VCO idea

Martin Czech martin.czech at intermetall.de
Fri Jul 31 09:03:29 CEST 1998


> I was just sitting in an electronics lecture and had an idea for a VCO. 
> Why not have a repeating waveform that has (say) 4 "lines" of which the 
> slope and duration can be adjusted, either by a control knob or voltage 
> control. Like this :
>         |         |
>        / \_       |
> \     |    \_     |
>  \   /       \_   |
>   \  |       ^ \_ |
>    \/        |   \|
>  ^    ^      |     
>  |    |      |    ^
> Line1 |     Line3 |
>       |           |
>     Line2        Line4 
> 
> 
> So, in other words, you would have a waveform which has a  constantly 
> changing waveform. You could modulate the amplitudes and slopes of the 
> lines in any way you wanted creating very strange waveforms. How would 
> one go about building a circuit that could do this? Could it be done 
> using analogue electronics? Has anything like this ever been incorperated 
> in a Serge, Buchla, or Moog modulars?

I don't want to discourage you, but ... I toyed arround a lot with hand
made waves and wavetables for the Waldorf Microwave I (No, I won't swap
it against a Microwave II or XT) and came to the conclusion that in a
lot of cases the results are not very exciting, certainly there are
differences in sound, but I expected something to knock me off my
chair, and this really happens, but not very often.

What you are proposing is a "piecewise linear function" PLF, if I got
it right.  A sawtooth wave is feed into some black box, and a PLF comes
out.  This can be done using half wave precision rectifiers HWPR. This
is a standard circuit you'll find in any good book about opamps and
analogue computing.  You need one HWPR for every break in your curve.
You feed the sawtooth wave into each HWPR and a some offset (at the
desired breakpoint)  at each HWPR. The result is that the HWPR turn on
one after the other, just as it is specified by the offset voltages.
You now take the output of each HWPR and go into "bipolar coefficient"
boxes. These are analogue computing modules that multiply the input
with a constant k , -1 < k < 1. Finally all these voltages are added
together and voila there's the PLF.

Problems: Most PLF won't be spectacular. If you want a non monotonic
PLF (like a triangle, first up, then downwards) this means the the
coefficient for the first breakpoint is 0.5, and for the next it has to
be -1. Remember that both coefficient outputs are added together, first
you have only a factor of 0.5 then the second HWPR turn on (0.5 +
(-1))=-0.5.  This example shows some limitation, if you need wild
nonmonotonic breaks (and these waves sound exciting, there is a
tendency for smaller coefficients.  And: every segment depends on all
predecessor segments. It's not like drawing the waveform with the
mouse.

As I said, the nonmonotonic PLF are of most interest, because they can
produce really strange harmonics. Take for instance a PLF that looks
like a long piece of triangle wave. Say 7 cycles. If you vary the input
amplitude of the incomming saw, you first get a  saw wave, this changes
into a tri, and later you get waves with very small fundamental but
very strong harmonics, up to 7th order, the final stage is a tri wave
seven times faster then the incimming saw.

Is this the principal of the Serge waveshaper?

Anyway, you can try this with the Nord Modular, it comes close to "FM"
sounds, when the carrier is set to 0Hz fixed.

Hope this helps.

m.c.

------------------------------------------------------------------------
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