[sdiy] Module idea - you guys get to work (for me!)

[Oren Leavitt]

Howdy-howdy,

A 'summing node' is a point, typically the (-) input of an inverting op 
amp, where input signals and feedback from the output are mixed together 
thru resistors to produce a sum of the signals, as in a mixer, or where 
control voltages are summed together in a VCO, VCF, VCA, etc.

A quick primer here:
http://www.play-hookey.com/analog/analog_addition.html

A not-so-quick (but very good) primer:
IC Op-Amp Cookbook, 3rd Ed, by Walter G. Jung
ISBN 0-13-889601-1

The patch:

Feed the output from two VCOs into a ring modulator (4 quadrant 
multiplier, balanced modulator).
VCOa to the X input, VCOb to the Y input.
The ring modulator output will produce the two sideband frequencies: 
f(VCOa)+f(VCOb) and f(VCOa)-f(VCOb)
If the oscillators are near unison, the upper sideband will be nearly 
twice the freq of the two oscs. The lower sideband will just a few Hz - 
the rate at which the VCOs are 'phasing' with one another.
Add a very low-pass filter (infra-red filter to shed a little light on 
the subject :-) to the output of the ring mod. (fixed frequency - 
doesn't have to track anything)
We can now filter out that upper sideband and have just the low freq 
part to use for modulation - or panning the two VCOs as Peter is after.

'nuff talk - now off to my synth to play around with this myself!

Oren

john mahoney wrote:

>----- Original Message ----- 
>From: "Peter Grenader"
>  
>
>>Let's say you've got a patch going with two VCOs tuned 'close' to unison.
>>If you do NOT sync your VCOs (I rarely do this) and tune them just so to
>>    
>>
>get
>  
>
>>that marvelous sweeping phase cancellation, wouldn't it be nice to pan the
>>sound across the stereo field with that?  Or sync a filter opening with
>>these?
>>
>>That sort of thing.  This patch allows you to do this.
>>    
>>
>
>Ahh, I think I'm catching on. But what exactly *is* the patch?
>
>Oren wrote, "Multiply the signal from another VCO with its own, filter it,
>and feed the offset back to the summing node."
>
>Do I have a multiplier? Perhaps a "ring" (balanced) modulator does that, or
>just an op amp?
>
>As for filtering, are you using a lowpass VCF that tracks the pitch?
>
>Finally, I believe that "summing node" typically means mixer module, but I'm
>confused by the phrase "feed the offset back to the summing node." *Back* to
>the summing node? Were we at the summing node somewhere else in the patch?
>
>Enlighten the unwashed masses, please! :-)
>--
>john
>
>
>  
>

-- 
Oren Leavitt
oleavitt at ix.netcom.com