Alternative to MIDI-CV

gstopp at fibermux.com gstopp at fibermux.com
Thu Feb 8 22:38:44 CET 1996


     Actually the purpose of the D flip-flop at the input is to make the 
     converter *insensitive* to the recovered pitch duty cycle. Remember if 
     you feed a train of pulses of *any* duty cycle into a toggle 
     flip-flop, the output will be a 50% duty cycle square wave at one-half 
     the frequency of the input, as long as the edges don't jump around 
     much. Therefore this converter "converts" on the even cycles, and 
     "rests" on the odd cycles, so it does depend on at least three pulses. 
     If the input signal has little or no cycle-to-cycle jitter, the 
     conversion duty cycle will be exactly 50%, and the conversion will be 
     correspondingly perfect in the typical digital fashion.
     
     Once again it works great on the bench, but no live signals have been 
     tried. This of course would be the bottom line. Good idea about the 
     exponential discharge converter!
     
     And like you say, most of the work will have to happen in the input 
     signal conditioning anyway.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: Alternative to MIDI-CV
Author:  don at till.com at ccrelayout
Date:    2/8/96 2:18 PM
     
     This approach seems suboptimal.  For one thing, the digital counter & 
     DAC aren't necessary; they can be completely replaced with an analog 
     ramp and S&H.  Either way the output still needs to be logged.  And 
     this circuit, as described, will be completely sensitive the duty 
     cycle of the input waveform.
     
     There's a better way.  Start the beginning of each cycle of the input 
     signal with a cap charged to a reference voltage.  Discharge that cap 
     exponentially with a simple resistive load.  S&H that voltage at the 
     end of the input cycle.  You now have an volts-per-octave F-to-V. 
     Simple, all analog.  (No, I haven't built it yet.)
     
     What's really important is how well the circuit handles real-live 
     input signals from the whatever the source is.  Any real-live source 
     is going to create lots of glitchy conditions (with the guitar 
     probably being an absolute worst case situation) and it's very 
     important that the F-to-V's response to these isn't unmusical.
     
     




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