AW: Re[2]: Pitch -> CV conversion

[gstopp at fibermux.com]

     I believe that this is called an "adaptive filter" similar to what is 
     used in some deltamod PCM systems. It sounds like an interesting thing 
     to try! In a PLL situation it could be implemeted by using a VCF for 
     the loop filter and controlling the cutoff with some kind of circuit - 
     this is a case where one can really speculate on exactly what to use 
     here. I personally would not commit to anything without bench testing. 
     Maybe when I finally put my PLL stuff in a case (someday...) I'll mess 
     with these concepts again.
     
     - Gene
     gstopp at fibermux.com
     
     p.s. as you can probably tell I'm back in the US again. A new ASM-1 
     order will probably be placed soon. I was off on a surprise business 
     trip, on the broadcast crew at the Brazilian Grand Prix! Hi-tech has 
     its perks sometimes....


______________________________ Reply Separator _________________________________
Subject: Re: AW: Re[2]: Pitch -> CV conversion
Author:  chordman at flash.net at ccrelayout
Date:    4/1/97 11:12 AM


I am assuming that the major objection to a PLL is that the filter adds 
a 'portamento' to the derived CV that might not be desirable.  I have 
played with the loop filter by using a pot instead of a fixed damping 
resistor.  This gives some control over the response to pitch changes, 
but it is certainly not instantaneous and can cause over-undershoot 
"sproing" when set too small.
     
I was just thinking that it might be possible to improve the tracking 
by using a method similar to the GR-50 MIDI guitar box (Roland).  They 
use an 'intelligent guess' by examining the dv/dt of the first quarter 
cycle of the picked string.  This guess is sent as a note on message 
and then quickly adjusted by a series of pitch wheel messages.  I am 
not suggesting this method be used directly, but applying the same 
sort of idea to a PLL system.  Suppose one designs a PLL with a loop 
filter that feeds one input of a dual input summing node before going 
to the PLL's VCO.  Then a guess mode circuit could watch for input 
pitch changes. (like a rectified low pass filter and then sense the 
change in DC output... bypass cap?).  Whenever the pitch changes, a 
'helper' current is applied to the other summing node input for a 
short period of time to increase the speed of response to the pitch 
change.
     
Ok, I'll put the mushrooms away now...