Some phasemodulation thoughts

[Magnus Danielson]

From: Rene Schmitz <uzs159 at uni-bonn.de>
Subject: Re: Some phasemodulation thoughts
Date: Mon, 04 Oct 1999 14:38:42 +0200

> Hi Magnus, and all!
> 
> >Then, I have also been thinking about a similar way of doing things.
> >For instance, if you look at http://home.swipnet.se/cfmd/synths/schematics/
> >you will find a very crude schematic on a sawtooth frequency doubler idea
> >that I got once. 
> 
> I've had a similar idea, a cascade of full wave rectifiers along with
> similar scaling/offset circuitry. I posted it a while ago, the circuit is
> online, but not linked to: http://www.uni-bonn.de/~uzs159/phsfwr.gif
> This is already capable of +/-180deg phasemodulation. Originally intended
> as a 90deg/variable-shift two phase signal for LFO modulation. 
> Adding more foldover points is straightforward, just add some more
> sections. The two blocks labeled X1 and X10 are fullwave rectifiers. I
> found a very good rectifier in "The Art of Electronics". Its the one I now
> use in my VCO3, this would fit nicely in there. Or use the 1 transistor
> rectifier a la moog.

The rectifier part is a quickie. It's not a precision rectifier or anything.
One should probably be well of using just a few resistors more or something.

> Or one can go along the digital way and calculate the difference between
> the input voltage and the same voltage run thru a AD/DA conversion. The
> signal would be wrapped arround the LSB step voltage. "Indices" of 0-255
> should be possible. Properly scaling this voltage back to +/-pi.

This is basically the same thing as I proposed, just a diffrent implementation.
Not that you end up with trouble as you want to go deep.

> >This form of phase modulation is limited in the number of cycles you may
> >modulate, those the depth. However, for small cycle counts it will work
> >pretty well.
> 
> There is always a certain limit, either in current or voltage (or number of
> bits to represent the index). 
> The ammount of hardware necessary for modulation indices of 10 and up would
> be immense (without the AD/DA).  It could be relaxed somewhat if one uses a
> sine generator which can go wider than 360 degrees. With a circuit that
> does +/-540 degrees the count of rectifiers can be cut into half. The AD639
> can do that. (Although it will be less precise then.)

What? Don't you build PERFECT curcuits? ;)

> >Another method of doing it would be to do some brain surgery into the VCO
> core.
> >If we take a VCO core like the ASM-1 VCO (which is quite similar to many
> >others) you can do it like this:
> >
> >1) Create a diffrential curcuit through which you send your modulation
> signal.
> >
> >2) Add the output modulation signal to the load current signal of the
> >   capacitor.
> >
> >3) Add a negative reset curcuit to the VCO, core. This is necessary for
> taking
> >   the oscillator multiple of cycles backwards. This basically means another
> >   LM311 comparator with associated curcuitry (including another reset JFET).
> 
> I think the result of this would be similar to putting a differenciator
> prior to a thru zero FM VCO.

Well, it *migth* be the same. It all depends on the implementation. Here you
may reverse on any point of the waveform. It really comes down to your thru
zero FM VCO. In a way, this is actually what I describe.

> When your oscillator goes forward everything is fine, the cap gets
> discharged. But when the VCO is made run backwards, one would have to
> charge the cap to the reset level of the other comparitor. I doubt that
> this will work with a integrator type VCO like the ASM1s. But it could work
> for a VCO where the cap goes to Vcc.

Depends on the design. There is nothing fundamentally diffrent, just that
I bit my finger as i described that my method was in a way wrong. In intention
is was right.

> It may not be so straightforward/straightbackward (phase ambiguity ;->), as
> it seems but it sure is possible. I think I'll try something with my
> triangle TZVCO. Adding the PM signal prior to the schmitt trigger should
> work. At least in theory. If I got you right this is what you proposed in 
> your other mail. An adder is already there. But the direction sensing could
> be tricky.

Right. But if you want to go deep, expect special equipment ;)

> >> This circuit will make high demands on the sawtooth input, to avoid
> >> glitches. But they could be compensated like in Jorgen Bergfors'
> sineshaper.
> >
> >Which then built on my idea ;)
> 
> Ok, then I stole it from you :-)

Ah, how nice ;)

Go ahead and use it ;)

> Works nice in my VCO3.

You just did ;)

Cheers,
Magnus