[LAD] Plugin 1/oct frequency controls (AMS/MCP/VCO)

David Robillard d at drobilla.net
Tue Aug 21 22:52:28 UTC 2012

On Wed, 2012-08-22 at 00:11 +0200, Nick Copeland wrote:
> > Subject: Re: [LAD] Plugin 1/oct frequency controls (AMS/MCP/VCO)
> > From: d at drobilla.net
> > To: nickycopeland at hotmail.com
> > CC: jef at synthedit.com; linux-audio-dev at lists.linuxaudio.org
> > Date: Tue, 21 Aug 2012 16:59:03 -0400
> > 
> > Yes, but as already mentioned, for a modular to be usable a standard
> is
> > required. The analog synths on which this was modeled have such a
> > standard. As it turns out, that was based around A as well, just dug
> up
> > by Robin Gareus on IRC:
> > 
> > https://en.wikipedia.org/wiki/CV/Gate
> > 
> > Even volts on A in both schemes.
> > 
> That is a nice page but it definitely does not advocate any attachment
> of
> semantics (eg, frequency) to any specific voltage. It gives some
> tables making
> voltage comparisons between V/O and H/V using integral values however
> these are given as examples of the translations between the two
> systems
> based on the preconfigured generation of a given frequency by an osc.

It clearly shoes a mapping between volts and absolute frequencies.  That
there is tuning involved which can modulate the center slightly doesn't
matter.  The plugins in question also have frequency modulation inputs
that can be used for tuning.

In the synthesizer as it is working, a voltage is representing an
absolute frequency.  This is inevitable, because... well, you need to
represent an absolute frequency with a voltage when all you have is
voltage to convey information.  You are correct that it is not set in
stone, but it is representing an absolute frequency regardless.

Fact: In a real-life genuine analogue Moog, even volts are A, e.g. 4V is
A4, or 440Hz +/- tuning.

How about a diagram:

note.frequency_out ------A------> osc.frequency_in

There are 2 possible scenarios here:

1) A carries a representation of an absolute frequency, and both "note"
and "osc" agree on what that representation is

2) Your synthesizer does not work

It doesn't matter, or even really make sense, to argue about where the
semantics of the signal on A "came from".  They are there.


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