Alternative to MIDI-CV

[gstopp at fibermux.com]

     Coincidentally, I built a frequency-to-voltage converter on a 
     protoboard a couple weeks ago. It's entirely digital. But there's more 
     to the story -
     
     The first thing to ask about a F-V converter is - what is it supposed 
     to do in a musical application? I would like to see it do the 
     following:
     
     1. Accept a (monophonic) musical tone, created by any fairly clean
        tone source
     
     2. Convert that tone into a voltage such that an exponential VCO
        controlled by that voltage can reliably match the input tone
     
     3. Respond quickly to changes in the input tone's frequency with a
        minimum of lag
     
     4. Hold the last reliable voltage when the input tone drops below an
        intelligable level
     
     In order to do this you need the following:
     
     1. An input conditioner circuit that derives a rectangular waveform
        from the input signal that has the same frequency as the percieved 
        pitch of the input signal. Note that pitch and frequency are NOT 
        the same thing in all cases.
     
     2. A reliable frequency-to-voltage conversion circuit
     
     3. An exponential converter circuit to create a 1 octave/volt
        relationship
     
     4. Some kind of latching function that halts conversion when the input
        falls below a certain "squelch" threshold, and maintains the last 
        reliable voltage
     
     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. The audio input square wave that is to be converted 
     into a voltage goes into a D-type flip-flop. On the high state, the 
     counter clock is blocked and the counter is preset to all "1"'s. On 
     the falling edge of the flip-flop's output the counter clock is 
     allowed to pass to the counter, which starts to count down from all 
     "1"'s to all "0"'s. When the flip-flop output goes high again, the 
     counter clock is blocked, the 16-bit word present on the counter at 
     that instant is latched into the D-to-A, and the counter is preset 
     back to all "1"'s and the whole thing starts over again.
     
     The result is that the voltage on the D-to-A will be a voltage 
     corresponding to the time interval between the last two input 
     transitions. Notice that this is an "instantaneous" converter, with 
     the theoretical best possible conversion rate of one cycle time! I 
     tested it with a variable audio-range oscillator, and on the scope it 
     appeared to work perfectly. I have not done any spec tests, but it 
     does go down to about 10 hertz and up beyond the audio range. If 
     resolution is a problem then more bits can be added.
     
     I have not yet built the exponential converter, but the schematic is 
     readily available in Electronotes. It may also be possible to use some 
     kind of exponential DAC to eliminate any analog errors completely - hey 
     I just thought of that! Whoa Brainstorm!
     
     The hard part would be the input conditioner. Getting a clean square 
     wave out of any possible input pitch is gonna take some 
     experimenting.
     
     Notice that if you can derive a reliable squelch signal from the 
     input conditioner, it would be easy to provide the "hold" function 
     since all you have to do is stop updating the latch. Big "if" there 
     however.
     
     This circuit was not popular back in the Electronotes days, since it 
     uses so many chips (they preferred some kind of charging cap/sample & 
     hold type F-V converter, which of course would be subject to drifts 
     and offsets). I stuck the whole thing on one chip here at work (a 
     programmable logic array). Yeah I know these things are not for the 
     average home-builder, but I can't resist the occaisional after-hours 
     experiments :-)
     
     - Gene
     gstopp at fibermux.com
     
     
     
     
______________________________ Reply Separator _________________________________
Subject: Alternative to MIDI-CV
Author:  Kimmo Koli <kimmo at clara.hut.fi> at ccrelayout 
Date:    2/8/96 6:12 AM
     
As most of the synth-DIY folks may not me very thrilled to work
with EPROMs and programming instead of the good old solid hardware, also 
other possibilities to control analog synths shoud be considerer.
     
I'm talking about frequency to CV conversion. For example Korg MS-20 
had a F to CV converter. A friend of mine used a crappy sounding old 
DCO-based synth to drive the MS-20 frequency output and it works fine.