[sdiy] Imperfect VCO

[Czech Martin]

Ok, let me think:

a discharge time of 100ns to 1us should be doable
with any sawtooth core. A 1kHz Wave has ~1ms charge time,
so the discharge/charge ratio will be in the 0.1% - 0.01% range.

Above 1kHz we can hear less partials, at 10kHz only one.
At lower frequencies the discharge/charge ratio get's 
even smaller.

So it is hard for me to understand why a normal VCO core
should have such a bad ratio like 1% or so.
So I expect that all "vintage" saw cores are much better then that,
and that no audible difference in this respect will be.

Of course, if you want you can build such a 1:99 thing,
and then your observation will hold.

Your second example reads like combining a saw with double frequency
plus a sqa in order to get a fundamental freq saw.
I mean this is basically the circuit for a tri core to saw converter.

>From this construction it is clear that an imperfect solution will
somehow disturb e.g. the 2nd harmonic in a way that my masking argument
will not work.

So perhaps I did not sketch up the "right" wave distortion in my
experiment.

m.c.

-----Original Message-----
From: Oakley Sound [mailto:oakley at techrepairs.freeserve.co.uk]
Sent: Freitag, 1. August 2003 13:58
To: synth-diy
Subject: Re: [sdiy] Imperfect VCO


> 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.

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.

Regards,

Tony Allgood                       Penrith, Cumbria, England

Oakley Sound Systems                   www.oakleysound.co.uk
Modular Synthesisers      www.oakleysound.co.uk/projects.htm