[alsa-devel] Split a 7.1 device into 4 stereo devices

[Raymond Yau]

>
> I have a 7.1 sound device and speaker-test -c8 can output on each
> speaker. But what I would like to do is using it as 4 stereo devices so
> I could play different sounds on each of them.
>
> Ideally I would like to avoid dmix and friends to be able to have
> gapless and bit-perfect output.
>
> On IRC "debianuser" said it is possible with hdajackretask to make it
> appear as 4 different hw devices in "aplay -l" but we didn't succeed
> doing that. Anyone knows what configuration is needed for that?
>
> Here is my setting:
> http://www.alsa-project.org/db/?f=bf0744873d5e6d42d3adc0076c5f66189e9191b7
>
The easy way is to use jackd and alsa jack plugin

If you really need  need four stereo devices, you need to customized
snd_hda multiout_analog_open, snd_hda multiout_analog_prepare and snd_hda
multiout_analog_cleanup similar to those  alt_analog_capture_xxx function
to use substream->number to provide four stereo devices

static const struct hda_pcm_stream pcm_analog_playback = {
.substreams = 4,
.channels_min = 2,
.channels_max = 2,
/* NID is set in build_pcms */
.ops = {
.open = playback_pcm_open,
.close = playback_pcm_close,
.prepare = playback_pcm_prepare,
.cleanup = playback_pcm_cleanup
},
};

Since only those four retaskable jacks support HP, you may need to use
input jacks if you want four headphones instead of four line out jackd