[Pd] Pd to CV for a Moog (OT)

[Kyle Klipowicz]

You may want to query the guys on the music-dsp list also.  There seem
to be some real analogue gurus on that one.

~Kyle

On 9/11/06, Martin Peach <martin.peach at sympatico.ca> wrote:
> Chuckk Hubbard wrote:
> > Instead of adding more portamento for finer pitch resolution, would it
> > be possible to do different registers separately?
> I don't get it. I didn't mean to suggest adding portamento. That's a
> drawback of PWM: you can't switch quickly between levels and have a pure
> DC level at the same time: the voltage ramps up or down.
> >   Since I intend to
> > compose the entire piece before passing it through the Moog, and my
> > patch handles all the tempos, it would not be a problem to record the
> > audio in segments and line them up in Pro Tools.  I suppose the
> > wandering tuning of the synth itself will be an issue.
> >
> You could use pd to tune the synth: count the frequency and compare it
> to what it should be, change the CV. The Oberheim Xpander does that to
> overcome the limitations of its 8-bit main DAC.
>
> > I like your very last suggestion.  I realized through some preliminary
> > investigations with a multimeter that my soundcard removes DC pretty
> > quickly.  I guess your point about the 22kHz constant frequency is
> > that this would not happen.
> It still happens but it's easier to filter out the 22kHz than the 22Hz
> or so that PWM would need.
> You have to add the DC back in by passing the signal through a diode
> somewhere, the easiest circuit looks like this but it will be too noisy:
>        diode
> in --->|----+-------out
>                  |
>                 = capacitor
>                  |
>                 gnd
> Basically the diode forces all the voltage to be positive relative to
> the average voltage held on the capacitor. With a high enough input
> frequency and a large enough capacitor you get a steady DC voltage. The
> capacitor stores charge, the larger it is the slower the output changes.
> If it's too small the input frequency passes through, which you don't want.
> > For the op-amp idea, would a signal not be sufficient?  I thought
> > [sig~ .4], for instance, would output a constant value of .4?
> Probably that would work. The actual voltage you get from a soundcard is
> probably too low by a factor of ten so you need to amplify it somehow,
> usually with an op-amp.
> >
> > Another thing I didn't mention is that the only computer in the same
> > room as the Moog is a university Mac, which I won't be able to tear
> > open.  I don't yet own a working laptop, so if anything like that
> > needs to be done, it will be me bringing my PC to school.  I think
> > this will still be easier for me than the PWM squarewave, which I
> > still don't understand.
> >
> The PWM idea is simple: it's to average the wave. If it's 25% on, 75%
> off you get an average level of 25% full-scale. To get the average level
> you low-pass filter it at a very low frequency, which then limits how
> fast you can change the level. And if you're sampling at 44100Hz and
> need 1000 different levels you have to use a frequency of 44.1 Hz for
> the PWM frequency.
>
> Martin
>
>
>
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