[LAU] turning a consumer soundcard into "prosumer" w/ quasi-balanced outs

[fons at kokkinizita.net]

On Mon, Jun 07, 2010 at 10:34:09AM -0700, Niels Mayer wrote:

> http://nielsmayer.com/db60xg-jaaa-L-noise.png
> shows the noise level at -51db whereas the
> right channel is the more-reasonable -65db
> http://nielsmayer.com/db60xg-jaaa-R-noise.png

Both figures are without any meaning for three reasons:

1. To have a correct idea of what is going on you have to
   use a smaller bandwidth (around 1 Hz) will do). As you
   decrease the BW, you will see the 'noise level' go down.
   showing that this number has no meaning at all.

2. The only valid figure for noise at a particular frequency
   is *noise density*, the noise power per Hz. For your
   measurement the BW was 16.7 dBHz, so the noise density at
   the left (assuming this is noise and not a DC offset or
   discrete LF frequencies) is -51 - 16.7 = -67.7 dB/Hz.

   You can get this figure without calculation by putting
   a noise marker at the required frequency. To get a stable
   value use the 'Video Average' function. If the signal 
   around the marker frequency is truely noise, the noise
   density will not depend on BW.

3. Using 'Peak Hold' on noise-like signals produces nonsense.

Ciao,

-- 
FA

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E guerra e morte !