[LAD] A little quiz about audio measurements...

[Gabriel M. Beddingfield]

Dammit, Fons!  I'm trying to get some work done today and go 
off and nerd snipe me.....

On Fri, 28 May 2010, fons at kokkinizita.net wrote:

>> Noise and interference on the voltage reference causes
>> amplitude modulation, and jitter on the clock causes
>
> You're close. See the extra hint in a previous post.

OK, here's my guess (standing on the shoulders of guys
like Neil)...

The signal is being amplitude-modulated by a 100 Hz
saw-tooth signal.  This signal by itself produces a
tight frequency series in multiples of the fundamental
(i.e. 100, 200, 300, 400 Hz...)  and constant
amplitude.  When used to modulate the amplitude of the
input signal, it results in a peak at the input
singal's frequency... with symmetric, attenuated peaks
@ 100 Hz spacing (like the ones that you show).

If you change the frequency of the input signal, the
surrounding peaks will move with it to maintain a 100 Hz 
spacing.

The 100 Hz (being 2x 50Hz, the power freq. in Italy)
suggests that it is probably related to some manner of
power supply.  However, I have no theory why we're
getting 2x 50Hz (and I think I need one :-)).

That the signal is (or is approximately) a sawtooth wave 
suggests a capacitor somewhere... either corrupting the 
ground signal or creating a ripple in the DC power supply 
(rectifier circuit or a broken voltage regulator).  I would 
probably start by checking the MOBO power supply.

-gabriel

p.s. Nerd Sniping... http://xkcd.com/356/