[sdiy] Imperfect VCO

[Magnus Danielson]

From: "Oakley Sound" <oakley at techrepairs.freeserve.co.uk>
Subject: Re: [sdiy] Imperfect VCO
Date: Fri, 1 Aug 2003 12:57:59 +0100

> > To get something interesting you really have to draw large distortions
> of the original curve...
> 
> But try this one. Alter the rise time vs fall time of a triangular
> waveform. The difference between a 'sawtooth' with a rise time to fall
> time ratio of 1:99 and 2:98 is very noticeable. I tried this with the
> Ion, and was suprised how 'digital' the 0:100 setting was and how much
> warmer the 2:98 setting was. I was hearing [I think] the fundamental
> becoming stronger. Perhaps a significant part of the 'tone' of the
> sawtooth output on VCOs could be controlled by the reset time. But
> thinking about it, is it that we describe a powerful VCO by the strength
> of the fundamental? A perfect sawtooth being that little bit too bright?
> 
> I should do some basic fourier maths on sawtooth waveforms with
> differing flyback times.

If you model it as a PWM followed by a integrator you can easilly
modify the text-book formulas for a pulse to do the right thing.

Also, maybe this helps you to understand what is happening, yes... the
fundamental gets significantly stronger as the reset-time increases. Also,
such "imperfection" calls for more "imperfection" in the form of Franco
compensation, but unless you do very specific waveshaping I think it is most of
the time a fair deal.

Note that the integrator provides a -6 dB/Oct slope, so lower frequencies will
be stronger to start with for the ramp-form, so just a little fiddling with
reset-time/PWM-factor will indeed make a rather high impact on fundamental.
I think this is really what you wanted to hear, right?
 
> Also there is quite a difference in tone when there is a discontinuity
> in the falling ramp of the sawtooth. When setting up Polyfusion VCOs
> [triangle based core], you need to adjust the symmetry of the waveshaper
> to give you zero glitching on falling ramp. A small glitch as seen on
> the scope does give a different tone.

Interesting. I will contemplate over this as I go and get some REAL beer.

Cheers,
Magnus - off into the big city jungle