[Bug 268136] [snd_uaudio] Distorted audio from MOTU M4 USB interface

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268136

--- Comment #12 from mburetorp <markus.buretorp@gmail.com> ---
virtual_oss reported it couldn't initialize the sample format (-b 32). But
changing to -b 16 fixed that but only the first two channels worked
(dsp_secondary didn't do anything). I believe this was related to vchanformat
which defaults to s16le:2.0. Changing vchanformat when in bitperfect mode
however always changes it to s32le:4.0 regardless of what the argument is (see
command in previous reply). And when vchanformat is s32le:4.0 nothing works.

BUT then I found out that disabling vchans fixes the problem and virtual_oss
works as expected with -b 32 and all four channels (both dsp and dsp_secondary
works). And the good thing is that now the distortion is gone as well.

So I simply do this in /etc/sysctl.conf and initialize virtual_oss like you
said.

dev.pcm.4.play.vchans=0
dev.pcm.4.rec.vchans=0
dev.pcm.4.bitperfect=1

Not sure what the conclusion is, but something is fishy with OSS and vchans in
my case. But then again with virtual_oss you don't need vchans anyway if I
understand correctly.

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