[alsa-devel] 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/aslactl -f - store)

I originally asked about this on the pulseaudio-discuss list [2] because 
it uses "dB data" [3].

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% 
(using PulseAudio). 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 [4]) -- this has 
previously been handled by patching the USB audio driver [5] ... 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] 
https://lists.freedesktop.org/archives/pulseaudio-discuss/2017-August/028600.html
[3] http://0pointer.de/blog/projects/decibel-data
[4] https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/637066
[5] https://bugs.freedesktop.org/show_bug.cgi?id=93193#c14