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

On Mon, Jun 07, 2010 at 01:38:59PM -0700, Niels Mayer wrote:
> Fons:
>
> Thanks for your suggestions and input. Here is are some more outputs
> from a different dynex dx-sc51 soundcard:
>
> http://nielsmayer.com/npm/jaaa-db50xg-L-fullband-captureoff.png
> http://nielsmayer.com/npm/jaaa-db50xg-R-fullband-captureoff.png (note
> interesting anomaly at ~14K)

OK, this is the performance of the A/D itself and its driver.

Noise is at around -145 dB (you should have used Video Average
and a noise marker really). With a BW of -1.35 dB this results
in a total noise level of around -145 + 1.35 + 43 = ~ -100 dB.

The stuff around 14 kHz in R is strange. It's noise, not a
single frequency interference. No idea where it comes from.

The noise rises below 100 Hz or so, this is normal. All analog
circuits have some 1/f noise. There may also be mains related
frequencies but to be sure you need to zoom in and use VA.

> http://nielsmayer.com/npm/jaaa-db50xg-L-fullband-captureon.png
> http://nielsmayer.com/npm/jaaa-db50xg-R-fullband-captureon.png
> (anomaly disappears in the noise)

Here we have the noise at around -117 dB, which works out to
a total noise level of around -72 dB. So you have 72 dB
dynamic range (unweighted), not really suberb. To find out
if it is the card itself or whatever is connected to it,
you need to measure this without anything connected and
the input short-circuited.

> http://nielsmayer.com/npm/jaaa-db50xg-L-lowfreq-captureoff.png
> http://nielsmayer.com/npm/jaaa-db60xg-R-lowfreq-captureoff.png

All the interesting stuff is below the lower edge...

> http://nielsmayer.com/npm/jaaa-db50xg-L-lowfreq-captureon.png
> http://nielsmayer.com/npm/jaaa-db50xg-R-lowfreq-captureon.png

Again you should use Video Average, and a F range of 0 to 500 Hz
or so to see if there are any mains related frequencies.

> Is the -62db/hz&-66db/hz at 0.1hz an anomaly?

Looks like a DC offset.

> In 'jaaa' the 0.1 hz value "jumps around" as if there were a shifting
> DC level.

What you probably see is DC + noise. Again use VA for a stable
reading.

> It can be seen in 'yass' for example... "capture off" shows
> a flat line with AGC turned on. "capture on" shows a noise that is
> entirely positive voltage, which can be raised higher by adjusting the
> capture gain above 0db.

Definitely DC offset.
 
Ciao,

-- 
FA
O tu, che porte, correndo si ?
E guerra e morte !
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