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

[Raymond Yau]

>
> >Do the automatic gain has any side effect when it normalise the volume?
>
> I am unsure of how to go about testing that, 'alsamixer' does not appear
> to allow me to modify automatic gain.
>

You have to specify card number

It affect your measurent if you are using the mic on your usb audio if the
automatic gain control is switch on

>
> >Do you mean the sound card does not has 7248 steps?
>
> I am not familiar with how to properly check, however dbverify indicates
> 'Element volume range is 0..151 (-28.37..-0.06 dB).'
>
> Instructions for dbmeasure on the wiki indicates the need for a Line In,
> the sound card only provides Line Out and Mic In. Will it automatically
> make use of 'Mic In' if 'Line In' is absent? Or do I even need to use it?
>

Background noise affect the result if you are not using line in and line
out with a loopback cable , this mean you have to minimize the background
noise

>
> >You need to find out the value of c->res
> >As you complain dB range -28.37 to -0.06 are not correct
>
> Quick testing done with dbverify:http://ix.io/eyi
>
>
> >You need to calibrate the dB range
>
> Volume Steps <=3 are mute while volume step 4 is audible,
> based on dbverify testing. This 'seems' to indicate that the
> minimum volume is somewhere between -27.81 dB to -27.63 dB?
>

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?

http://mailman.alsa-project.org/pipermail/alsa-devel/2010-September/032016.html

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

> Additional info logs:
> Output of 'amixer -c2 contents':http://ix.io/eyq
> Alsa info script output:http://ix.io/eyn