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What is the value of the "Headphone Playback Volume" mixer control (as shown by "amixer contents") above and below this threshold?

using "amixer --device hw:Set contents" 36% yields "values=8,8" 34% yields "values=0,0"

interestingly, using "alsamixer" shows that while (default) reports 36%, the C-media device reports 3%

Card: C-Media USB Headphone Set at usb-0000:00:12.0-1, full speed Unit: 9 Control: name="Headphone Playback Volume", index=0 Info: id=9, control=2, cmask=0x3, channels=2, type="S16" Volume: min=-7264, max=-16, dBmin=-2837, dBmax=-6

Unit: 10 Control: name="Auto Gain Control", index=0 Info: id=10, control=7, cmask=0x0, channels=1, type="BOOLEAN" Volume: min=0, max=1, dBmin=0, dBmax=0

Do the automatic gain has any side effect when it normalise the volume?

Do you mean the sound card does not has 7248 steps ?

http://www.freedesktop.org/wiki/Software/PulseAudio/Backends/ALSA/Decibel/

You need to find out the value of c->res

https://bugzilla.redhat.com/show_bug.cgi?id=1065097

As you complain dB range -28.37 to -0.06 are not correct

You need to calibrate the dB range

https://bugs.freedesktop.org/show_bug.cgi?id=46236