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

[Allen M.]

Raymond Yau wrote:

>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]


>You many need to change the name of the volume control to headphone
>playback volume in alsamixertest

Done. alsamixertest output: http://ix.io/ezd