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

Fons Adriaensen fons at linuxaudio.org
Tue Aug 21 23:02:03 UTC 2012

On Tue, Aug 21, 2012 at 06:29:22PM -0400, David Robillard wrote:
> What is this signal?  It either means some absolute frequency, or it is
> useless.  This is what I mean by: in reality/practice, signals that
> represent absolute frequencies *do* exist here, whether anyone likes it
> or not.  They must exist, since the ability to make an oscillator play
> the appropriate frequency is obviously something you must be able to do
> in order to build a synth.
> In AMS, this is true.
> In actual analogue modular synthesizers, this is true.
> In every modular synthesizer ever, this is true.
> It has to be true, because otherwise you can't build synths.

It doesn't have to be true... you can always see the
absolute part as a property of whatever receives the

Suppose you have a VCO with two 1/octave control ports.
One is used by a GUI element, say a slider which could
have a scale in Hz, note names, or octaves. The second
is the 'voltage' from a keyboard.

The actual frequency of the VCO would be

 F = exp2(V1 + V2 + C)        (1)

where C is some constant, e.g. log2(440).

Each of these two ports can be used without the other,
that depends just one what you are doing. In case you 
use only V1, you could say that C is a property of the
port that delivers V1. Same for V2.

But if you use both, which one, keyboard or calibrated
slider, is the absolute one and which one is the offset ?
That, I'd say, is just 'in the eyes of the beholder'. 

That's why I'd say that both are offsets, and C is a
property of the VCO.

Ask yourself this: in what way would (1) be different
if you'd consider either port to be 'absolute' ?

Even if you'd move C 'into a port', the net result
would still be the same sum, in which all terms have
equal status.



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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