Re: [webrtc-svc] [Question] Difference between SVC and Simulcast (#60)

1.  In AV1, it is possible for an encoder to generate multiple encodings within the same bitstream.  AFAICT this capability is not supported in other codecs.   Currently, the 'S' modes defined in AV1 are not supported within WebCodecs (or WebRTC). 

2.  VP8 does not support spatial scalability, only simulcast.  So the paragraph was trying to make clear that spatial modes such as `L3T3` are not possible with VP8.  

3. The difference between simulcast and SVC can be seen in the dependency diagrams.  A frame in a simulcast layer depends only on other frames in that same layer, whereas in SVC a higher layer can depend on a lower layer.  

In WebCodecs, simulcast can be supported by creating another encoding, so any codec that WebCodecs can encode can support simulcast.  Within WebCodecs, SVC is supported in VP8 (temporal only), H.264 (temporal only), VP9 (temporal at the moment with work on spatial scalability in progress) .

4. The example uses `L1T3` because it illustrates the use of simulcast along with temporal scalability.  Each simulcast stream is using a different resolution so there is no need to also employ spatial scalability - only temporal scalability is needed. 

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Received on Friday, 7 January 2022 19:13:37 UTC