> pulseaudio assume surround21 is four channels
For PulseAudio, surround21 is now three channels. It's mapped to four channels in alsa-lib, exactly which four channels is based on chmap information from the kernel.
> Does it mean that test with 5.1 downmix to surround21 ?
Yes I believe 5.1 would be downmixed to 2.1.
> pulseaudio assume surround21 is four channels
For PulseAudio, surround21 is now three channels. It's mapped to four channels in alsa-lib, exactly which four channels is based on chmap information from the kernel.
> Does it mean that test with 5.1 downmix to surround21 ?
Yes I believe 5.1 would be downmixed to 2.1.