[alsa-devel] wrong decibel data?

[Raymond Yau]

2010/5/27 Colin Guthrie <gmane at colin.guthr.ie>

> 'Twas brillig, and Clemens Ladisch at 27/05/10 14:48 did gyre and gimble:
> > Colin Guthrie wrote:
> >> state.Intel {
> >>      control.1 {
> >>              comment.access 'read write'
> >>              comment.type INTEGER
> >>              comment.count 2
> >>              comment.range '0 - 31'
> >>              comment.dbmin -4650
> >>              comment.dbmax 0
> >>              iface MIXER
> >>              name 'Master Playback Volume'
> >>              value.0 30
> >>              value.1 30
> >>      }
> >
> > This is the hardware volume control.
> >
> >>      control.11 {
> >>              comment.access 'read write user'
> >>              comment.type INTEGER
> >>              comment.count 2
> >>              comment.range '0 - 255'
> >>              comment.tlv '0000000100000008ffffec1400000014'
> >>              comment.dbmin -5100
> >>              comment.dbmax 0
> >>              iface MIXER
> >>              name 'PCM Playback Volume'
> >>              value.0 253
> >>              value.1 253
> >>      }
> >
> > This is the emulated software volume control that is created by the
> > softvol plugin.  This control gets recreated by "alsactl restore" even
> > when the plugin is not running.
> >
> > Might it be possible that PA is trying to use this, but that it doesn't
> > have any effect because PA is using PCM device hw:0?  (Try unloading
> > snd-hda-intel and then deleting that entry from /etc/asound.state.)
>
> PA should play nice with the softvol plugin so I don't think this is the
> bit that is at fault.
>

For AC97 codec

PCM -34.5dB to +12 dB
Master -46.5dB to 0dB

The total dB range is -81dB to +12dB

For HDA

No idea how to know the master volume control is a virtual master volume
control

The softvol plugin -51dB to -0dB is software atten of the input digital
signal before pass to the HDA sound card

PA seem has its own software atten/gain and mixing of the digital signal
from the PA clients



>
> I strongly suspect that the reason has already been correctly identified
> a while ago, which is that this card considers -48dB silent where as PA
> assumes this level is -200dB. I believe it was Raymond who pointed out
> the -48dB level in the HDA spec before on this list.
>

Different HDA codecs have different dB range according to the HDA spec


>
> I'm not sure of the internals, but things do indeed go silent when the
> volume reaches the magic -48dB mark (which is around the 14% mark with
> the current cubic mapping).
>
> I suspect that if I were to define infinity to be 48.0 in PA, everything
> would work nicely.
>

I have doubt since I can still hear sound when using ac97 codec of my au8830
below -48dB and using  baudline (require OSS emulation) also indicate that
there is signal recorded by using loopback of ac97 codec


>
> What I think is then ultimately needed is a way to ensure that everyone
> sings from the same hymn sheet regarding the real world value of -inf dB.
>
>
> I was hoping Lennart would have commented on this thread by now, so I'll
> try and prod him to get some proper input as I'm very much flailing
> around wildly in the dark!
>
>
> Col
>
>