[sdiy] Moogey jitter

[René Schmitz]

Hi Ian and all,

Ian Fritz wrote:

> When I first listened to the file I could hear a small difference 
> between the two instruments.  However, there is about a .5 dB amplitude 
> difference between the two.  I leveled this difference with an editor 
> and now I can't hear any difference between the two signals.

I've listened to it while the tone controls of my soundcard were turned 
down, and it gave a very pronounced difference. Repeating that with the 
controls on the mixing desk it wasn't pronounced anymore. My conclusion 
is that the tone controls on my soundcard suck. (If I'm not mistaken 
then it uses its DSP for that.)

So even when I can't hear much of a difference on the plain wave, there 
might still be things that show up after processing. (The digital wave 
seems to have a little "more bottom", while the VCO is a bit more "bright".)

> The amplitude variations seen in the Moog waveform are due to small 
> transients at the switching point of the waveform.  These come and go at 
> a rate of ~5 Hz.  I can't hear any modulation at that frequency, and I 
> believe the spikes are inaudible artifacts. But people will always say 
> your data has am noise on it anyway, I'm sure.

Could be a sampling artefact. Some of the high frequencies of the fast 
transition might be aliased down.

> I used Sound Forge to resample and interpolate the waves at 4*44.1 kHz.  
> With this done there are transients at all the switching points and they 
> are all nearly the same.  So there is very little amplitude variation.  
> I measured the Moog period at five different points with a resolution of 
> .01 ms.  The periods range from 10.28 ms to 10.30 ms, a variation of 
> 0.2%.  If the jitter is a fixed amount of time, then it would be 2% at 1 
> kHz, which might be audible.

But it will surely turn up if you use several oscs beating against each 
other.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159