[SIP Beyond VoIP] Opus at lower sample rate

Juha Heinanen jh at tutpro.com
Wed Aug 27 16:34:42 CEST 2014

Saúl Ibarra Corretgé writes:

> Opus is always advertised at 48KHz but it self adjusts. IIRC you
> always need to specify it at 48KHz.

That is correct, but the problem is that number of channels=2 is
missing.  According to


      The media subtype ("opus") goes in SDP "a=rtpmap" as the encoding
      name.  The RTP clock rate in "a=rtpmap" MUST be 48000 and the
      number of channels MUST be 2.

Blink generates

a=rtpmap:113 opus/48000

when it should generate

a=rtpmap:113 opus/48000/2

Now when /2 is missing, blink is not able to negotiate opus with webrtc
clients such as google chrome browser.

-- Juha

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