[Bug 746617] opusenc: headers are never sent

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=746617

--- Comment #25 from Sebastian Dröge (slomo) <slomo at coaxion.net> ---
(In reply to Olivier Crête from comment #22)
> I think setting the rate in the caps from the depayloader is incorrect, the
> rate is always 48khz for Opus. It's just a "hint", not a guarantee. Same for
> sprop-stereo, it just indicates that it can be safely downmixed, not that
> the data will never be stereo.

What is the difference between "safely downmixed" (or "safely downsampled") and
actually doing that?

What do you suggest should we do here?

> Also, in section 7, the draft says "the number of channels MUST be 2". 
> 
> Ie, please read the RFC before doing patches, and please just revert all of
> this.

I was reading the RFC and as you quote above it says you can safely downmix.
That's what we do now to not produce useless data and cause unneeded processing
overhead.

Why would anything put the sprop- things into the SDP if it wanted to send
stereo or higher sample rates? And what would anybody be supposed to do with
those if they mean nothing?

> Opus was carefully designed to be used without headers with RTP, so
> the mistakes of Vorbis were not repeated.

Right, and it also works without any headers and information from the SDP.

The only difference now is that we actually put this information into the RTP
caps in the payloader so someone can do something useful with them (whatever
that may be), what is missing in the payloader is negotiation (to tell upstream
to produce e.g. mono if downstream only wants mono). And in the depayloader we
put this information in the caps again so that the decoder can do something
with it.


Also we now put the channels and original rate into the audio/x-opus caps now
so that something like "opusenc ! opusdec" behaves like an identity for caps
instead of resampling and upsamling. If the information from the SDP are
supposed to be useless then the fix here would be to not have the depayloader
translate them into caps fields for audio/x-opus.

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