[alsa-devel] What does 0dB refer to? (Logitech USB Speakers)

[Takashi Iwai]

At Sun, 20 Apr 2008 02:00:51 +0200,
Lennart Poettering wrote:
> 
> On Tue, 15.04.08 12:17, Takashi Iwai (tiwai at suse.de) wrote:
> 
> > > 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.
> > 
> > This is typically a problem of your usb hardware.  The USB descriptor
> > gives the dB level information and the usb-audio driver simply passes
> > the given data as dB scale information.  But, the hardware actually
> > doesn't behave as it states.  Unfortunately a frequent problem with
> > cheap USB audio devices.
> > 
> > So, it's definitely no universal ALSA problem.  Please don't mix up.
> 
> My understanding is that the dB values of this USB device are relative          
> to its maximum gain, and as such consistent in itself. Does the USB
> spec define the reference level for the dB values?       

Not clearly.  IIRC, it's against the line level...

> If not there's probably nothing wrong with the USB device. However,
> then I guess I am doomed with my task to find good defaults in a
> generic way for mixer controls.

I'm afraid so, too.


Takashi