I’ve landed a change that allows using a combination of opus and RFC 2198 redundancy which may improve audio quality in a number of use-cases:
This enabled RED by default in M95 but it is toggled off via a killswitch since it was missing the fmtp line which took a bit longer to ship (the final bit landed in 96.0.4663.0).
Using Chrome Beta (M96 currently) you can try it on
Since the difference is a bit hard to hear see https://webrtchacks.com/red-improving-audio-quality-with-redundancy/ for how to evaluate RED, e.g. with simulated packet loss as well as https://webrtchacks.com/implementing-redundant-audio-on-an-sfu/ and https://www.meetecho.com/blog/opus-red/ for how a server might utilize RED.
audio/red will be using a payload type 63 by default. Note that libwebrtc (in M96) will require a RFC 2198 fmtp line. This does not influence the amount of redundancy the encoder generates which defaults to 1 for the time being. Note that M96 will not negotiate RED from M95 due to the missing fmtp line (which is the reason this got disabled via the killswitch so it should not show up normally).
opus+red is not active by default but can be activated through the setCodecPreferences API by changing the order of codecs such that red appears before opus. Note that in getStats opus+red will still be identified as “opus”.
The encoder is configured to encode a single redundant packet. This greatly improves the POLQA-MOS score for low packet loss without excessive bandwidth overhead. It is possible to use the encoded insertable streams API to write a custom encoder that wraps opus frames in the RFC 2198 format for applications that require more flexibility with respect to the amount of redundancy.
On the receiving side, there is no practical restriction on the amount of redundancy (the hard limit is 32), tests were done with up to four redundant packets.
See the issue tracker for more details.
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