[sdiy] Questions about Pulse Width Modulation circuits, VCF Cutoff Freq & Resonance controls

[harrybissell]

Hello Logan

#1 -  The triangle is the carrier (from the VCO)
The modulation input can be anything... an LFO, ADSR, any control
voltage...
For max mayhem even another VCO could be used (though uncommon...)

#2 -  From your asciimatic... the four resistors in the cascaded RC
networks
need to be voltage controlled... simultaneously.  Feedback would be from

the output of the filter, to the input. Your CV input is totally
perplexing to me...
I can't see what it would do except move the output DC level up and down
(probably
not useful).   CV of four resistors is not simple.  Also, those four
stages probably
need to be buffered from each other so they will be independant.

See Juergen Haible's 2040 filter clone... or Rene Schmitz version of the
same...
for an idea of how your filter 'might' be voltage controlled. Its really
about that complex.

H^) harry

Logan Mitchell Sr wrote:

>
> 12/13/2005
>  Question #1: Pulse Width Modulation - I want to build a PWM circuit
> to use with my home-made VCO's. Two of the schematics that I have for
> PWMs show a triangle wave input & a modulation input. What type of
> waveform gets injected into the modulation input ? Question #2: VCF
> Resonance & Cutoff Frequency controls - I made my own VCF and want to
> add Cutoff Freq & Resonance controls. The basic filter design is as
> follows below. Where in the circuit would I place the Cutoff Freq &
> Resonance pots
> ? Logan
> R6
> ----/\/\/\/\/\/\--------
> |      ----            |                     R1          R2
> R3         R4           R5     |     |      \         |Audio In
> o----/\/\/\/\--.--/\/\/\/\--.--/\/\/\/\--.--/\/\/\/\/\--.--/\/\/\/\--.|.----|
> --     \       |                            |             |
> |              |                   |   741
> \......|......o                      C1 ==   C2  ==    C3 ==     C4
> ==            |----| +      /         Audio
> |             |            |              |              \    |
> /           Out
> ------------------------------------------              /
> -------
> |                             R7
> \
> --------
> /
> -----
> |
> ---
> o
> Control Voltage
> Input
> from Envelope Generator  &nbs! p;
> -----------------------------------------------------------------------
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