Mumble: Opus codec: disable phase inversion

[streaps]

This issue came up in #4209. It's maybe a good idea to disable "phase inversion" in the Opus encoder and decoder to prevent terrible audio artefacts when downmixing from stereo to mono.

See also
https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/409
for an explanation and audio example.

This would be set by

opus_decoder_ctl(enc, OPUS_SET_PHASE_INVERSION_DISABLED(1))
opus_encoder_ctl(enc, OPUS_SET_PHASE_INVERSION_DISABLED(1))

[Krzmbrzl]

Do you know what phase-inversion is used for at all?

[streaps]

As far as I understand it, it is some psychoacoustic voodoo magic for low-bitrates. Here is a in-depth explanation: http://dirkmittler.homeip.net/blog/archives/7999

The good news: since libopus 1.3 phase inversion is automatically disabled, if the decoder is configured for mono output:

https://github.com/xiph/opus/blob/d8b1fd4ad0233fc69a58ac6f91e1d154f22c481b/celt/celt_decoder.c#L167

[Krzmbrzl]

Yeah I guess we can disable that. Could you submit a PR for this? :)

[TerryGeng]

I think this argument starts to make some sense to me. I will try enable this flag in #4209 , but just to remind you guys that the mono down mix now is of pretty good quality so I doubt how much improvement this idea may achieve.