[alsa-devel] FIXME for Rear Channels of CMI8738, how do i write a softvol to fix this defect?

[John L. Utz III]

On Wed, 09 May 2007 15:03:32 +0200
"Takashi Iwai" <tiwai at suse.de> wrote:

> At Mon, 7 May 2007 14:10:18 -0700,
> John L. Utz III wrote:
> > 
> > Hello;
> > 
> > the CMI8738 allows for 4 channel output by providing a second DAC,
> > but sadly the hardware has no volume control for said second DAC.
> > 
> > A review of /usr/share/alsa/cards/CMI8738-MC6.conf shows that the
> > implementor of said file was aware of the need to fix the problem:
> > 
> > # 2nd DAC
> > # FIXME: we need a volume attenuator for rear channel.
> > CMI8738-MC6.pcm.rear.0 {
> >         @args [ CARD ]
> >         @args.CARD {
> >                 type string
> >         }
> >         type hw
> >         card $CARD
> >         device 1
> > }
> > 
> > 
> > 
> > I'd really like to fix this!
> > 
> > Is it fixable by creating a softvol plugin?
> > 
> > http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html#pcm_plugins
> > 
> > so, if i have to invoke speaker test with -Dhw:0,1 to get 4 channel
> > out, what does that tell me about how i need to configure a softvol
> > for this item?
> > 
> > Or am i missing the point and it's actually the *driver* that needs
> > to be doctored up to provide a softvol implementation. If so, how?
> 
> For using the rear PCM defined in the above, you must use "rear" PCM
> as it's named, not hw:0,1, such as
> 	% aplay -Drear foo.wav

thankyou for pointing that out!

however, i wasnt planning on using -Drear, i included the snippet to
provide context concerning the fact that a set of folks larger than me
agree that the problem needs fixin! :-)

i need to have four channel output, thus my use of hw:0,1.

So, fixing rear volume via softvol or something else has to work in the
context of four channel output.

if there is a way to get 4 channel output (as defined as successful use
of 'speaker-test -t sin -c 4 -Dhw:0,1) *besides* using hw:0.1, i would
be interested to learn of it.

> The softvol implementation is easy.  See CMI8738-MC8.conf as a
> reference.  Note that you should use a different mixer name instead of
> "PCM Playback Volume".  If an existing control name is specified,
> softvol will be skipped automatically.  Use "Rear Playback Volume"
> instead, for example.

I will absolutely use Rear Playback Volume! :-)

> 
> Takashi
> 

tnx for your help!

johnu

(ps: it's so nice to be able to use a proper email client again! i am
really grateful that our windows centric admins where willing to make a
special exception and turn on IMAP for me!)