[sdiy] analogue phase modulation

[JH.]

Hi,

I know we've talked about analogue phase modulation before,

but I'm not sure if the following was covered already:

When I build a quadrature VCO, with one output producing

x(t) = sin(wt) and the other output producing y = cos(wt),

then I can get an output signal at "any" phase with a linear

combination of these:

out(t) = sin(wt + phi) = A * sin(wt) + B * cos(wt) = A x(t) + B x(t)


So I could modulate phi by modulating the factors A and B:

out(t) = sin(wt + phi(t)) = A(t) * x(t) + B(t) * y(t)

That would be phase modulation by "post processing",

without interfering with the actual VCO core.

Of course we want this to work for any (continuous) input signal,

where "input signal" means a phase shift phi, so we need functions

A(phi) and B(phi), instead of generating A(t) and B(t) directly.

(We want an analogue input for phase modulation, after all.)

A(phi) and B(phi) are nonlinear functions, and periodical in 2Pi.

Something that produces sin(CV) and cos(CV) where CV is a control voltage.

So we'd need a wave folder circuit, in the broadest sense, here.

And the function becomes more complex, the more periods of Phase

Modulation we want to cover.

Now, after much talking, my only question is:

What amount of (maximum) phase modulation depth (number of periods), do 
typical

digital PM sounds a la DX7 etc. have ?

JH.