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

[Raymond Yau]

2010/2/17 Lennart Poettering <mznyfn at 0pointer.de>

> On Tue, 16.02.10 23:38, Mark Brown (broonie at opensource.wolfsonmicro.com)
> wrote:
>
> >
> > On Tue, Feb 16, 2010 at 11:42:12PM +0100, Lennart Poettering wrote:
> > > On Tue, 16.02.10 22:39, Mark Brown (
> broonie at opensource.wolfsonmicro.com) wrote:
> >
> > > > 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.
> >
> > > "line reference level"? What is that?
> >
> > As I understand it (and bear in mind that I'm a software rather than
> > electrical engineer) it's not 100% fixed, though since the actual result
> > depends very much on what it's connected to an absolute answer isn't as
> > important as it might be.  1V RMS is very standard in consumer stuff at
> > least.
> >
> > If you could frame exactly the information that you're looking for then
> > I should be able to ask a few people who are electrical engineers and
> > get a more coherent answer for you.   I guess you're looking at trying
> > to have PulseAudio set sensible levels by default?
>
> Nah. I actually don't need this value for PulseAudio. In PulseAudio I
> chose to shift all scales so that maximum amplification becomes 0 dB,
> so that the UIs are similar to how most hifi racks do it, and which I
> think is a nice, easy and understandable logic. That way my volume
> scales range from -inf dB to 0 dB in all cases, regardless of the
> hardware/drivers in use. Of course, that means that ALSA's own 0dB is
> not directly visible on the scale. Because it might be sometimes
> useful to know where it is, we export that position as "base volume"
> and the UIs show it with a little marker at the side.
>
> The reason I was asking how Jaroslav chose the 0dB position for his
> measurements was purely because I wanted to do my own measurements for
> that Aureon card. The dbmeasure tool I wrote for stuff like that puts
> 0dB at max amplification, however Jaroslav didn't, so I was wondering
> how he chose 0dB then.
>
> Lennart
>
> From the viewpoint of user, they only care how easy to find the point which
is no software gain or software auttentation  since software gain lead to
clipping