Re: [LAD] Calibrating sound cards

On Mon, Apr 14, 2008 at 10:15:50PM +0100, James Courtier-Dutton wrote:

> Is there a recognized method for calibrating a sound card?
> e.g.
>
> For Playback, one sends a sample .wav PCM file to the sound card,
> then measure the analog output from the line-out.

It all depends on what kind of sound card you have

- Consumer
- Semi-pro
- Pro

The latter usually are calibrated, or have a calibrated
gain setting.

Some semi-pro's as well, e.g. my Terratec 88MT is matched
rather precisely to my mixer when all levels are set at the
+4dB setting and gains are set to 0dB.

For consumer type cards there are no standards. The only
thing you can do is connect it to a line/aux/cd input on
your amp, play back a file with rather high levels and
adjust both the card's mixer and your volume control until
it's loud enough and sounds right.

It also depends on what your card is connected to, what you
use it for, and in particular if you have and/or want to use
a volume control after the card, e.g. on the amplifier. For
'entertainment' this is perfectly OK, but in a studio setting
you may want fixed levels, which means that gains are set so
that the card will not overload on the highest peaks you ever
want to reproduce, and volume is set in software. The typical
result of doing this is that things like 'desktop sounds' will
be much too loud. K3B almost blew out my speakers that way
some years ago, which made me ban all KDE apps.

> If the sound card itself provides 0dB of Gain, what should be the
> measured figure?

What is OdB of Gain ? One side of an ADC/DAC is numbers, the
other is a voltage. You can't compare apples to oranges...
You could speak of a 'gain' if you have a defined 0dB voltage
and compare it to maximum digital level, but it could be quite
confusing...

For audio the semi-pro level is -10dBu, and the pro level
is +4dBu, with 0dBu being 775mV RMS. But these are 'normal
program levels', and not the maximum. Normally these levels
are set to correspond to around 12dB below the maximum
digital level. Which means that a semi-pro card must be
able to produce +2dBu (about 1 Volt RMS), and a pro card
must be able to handle at least +16dBu (about 5 Volt RMS).

> Can this be done with a simple AC volt meter?

Most AC voltmeters will give a completely wrong reading
for typical audio frequencies. If you don't go too much
above 100 Hz it will work.

Ciao,

-- 
FA
Laboratorio di Acustica ed Elettroacustica
Parma, Italia
Lascia la spina, cogli la rosa.
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