opus: register Opus as a SIP/SDP codec

[inechay]

What

Register Opus as a SIP/SDP codec so it can be offered and negotiated on the RTP (telephone) leg.

Why

Opus is implemented in this repo (opus/opus.go) but only wired into the internal WebRTC leg — it is never registered as a SIP/SDP codec (no init() / RegisterCodec), unlike g711 / g722 / amrwb. As a result livekit/sip can only offer G711/G722 on the telephone leg, capping wideband calls at G.722 16 kHz even when the softphone offers Opus.

This is the media-sdk side of livekit/sip#281.

Change

opus/opus.go — register Opus the same way g722.go / amrwb.go do:

• CodecInfo{SDPName: "opus/48000/2", SampleRate: 48000, RTPClockRate: 48000, RTPIsStatic: false, Priority: 100}
• mono encode/decode adapters over the existing Encode / Decode
• encoder tuned for telephony: inband FEC + a packet-loss estimate (so lossy mobile paths degrade gracefully), max complexity, a generous bitrate — kept separate from the shared Encode so the WebRTC leg (its own bandwidth estimation) is untouched
• codec-init failures fail the individual call (via a small failWriter) instead of panic-ing the whole process

The RTP media path (rtp.EncodePCM / rtp.DecodePCM) is already generic over CodecInfo + AudioCodec, so this registration is sufficient end to end — no per-codec wiring. Happy to make the encoder settings configurable / less opinionated if preferred.

Companion change

livekit/sip needs a one-line change to enable opus in its default codec set (pkg/sip/media_codecs.go); I'll open that PR once this lands.

Testing

Built into a custom livekit-sip image and verified on a live inbound call from a softphone (Zoiper with Opus above G722): SDP answer a=rtpmap:106 opus/48000/2, log using codecs … audio-codec=opus/48000/2, clean 48 kHz media (expected_pcm_hz=48000, packets_failed=0, write_errors=0, no decode errors), DTMF/telephone-event intact, and audibly better than G.722.

Refs: livekit/sip#281

[CLAassistant]

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[inechay]

Closing in favor of #69, which takes the same approach (register Opus in the media-sdk opus package via init()) and predates this — I hadn't seen it when I opened this one. Thanks @thetillhoff for already carrying this (and the companion livekit/sip#738).

I've left notes on #69 with live-test results and an optional telephony encoder tuning that could go on top once it lands.