[alsa-devel] Verifying mixer dB data/Invalid dB data from USB cards, especially Aureon 5.1 MkII
linux at audioscience.com
Wed Feb 17 01:52:11 CET 2010
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
> 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
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
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.
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