[alsa-devel] wrong decibel data?

Lennart Poettering mznyfn at 0pointer.de
Sun Jun 13 15:53:49 CEST 2010

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


Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

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