What does 0dB refer to? (Logitech USB Speakers)

[Lennart Poettering <mznyfn@0pointer.de>]

Heya!

I think I discussed this with both James and Takashi a while back: it
would be great if we could initialize the sound card mixers in a
better way by default and not everything set to 0. i.e. for output
only PCM should be active, at a sensible volume, no bass/treble/3d
enabled or whatever. All other sources should be muted. And that for
input the mic jack should be active and nothing else, again. Right now
people too often have to invest a lot of time to find the right
controls to toggle/change in the mixer to get something sensible out
of the speakers.

Some distributions have started to ship special scripts that try to
initialize the volumes properly. I know for the Fedora case that this
tool is very very simple, and just initializes PCM and Master to 70%
and that's basically it. It would be highly preferable if we had a
sane tool like this from alsa upstream, which would set the default
values to 0dB, possibly as an extension to "alsactl".

I started to hack something similar to this as part of PulseAudio: if
PA finds that the PCM slider is not really useful for volume control
(no attenuation to at least -60dB, too few steps, no seperate controls
for all channels, ...) it falls back to software volume control,
however tries to initialize the the slider to 0dB first.

Now the thing is, this mostly works fine, however on one of my
devices, a pair of external Logitech USB speakers (ID 046d:0a04
Logitech, Inc. V20 portable speakers), the dB scale seems to be
totally bogus. According to ALSA volume ranges from -41 dB to
+3dB. However, Every setting > -41 dB makes audio awfully
loud. Really, really fucking loud that is. Disco loud. As loud that
the speakers start jump around centimeters due to the massive
vibrations. Setting the volume to 0dB with those speakers really makes
you fear you are trashing them.

I somehow expected that the magic "0dB" setting in ALSA would refer to
some sane default, where the final output would have a sensible
volume. In this case this is not so. I am thus wondering how ALSA
actually defines 0dB? Is this a bug in the hardware that should be
worked around in ALSA? Is this an ALSA bug?

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
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