[pulseaudio-discuss] USB dB data

[Tanu Kaskinen]

On Tue, 2017-08-29 at 13:47 -0700, Jack Bates wrote:
> 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

While it's possible that someone reading this list might know, you have
a better chance asking on the alsa-devel list.

-- 
Tanu

https://www.patreon.com/tanuk