[pulseaudio-discuss] USB dB data

[Jack Bates]

I confirmed with Wireshark that my USB sound card responds to GET_MIN 
with 0x0000 and GET_MAX with 0x6300 ... The audio device class 
definition [1] says the units are 1/256 dB (section 5.2.2.2.3: Mixer 
Control) ... So by my calculation, the min and max are 0 and 99 dB 
(0x6300 / 256 == 99) ... Why then does alsactl print:

> range '0 - 99'
> dbmin 0
> dbmax 38

(/usr/sbin/alsactl -f - store)

 From what I've read, this community knows the most about this, because 
it uses "dB data" [2] (though it's not really a PulseAudio issue) ...

The "0 - 99" makes sense -- it lines up with my calculation ... Where 
does the 38 come from? (I expect it's obvious, I just haven't found the 
explanation?)

Background: I just got a Bose SoundLink Revolve ... When I connect it 
with USB, the volume is silent from 0 to 99% and deafening at 100%. 
Directly adjusting the volume with alsamixer works properly: It 
increases smoothly from 0 to 100% -- changes that are reflected in the 
PulseAudio volume as fine adjustments in between 99 and 100% (64587 to 
65536 PulseAudio volume).

 From what I've read there's a common problem where especially USB 
devices report the wrong dB data (don't follow the spec [3]) -- this has 
previously been handled by patching the USB audio driver [4] ... However 
I haven't figured out how my device deviates from the spec (0x0000 to 
0x6300 or 0 to 99 dB seem plausible) -- my understanding is incomplete: 
I haven't figured out where the value 38 comes from ...

[1] http://www.usb.org/developers/docs/devclass_docs/audio10.pdf
[2] http://0pointer.de/blog/projects/decibel-data
[3] https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/637066
[4] https://bugs.freedesktop.org/show_bug.cgi?id=93193#c14