[SIP Beyond VoIP] opus codec negotiation

Saúl Ibarra Corretgé saul at ag-projects.com
Thu Jun 12 19:08:00 CEST 2014

On Jun 12, 2014, at 2:23 PM, James Cloos wrote:

>>>>>> "JH" == Juha Heinanen <jh at tutpro.com> writes:
> JH> see below.  both baresip and google/firefox webrtc use this kind of
> JH> rtpmap for opus:
> JH> a=rtpmap:109 opus/48000/2.
> And blink used:
>    a=rtpmap:113 opus/48000.
>    a=fmtp:113 useinbandfec=1.
> The main difference is stereo vs mono.
> If you can convice blink to do stereo or the others to do mono they
> should inter-negotiate.

Opus is supposed to always support stereo, but downgrade to mono if necessary. Initially the draft stated that the codec was suppodes to not be offered as stereo (the /2) and then that changed. The testing I did back then showed that this way (without the /2) yielded better interoperability. With the proliferation of more Opus capable endpoints, maybe this doesn't hold true anymore.

Regardless, I thought I had tricked Blink into also accepting calls with the /2 in there :-S I'll take a look.


Saúl Ibarra Corretgé
AG Projects

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