online lecture. In our p2p streaming based live lecture broadcast system named PPClass, we designed a novel scheme to address this issue. The key idea is that when a new receiver joins the channel, it will try to retrieve the necessary reference frames from peers which have already been watching the channel. Moreover, a new type of frame named Sync-P-frame is introduced in the encoder in place of the periodical I frame, which has similar coding efficiency as normal P frame but also remain the semantic as normal I frame in the sense of providing random-access points. The initial experiments proved the effectiveness of this mechanism.