Hector Martin

Another codec tip: if you're encoding with opus, especially with libopus, at low-mid bitrates (96k or so, the default), use --no-phase-inv (apply_phase_inv=0 in ffmpeg libopusenc).

This is an encoding feature which fakes stereo width by putting both channels out of phase for certain bands. Saves some bits, sounds fine... ... until you downmix to mono. Then we're back in crappy MP3 land, as entire frequency bands appear and disappear randomly.

IIUC from what you're saying, the feature itself isn't bad per-se, it's just all decoders that downmix to mono don't do it correctly so it's easier to turn it off? Is it only a problem with a mono source going to stereo and back again, or can come up in other places?

Take a stereo song, downmix it to mono, it sound good. Take the song, encode it to Opus, decode it in stereo, downmix it to mono, it sounds bad. The Opus guys say you're supposed to let the Opus decoder do the downmixing for you to avoid this problem, but that's not practical.

So the feature is bad because it breaks a property of the audio called mono compatibility. And saying the decoder can fix it if you ask it to is not good, because you can't know ahead of time that the audio will be downmixed later in most practical use cases.

Fair enough. I'm curious what professional stereo mixes do differently to ensure compatibility with mono that the algorithm doesn't? Does it change the signal average?