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

[Eliot Blennerhassett]

Lennart Poettering wrote:
> On Wed, 17.02.10 12:47, Eliot Blennerhassett (linux at audioscience.com) wrote:
> 
>> For our cards this is adjustable, but lets say for consumer cards it is
>> fixed.  This information is not currently available via the ALSA
>> API.
> 
> I wonder how realistic it would even be to add an API for
> this. i.e. can we even know the refernce level? On many cards most
> likely not. For example, for my integrated USB speakers speaking of a
> reference voltage would already be misleading...

Certainly. Same for laptop speakers.

Perhaps could define 0dB as the highest valid volume setting that
doesn't cause distortion in the audio when playing a full scale digital
signal.

Otherwise the absolute reference might be dBSPL at X metres from the
speaker ;)  (before anyone answers - in this context I'm joking.)

> Yepp, that's the exactly what I was asking, Jaroslav's measurements
> suggested to shift one of the mixer controls from 0..x dB to -x..0 dB
> and I was just wondering why, since we don't define the ref level in
> ALSA, hence the shift is kinda arbitrary...
> 
> Anyway, I am not pushing for adding an API for reference levels to
> ALSA. 

We define controls with names like "LineIn0 Level","LineOut1 Level"
which we document as having units of dBu.

Given that no other driver has this, its the de-facto standard ;)

> As mentioned for the PA case I decided to shift 0dB to max
> amplification in any case, which I think is a workable way to avoid
> this problem.

Sure.