[alsa-devel] wrong decibel data?

[Raymond Yau]

2010/6/13 Lennart Poettering <mznyfn at 0pointer.de>

> On Thu, 10.06.10 17:11, Colin Guthrie (gmane at colin.guthr.ie) wrote:
>
> >
> > 'Twas brillig, and Raymond Yau at 10/06/10 04:16 did gyre and gimble:
> > > Can you explain how PA handle the volume controls of ac97 codec ?
> > >
> > > PCM -34.5dB to +*12* dB
> > > Master -46.5dB to 0dB
> > >
> > > The total dB range (PCM + MASTER) is -81dB to +*12*dB
> > >
> > >
> > > Most user concern about recording without distrotion. (i.e. best result
> when
> > > Capture Volume at 0dB , PCM and Master Volume at 0dB ) and they need
> where
> > > are 0dB points
> >
> > I'm not 100% sure how this is handled, but I know it's not ignored.
> > You'll have to ask Lennart directly or dig in the code to see for sure.
>
> If the ALSA volume range is -x dB to +y dB, then the PA volume range
> will be -x-y dB to 0dB (i.e. shifted by -y dB). On top of that most
> volume controls should then mark the ALSA 0dB point as "base" volume on
> the slider, at what PA then calls -y dB.
>
> That way we will expose 0dB as maximum hw amplitude uniformly on all
> sound cards and have a special point on the slider that is hinted to be
> the "comfort" point.
>
> This is all explained on
> http://pulseaudio.org/wiki/WritingVolumeControlUIs#BaseVolumes
>


shifted +12dB to 0dB is completely wrong

+12dB is 4.00 if 0dB is 1.0 (floating point )

you can performed clubsoda 's  experiement as in
https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/581650

>> "This is important because if I use those maximum settings and play any
audio which peaks above -12dB, the output will clip and sound distorted."


if your sound card have ac97 codec ., you can use audacity to record the
output from hw:0,0 and you will see clipping occur when you set "PCM" volume
above 0dB