[sdiy] Multiplier vs VCA

[Scott Gravenhorst]

Derek Holzer <derek at umatic.nl> wrote:
>Howdy there SDIYers,
>
>my next design question for my new project deals with amplitude 
>modulation. I will construct a module inspired by the Buchla 259 
>Programmable Complex Waveform Generator, as well as the Cwejman VCO-2RM, 
>in that it will contain two oscillators (2 x Thomas Henry XR VCOs, using 
>the Bugbrand PCB layout) with a number of switchable modulation 
>possibilities (PWM, AM, FM...).
>
>I had originally intended to use a full-fledged VCA for the AM function, 
>but then I ran across Timothy Daugard's implementation of the diode 
>quadrant multiplier which says it is capable of "VCA type effects" when 
>the inputs are DC coupled:
>
>http://members.cox.net/synthfred/h_kmd101.htm
>
>So my question is: what would I gain by building a (more complex) VCA 
>for the AM section of this design, over using this simpler diode 
>quadrant modulator? The VCO-2RM uses a ring mod (most likely a 
>multiplier), as the name implies, and I'm not at all sure what the 
>Buchla 259 uses. Keep in mind that the module will not be set up to take 
>an envelope or any other signal for the AM, as I would use another VCA 
>module for any other AM needs later on.

An ideal true multiplier (4 quadrant) is also an ideal VCA.  I use multipliers as NCAs
(digital equivalent of a VCA) in my digital synths.  Imperfect multipliers will output
something other than a pure product, i.e. distortion - which is not always a bad thing.  

It depends on what you want to get out of amplitude modulation.  If you want to be
able to put in say 200Hz (sine) and 2Hz (sine) to get 202Hz and 198Hz sines, then you
want the best multiplier you can build/afford.  If you're looking for distortion, then
it would be good to hear samples of output with known inputs before making a decision.


-- ScottG
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-- Scott Gravenhorst
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