> Actually its not impossible to create extradata from raw Opus streams.
> The extradata only contains a magic, #channes, padding, a sample rate and
> a channel_family.
>
> We know the magic, the #channels can be found out via the packet's
> contents and whether it contains multiple packets, the padding would
> be 0 because its a stream, the sample rate is always 48000 (we purpusefully
> disrespect the container's sample rate in case of Opus, which this is),
> and the mapping family can be set to 255 to just let the decoder decide
> on the number of channels.
>
>
> > The RTP demuxer should start exporting extradata instead, if possible.
> > Either generated from available stream information, or taken verbatim if
> > present. Not sure how the RTP encapsulation for Opus is defined in this
> > regard.
> >
>
> Yes, I think that's the best solution. Just follow the steps I described
> above, and
> to write it out you can check out the opus_write_extradata() function in
> libavcodec/opusenc.c.
Thank you for the feedback everyone. I agree, the RTP demuxer is the best place
for this. I will resubmit the patch as such.
I have a question about "purposefully disrespecting the container sample rate".
In the case of SDP, it is possible to include the sample rate of the source in
the sprop-maxcapturerate parameter in the a=fmtp section. For example (Taken
from RFC7587 Section 7):
m=audio 54312 RTP/AVP 101
a=rtpmap:101 opus/48000/2
a=fmtp:101 maxplaybackrate=16000; sprop-maxcapturerate=16000;
maxaveragebitrate=20000; stereo=1; useinbandfec=1; usedtx=0
a=ptime:40
a=maxptime:40
In this case, would it be appropriate to set the sample rate in the opus header
to 16000? I think in any case, we are allowed to set it to 0, but if the source
sample rate is available we might as well use it.
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