Alternative to MIDI-CV

Don Tillman don at till.com
Thu Feb 8 23:08:14 CET 1996


   Date: Thu, 08 Feb 96 09:33:43 PDT
   From: gstopp at fibermux.com

   The thing I built the other day performs the frequency-to-voltage 
   part. It consists of a 16-bit down-counter clocked at a constant high 
   frequency, controlled by a timing generator, and followed by a 16-bit 
   D-to-A converter. 

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.

  -- Don






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