Alternative to MIDI-CV revisited

[Don Tillman]

   Date: Fri, 10 May 96 09:21:39 PDT
   From: gstopp at fibermux.com

   Don - great idea on the 4-chip converter - have you tried it? 

Yes, I'm got one breadboarded up on the livingroom carpet and driving
my Mellotron as we speak.  (I guess it's pretty clear I live alone!)

The big feature I like is that the circuit delivers a pitch voltage on
the very next cycle, so don't need nearly the ripple filtering you'd
need with other approaches.  
								 How well 
   would the log discharge curve comply to an exponential power-of-2 type 
   curve? Is there a "warpage" factor that can be added?

The curve runs higher than it ought to be for lower pitches, lower
than it ought to be for higher pitches, and very reasonable for a
couple octaves in between.  If you use a fixed-time portion of the
cycle to charge the cap, say 1/3 the period of the highest expected
pitch, the high pitch end of the curve gets corrected up a little bit
and you maybe get a three octave range.

I plotted these using a lovely little math program called "Theorist",
but any plotting routine will do.  It's not difficult.

   I like the "hybrid" idea, using a converter plus a PLL for fine 
   tuning.

There's a weighty fundamental issue with feedback loops going on here:
A feedback loop without a first approximation incorporated into its
design is always going to be inaccurate because the output is equal to
the loop gain times the *error*.  A feedback look with a first
approximation will be have an output equal to the first approximation
plus the loop gain times the error.

I think the audio fidelity implications of this are very important.
But in this case it affects the tactile response of the pitch-to-
voltage converter.

  -- Don