[sdiy] Analysis of frequency variation in analogue synths

[Tom Wiltshire]

Hello all,

I've just spent an interesting but rather confusing evening trying to  
analyse the minor variations in frequency that are supposed to  
account for the 'analogue character' of VCOs. I'm hoping to design a  
digital oscillator for my monosynth project, and had wondered about  
trying to include an 'analogue feel' parameter. As a precursor to  
this, I thought I'd better find out what 'analogue feel' really looks  
like.

Given that I haven't got a full suite of fancy equipment at my  
disposal, I sampled some tones from my Pro-One and SH101 into the  
computer and then looked at the results with software I wrote to  
analyse sound files. This has its limitations, chief among which is  
the 44.1KHz (or maybe 48KHz) input sampling of the computer's audio  
input.

The method I used to find frequency variations was to count the  
samples between zero crossings, so I'm really counting the time each  
half-wavecycle takes. I also used interpolation to try and make the  
results more accurate than simply rounding to the nearest sample as a  
simple count would do.

I would have expected some fairly slow, smooth variations, but in  
fact I saw fairly spikey variations, every few wavecycles. Many  
wavecycles are at constant frequency, then one half-cycle will be  
particularly short. Sometimes these short cycles seem to occur  
regularly, but not so regularly that it's a dead giveaway. Both  
synths I used are based on the CEM3340 oscillator, so these effects  
may be a characteristic of that oscillator. Even if that is the case,  
this is a highly regarded oscillator chip, and it would be  
interesting to know if that was a part of its sound. The Pro-One has  
two oscs, but I was only using Osc B's triangle wave for the  
analysis. On the SH101, I used the ramp wave, since it gave a more  
reliable result than the square, which isn't terribly symmetrical and  
makes a mess of half-wavecycle timings (one long pulse, one short  
space each time, you see).

As I said, it was a very confusing evening and I'm still not  
convinced that the results I got aren't a side effect of the  
experimental set-up, so I'm going to look into it some more. If I get  
some decent results I feel I can trust, I'll put them up online. What  
I wanted to know from the list is:

Does anyone know how 'analogue feel' has been implemented in the past?

Has anyone attempted the sort of analysis I was trying not entirely  
successfully to do this evening?


Thanks,
Tom