Some phasemodulation thoughts

[Rene Schmitz]

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.

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 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.)


>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.

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.

>I have been considering doing this to a ASM-1 VCO, but never come around to
>actually do it. Note that you can also add deep linear FM modulation
signal the
>same way, but add the FM signal after the diffrentiation. Those one would end
>up with a deluxe VCO having exp FM as well as deep lin FM and lin PM.
>
>Should we try this?
>
>Any killers anyone?
>
>I could enginneer some untested schematics if really needed.

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.

>> 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 :-) 
Works nice in my VCO3.

Bye 
 René



error:asciiartisnotfully  \ uzs159 at uni-bonn.de
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displaydriversettings...    \ http://members.xoom.com/Rene_Schmitz