[alsa-devel] Verifying mixer dB data/Invalid dB data from USB cards, especially Aureon 5.1 MkII

Mark Brown broonie at opensource.wolfsonmicro.com
Tue Feb 16 23:39:39 CET 2010

On Tue, Feb 16, 2010 at 10:07:03PM +0100, Lennart Poettering wrote:
> On Tue, 16.02.10 17:29, Jaroslav Kysela (perex at perex.cz) wrote:

> > Basically, if I remember correctly the discussion, 0dB on all elements 
> > should main unchanged volume, <0dB attenuation and >0dB gain.

> But you need some kind of "anchor" for that. What you write above
> applies only for elements that have some kind of analog input and
> output. But the most interesting components are probably the DAC and
> ADC (i.e. the "innermost" elements), and it's not clear what level they
> output, so the "anchor" for the relative dB values is missing...

Typically the DACs and ADCs will have a full scale signal at line
reference level - it should generally be a good approximation with
nothing else to go on.

> If you plot this it looks as if the card actually only implements 10
> or so discrete steps, and at around 10000 just becomes a constant
> function.

That's quite common.

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