Odp: [sdiy] analogue DX-7-style Phase Modulation using quadrature signals ?

[Roman Sowa]

I have a strong feeling that PM would be eassy in analog, maybe easier
than thru-0-FM. Simply use wave multiplier from Ken Stone, or any
similar circuit. It's straight phase modulation of saw wave.
Then saw-2-tri, tr-2-sin and we have PM sines. And they are locked
in phase thru entire CV range. It has linear phase/V relation and
not limited to 360 deg, as it simply wraps.
Am I missing something (beside waveform purity after saw-2-sin
conversion)?

Roman

----- Original Message ----- 
From: jhaible <jhaible at debitel.net>
To: diy <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, October 26, 2003 12:51 AM
Subject: [sdiy] analogue DX-7-style Phase Modulation using quadrature signals ?


> This is just an idea at the moment, and I wonder if this
> would work. It's about making DX-7-like Phase Modulation
> (often misnamed as Frequency Modulation) with analogue
> oscillators. From earlier discussions on synth-diy I know that
> FM of a VCO is easy, thru-zero FM is difficult but possible,
> but PM - easy in digital - would be most demanding on an
> analogue VCO.
> 
> But what about using a quadrature signal? Either using a
> quadrature VCO, or simply running an ordinary VCO thru
> a dome filter such as used in a Frequency Shifter. Then you
> have sin(wt) and cos(wt).
> 
> Now there will always be factors A(phi) and B(phi) such that
> sin(wt + phi) = A * sin(wt) + B * cos(wt).
> So we can modulate phi indirectly by modulating A and B
> with the right function, can't we?
> 
> We could run the outputs of the Dome Filter thru two
> multipliers (ring modulators) and sum the two products,
> just as a Frequency Shifter would do. But instead
> of using a Quadrature Oscillator for the modulation,
> we could derive both, A and B, from a CV input.
> 
> The difference to ordinary frequency shifting is that
> *there* (FS) you always go round the circle (0 ... 360deg),
> while this PM scheme would allow both a smaller or a larger
> range, performing partial circles depending on the
> modulator level  modulator envelope.
> 
> I haven't done the maths for this, but probably A and B
> would be sin and cos functions of an arbitrary CV input
> signal to get the desired result.
> If so, all we need is a sin(x) and cos(x) nonlinear network
> which works over a certain number of periods.
> I think I remember from earlier discussions that there are chips
> that do this, and of course such a thing could also be implemented
> with diode networks etc.
> 
> Two questions:
> 
> (1) Will this work at all? Is this a true emulation of PM a la DX-7 ?
> 
> (2) How many periods do we need for the nonlinear network?
>      (a) How many periods does a DX7 with maximum modulation depth
>            cover? And, if this number is too high,
>      (b) How many periods is the maximum PM depth in a typical "DX Piano"
>            sound on the DX7 ?
> 
> I have a feeling that it can't be so easy, but at the moment (early morning,
> time to go to bed) I can't see an error. Too tired to go and consult the
> Bronstein. Someone tell me if this will work, please.
> 
> JH.
> 
>