[alsa-devel] 2 speakers are assigned to the same DAC, this can't support 4.0/2.1 channles

[Takashi Iwai]

At Fri, 12 Jun 2015 08:07:20 +0200,
David Henningsson wrote:
> 
> 
> 
> On 2015-06-11 17:10, Takashi Iwai wrote:
> > At Wed, 10 Jun 2015 12:28:39 +0200,
> > Takashi Iwai wrote:
> >> OK, good to know. I'd like to test a bit more via hda-emu whether 
> >> this gives any ill effects. So far, this seems fixing a few other 
> >> machines, too, so it's a good thing to have in general. 
> > This change alone results in regressions on machines that are capable
> > of 4.0/5.1 surrounds.  For avoiding it, the badness for multi-io has
> > to be increased as well.  It's damn sensitive.
> >
> > But, now I wonder now whether blindly applying this is good.  Suppose
> > a machine with 2.1 speaker and one headphone, but the codec has only
> > two DACs.  With this setup, now the headphone and the speaker share
> > the same DAC, as the cost of having individual 2.1 speaker volume.
> > Is this more useful than having individual volumes for speaker and
> > headphone?
> >
> > Maybe the machine you're trying to support has a different situation.
> > So applying the new rule to limited devices is fine.  But if so, it's
> > not necessarily to be an adjustment of badness table, but just you can
> > provide the simple DAC/pin preference map explicitly in the fixup.
> 
> Oh, this is an interesting trade-off.
> 
> In the PulseAudio desktop scenario, we automute the speaker, and 
> PulseAudio remembers the individual headphone and speaker volumes. So in 
> this case, there is no benefit from having individual headphone and 
> speaker volume at the ALSA level.
> 
> However if a user wants to turn off automute, then there is a need for 
> being able to adjust headphone and speaker volume individually.
> 
> But it's not just a question of volume control for 2.1. Being able to 
> send a different stream to the subwoofer could be useful too, especially 
> if the hardware filter is bad or non-existing.
> 
> So my gut feeling leans towards using the second DAC for the subwoofer 
> speaker being the more useful default, but it's not a clear cut.

Right, this is indeed a difficult problem, and probably there is never
a clear answer.

Another interesting examples are:
A. 3 DACs, 2 HP, 2 speakers
B. 3 DACs, 2 HP, 1 mic, 1 speaker
C. 4 DACs, 2 HP, 1 mic, 2 speakers

For B and C, user expects the possible 5.1 outputs by retasking the
mic.  But then you'll lose the individual volume control with the
speaker.


Takashi