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

"This must mean that all SxTy modes refer to simulcast streams not SVC ones with multiple spatial layers."

[BA] SxTy means x simulcast streams, each of which have y temporal layers.  You can see the difference between SxTy and LxTy in the dependency diagrams. 

"This must also mean that there is no possibility to send SVC stream with at least two spatial layers using one RTP stream"

[BA]  That is not correct.  VP9 and AV1 support spatial scalability and their RTP payload specs only support sending spatial SVC layers on a single RTP stream. 

"all LxTy scalability modes mean sending spatial layers using separate RTP streams (different SSRCs)"

[BA] VP9 and AV1 codecs do not support sending spatial scalability layers on multiple SSRCs.  It is possible to send simulcast layers on multiple SSRCs but that is not the same thing.  

In general, it is important to distinguish what the output of a codec bitstream from how this is packaged within RTP.   Within WebCodecs, scalabilityMode can be used to configure an encoder.  How that encoding is subsequently packaged is an orthogonal issue. The packaging and transport could be done over WebRTC Data Channel or WebTransport in a manner under control of the application. 

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Received on Saturday, 8 January 2022 20:44:27 UTC