[sdiy] Analysis of frequency variation in analogue synths

[Nicholas Gregorich]

On May 3, 2007, at 2:41 AM, Tom Wiltshire wrote:

> Although I've heard much heated debate about VCOs being better than  
> DCOs (or not) and many presentations of the reasons that this might  
> (or might not) be the case, there isn't a great deal (notice I'm  
> not saying none) of actual experimental data showing measurable  
> differences. What does the frequency/phase drift on a VCO look  
> like? Show me a trace! How much comparator jitter is there on a  
> Moog VCO? Give a percentage! Unless I can measure and identify  
> these effects, I don't stand a chance of trying to emulate them in  
> a digital oscillator, which is my ultimate motivation for all this.

This reminded me of information from Dave Smith via Dave Bryce.  
Regarding the choice of DCOs instead of VCOs in the evolver, Dave  
Smith's claim is that a VCO and DCO output signal are  
indistinguishable on a scope. While this may or may not be true, the  
ears not eyes should be the ultimate judge. The sound of an evolver  
DCO (through open VCF, VCA etc) did not sound as warm/thick/analog as  
other synths I have here featuring VCOs (through open VCF, VCA).

[This brings up the point of testing a synth waveform with and  
without the VCF/VCA to see/hear if there is a significant amount of  
color added by those circuits even when they are all the way open.]

In addition Dave Smith's claim depends what VCO he was comparing to.  
If a CEM VCO (seems safe to assume this would be his baseline) is not  
as warm as other VCOs, it may be easier to make a DCO emulate them.

You might try and find information about the analog/slop parameter  
that is included on Dave Smith's evolver, it may give you a head  
start on what has been done to reduce the sterile sound of the DCO.

Regarding the last sentence quoted above, I think if you do  
successfully measure and document what makes a VCO sound like a VCO  
and implement it in software, you should be able to sell a lot of  
whatever you make. ;)

Nick.