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TrackWriter.Codec declared the encoder's channel count, so the default mono writer offered opus/48000/1. RFC 7587 signals Opus as opus/48000/2 whatever the stream carries, and that is what the SFU and browsers negotiate, so pion could not bind the track: SetRemoteDescription returned ErrUnsupportedCodec, the SDK logged "a track does not support the negotiated codec", ICE never connected, and nothing was published. examples/audio (which uses the default writer) and examples/publish (which hard-coded one channel) both failed this way against a live call. The capability now always says two channels; mono audio is a valid stream of that codec. A new loopback test publishes the default writer to a peer that negotiates Opus with pion's default codecs and checks that the track binds and its audio arrives; it fails without this change.
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Publishing with the default
TrackWriter(mono) never worked:Codec()declaredopus/48000/1, but RFC 7587 signals Opus asopus/48000/2regardless of the channels carried, and that is what the SFU and browsers negotiate. Pion couldn't bind the track (SetRemoteDescription→ErrUnsupportedCodec, logged as "a track does not support the negotiated codec"), ICE never connected, and no audio reached the call.examples/audioandexamples/publishboth hit it against a live call.TrackWriter.Codec()always declares two channels (mono audio is a valid stream of that codec);examples/publishlikewise.TestTrackWriterBindsToNegotiatedOpus: publishes the default writer over a loopback peer that negotiates Pion's default codecs and checks the track binds and audio arrives. It fails onmainwithErrUnsupportedCodec.TestTrackWriterCodecnow expects two channels for mono.Verified live:
examples/audio(unmodified) publishes into a call and a subscriber receives and decodes the audio; before the change it logged the codec error and never connected.make cipasses.