I made one of those - I'm sure it's in the archives.... I used an
exponential tri-square VCO of the Electronotes variety (excellent
tracking, MAT-03 matched pair, all that) plus a home-built phase
comparator made from a quad NAND gate. I just made up the comparator
design out of my head, tried it, and it worked great first time so I
didn't do any further research. The loop filter was likewise designed
by sheer fumbling around with various parts. My findings were that the
best tracking and response happens when there is *no* loop filter at
all (!) but the CV is un-useable for controlling an external synth. I
ended up with a compromise between good tracking and clean CV. As you
note, since the VCO in the PLL is exponential, the CV to an external
synth is 1V/oct.
It worked amazingly well over the audio range, with my wife playing a
flute into the mic and the CV controlling a Minimoog. The VCO idles
between notes at a couple of hertz so short release times worked best.
- Gene
gstopp at fibermux.com
______________________________ Reply Separator _________________________________
Subject: Re: Pitch -> CV conversion
Author: chordman at concentric.net at ccrelayout
Date: 3/12/97 7:27 AM
On Wed, 12 Mar 1997 13:07:44 GMT, you wrote:
>On Wed, 12 Mar 1997 08:20:48 +0100 (MET), you wrote:
>>
>>Ok, why not using an external VCO together with the 4046 phase comparator.
>>Somebody reported some time ago that this works pretty good, even
>>if the ext. VCO is exponential. The waveforms may be a bit distorted,
>>due to the permanent phase modulation.
Let's assume that you want to create a CV to drive a VCO of type
'XYZ'. If you build a PLL using the same type of VCO, it's linearity
curve will very closely match the VCO you are trying to drive. In
otherwords, the CV produced should allow the driven VCOs to track the
input signal rather well. Using the 4046 comparator circuitry, you
have two different comparators to choose from, both with different
characteristics. The drawback of pitch->CV conversion using a PLL is
the built-in portamento and overshoot, depending on the loop filter
characteristics. This can, however, produce interesting results.
YMMV.