[sdiy] VCO jitter, Slop ... (was: Re: Smith's Evolver Desktop's spikey VCO waveforms)

rsdio at sounds.wa.com rsdio at sounds.wa.com
Mon Jun 10 01:15:16 CEST 2013

I disagree with your hypothesis (Michael?), that analog VCOs sound better or
fatter due to jitter. My hypothesis would be exactly the opposite. Based on
many other circuits, I'd say that digital clocks, oscillators, and PLLs are far
more susceptible to jitter than analog VCOs and PLLs. In fact, the fix for
jitter in a digital system is to create an analog PLL with a presumably analog
VCO to smooth out the jitter of the digital, crystal-based clock.

Jitter involves fast changes in period or frequency, and generally makes audio
sound harsher, and possibly thinner. Drift is a much slower change in pitch,
due to changes in resistor values or silicon gain as temperature changes. There
are surely other flaws in a VCO besides drift, such as power supply noise or
waveform distortion, but I would expect jitter to be either totally nonexistent
or if it's even there then it would be the last and smallest contribution to
the "analog" sound.

If anything, it's the lack of jitter in an analog system that probably makes it
sound fatter than a DCO. If jitter made a VCO sound better, then we could
fatten up any CD by letting the jitter run out of control.

In other words, I agree with David Dixon that this sounds bull unless someone
can actually show jitter happening in a VCO, or even show a DCO with no jitter.

Not picking on you, Michael, since you said you can't prove your hypothesis. I
just suggest that people try to come up with a test for your hypothesis rather
than just ramble on without any basis.

Brian Willoughby
Sound Consulting

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