Re: [Alsa-user] OT? - Using a test tone to set correct signal levels

[Bill U. <un...@ph...>]

On Fri, 21 Jul 2006, Arthur Marsh wrote:

> I had some problems with *some* .wav files distorting badly when played
> in xmms.
>
> To check to see if sound levels were clipping, I downloaded the
> executable from http://sourceforge.net/projects/wavb then added a
> missing library (libpng3), and was able to easily produce a 30 second
> sample of a 1020 Hz sine was 100 percent modulated as a .wav file.
>
> Playing this .wav file in xmms revealed that I needed to set the Input
> level of "Wave" for my EMU10K1 based card to 60 percent, in order not to
> have clipping if the output levels "PCM" and "Master" were set to 100
> percent. (I used kmix, but alsamixer or similar would be fine).

Not a good test. Instead check the level of the recorded sound ( eg with
sox) to see how high you need to set the input level to get clipping.
It could well be that the out is clipping even with a not-maximal sound.

>
> It may be worth checking if one wants to eliminate distortion from
> signal levels being too high passing through the sound card.
>

Or directly measure the distortion. Often at least on cheaper cards
distortion starts climbing long before clipping starts. 
(eg www.theory.physics.ubc.ca/soundcard.html could for example be used.
Unfortunately it only uses the oss or alsaoss at present)

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-- 
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     un...@ph...
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/