[pulseaudio-discuss] Volume problems in Pulse and alsamixer.

[Lennart Poettering]

On Sat, 23.05.09 18:42, Jud Craft (craftjml at gmail.com) wrote:

> 
> Hmm.  I assume that when you say that ALSA gives no reference point
> for 0dB on a card, this is because the cards themselves have no such
> reference point, correct?

The ALSA drivers simply expose the dB information that is documented
in the hardware documentation. Often that documentation mentions
relative to which reference level those dB values are. However, ALSA
doesn't encode that information in any way. Also, a lot of
hardware/documentation does *not* document the reference level, for
example the USB spec doesn't do that at all.

We had longer discussions on alsa-devel about this. Ideally all
'inner' mixer sliders set 0dB to 'same amplitude on input as on output
of the element in question'. And all 'outer' mixer sliders set 0dB to
some defined reference voltage. However, realisitically it is an
incredible amount of work if not impossible to do that. For many cards
(especially cheaper ones) we'd have to physically measure the output
voltage. We'd then have to create a huge quirk database that includes
all that measured data. And then, what would we do about integrated
speakers (i.e. speakers with an USB cable)? Measure sound pressure? 

Given the limited manpower, the lacking equipment and the fact that we
have better things to do then spending all days with measuring sound
cards this is unlikely to be added.

But of course, you are welcome to contribute work in this area!

> Next question, for the curious:  how does PA know how to shift the dB
> scale to where 0dB matches maximum amplification, if ALSA itself
> doesn't know?

For a specific driver ALSA tells us that the volume control range goes
from (as an example) -64dB to +7dB. We have no clue what 0dB means in
this range data. However, what we know is that we can shift this to
-71dB to 0dB and then we know that 0dB is max hw amplification, -71dB
is min hw attenutation and -7dB is the 'base' volume.

Of course, this shifting doesn't add any information we didn't have
before. However it unifies behaviour across different sound cards a
bit and also makes the mixer a bit more like your stereo where 0dB is
max amplification too.

> So what you've said is basically -- "Yes, we know this is weird, but
> we have a new volume scaling formula in the current tree that should
> help balance out your volume control a little bit."  Right?

What I said is basically that it is *not* weird and *does* make a lot
of sense, although the 'percent' to loudness mapping in 0.9.15 can be
improved. Which I did in PA git. And that you should stick to the dB
information when comparing volume slider settings, and ignore the
'percentage' data, because it is very arbitrarily mapped.

Lennart

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