[alsa-devel] [BUG] Bad decibel data on C-Media USB sound card

[Raymond Yau]

>
> >It affect your measurent if you are using the mic on your usb audio if
the
> >automatic gain control is switch on
>
> Automatic gain control is now switched off.
>
>
> >Background noise affect the result if you are not using line in and line
> >out with a loopback cable
>
> I am using loopback, noise should not be an issue.
>
>
> >How many steps need to reduce amplitude of the recorded sine wave by
> >half when you play a sine wav through your usb audio and record it back
> >using audacity?
>
> Using recorded sine wav provided by 'speaker-test -t sin'
> amplitude of recorded sine wave reduced by half at step 77
>
> output of 'amixer -c 2 set Headphone 77':
>  Simple mixer control 'Headphone',0
>    Capabilities: pvolume pswitch pswitch-joined
>    Playback channels: Front Left - Front Right
>    Limits: Playback 0 - 151
>    Mono:
>    Front Left: Playback 77 [51%] [-13.94dB] [on]
>    Front Right: Playback 77 [51%] [-13.94dB] [on]
>
>

Does this number of steps remain the same when your starting volume are not
at 151?

Your value 77 seem quite different from  alsamixertest

> >You many need to change the name of the volume control to headphone
> >playback volume in alsamixertest
>
> Done. alsamixertest output:http://ix.io/ezd

You may need David to analyse the result

Do those measured value match with those in datasheet?

Seem the measured dB range is larger than 27 dB

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/log/sound/usb?qt=grep&q=cm106