[RE: VCDO]

[WeAreAs1 at aol.com]

Gene Z. wrote:

<<Guys,

    I have another idea, and there is probably somebody that can implement 
it.  Use
a simple voltage controlled high frequency oscillator and divide it down by 
the
number of samples you want to have per waveform, say 64.  Use a phase-locked 
loop to
lock the 64 times oscillator to the audio frequency you want to create.  The 
audio
frequency is taken from an audio exponential front-end VCO.  The result will 
have
the same accuracy and stability of the your audio VCO.  The audio VCO is not
intended to generate tones, just a square wave for phase locking and can be
consequently as simple as possible.  Any comments?

Gene Z (the other Gene)>>

This is how they do it in the Polymoog.  Well, sort of...  They use a 
Minimoog-type VCO to drive a 4046 PLL.   The  4046 drives a high frequency 
oscillator that is used to clock a 50240 top-octave organ chip.  

The things that make their application different are:

1)  They don't try to sweep the Minimoog VCO very far - maybe a couple of 
octaves max.  They weren't using it to get the entire keyboard range, just 
for pitcj bends and LFO modulation.  You'd need a much wider sweep range for 
a VCDO, and then you might have trouble with the PLL (sluggish response, 
maybe)

2)  The Polymoog high frequency oscillator didn't need to run as fast as the 
VCDO would have to.  (I could be wrong about this - what is the necessary 
frequency range for this VCDO design?)

Michael Bacich

PS - I was looking at the Rhodes Chroma schemos last night.  It has an 
intersting and unusual VCO, using a 4151 V-to-F chip.  They claimed that one 
of its advantages was having no high-frequency droop.  Maybe this could be 
modified for ultra-high freq. clock use?  It was a pretty simple design, and 
used parts that are available now.

Michael Bacich