Alternative to MIDI-CV

Kimmo Koli kimmo at clara.hut.fi
Thu Feb 8 08:27:35 CET 1996


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.

And it could work better: Because the MS-20 converter was designed to
convert a vocal sound to CV (= low and highpass filter before the F/V-
converter for waveshaping). If a simple (and fixed) synth waveform is used 
intead, conversion and possible input frequency doubling etc. is easier 
to do. 

Also the control frequency could be as high as possible in order to
minimize the ripple in CV (not possible with MS-20 due to the low pass 
filter). The F/V -converter follows vibrato and pitch envelope in the signal 
and by adding a amplitude follower also the amplitude envelope of the control 
synth could be used. And this means we can concentrate on VCOs and VCFs,
which are the parts that make the actual fat analog sound, not the ADSRs and
such.

The drawback: Yes, the CV is exponential. But you could anyway build a 
logarithmic amplifier or just add the CV in different way to the exponential
converter in VCOs or VCFs. Someting like that...

                    |
                   Res
                    |
                 ___|___
             b |/e     e\| b
  lin CV in ---|         |---gnd
               |\c     c/|
                |       |
                |       |
           exp CV in    |
                        |
                to VCO, VCF etc.

Other drawback: There is always a small amount of portamento in the control.
But after all, that may not be a bad thing, just more personal 
characteristics to the analog synth.

Thats all this time,
--------------------------------------------------------------------------------
  Kimmo Koli      			     Email: kimmo at clara.hut.fi 
  Helsinki University of Technology          URL:   http://clara.hut.fi/~kimmo
  Electronic Circuit Design Laboratory
  Otakaari 5 A 
  FIN-02150 Espoo     			     Tel:  +358 0 451 2273
  Finland      				     Fax:  +358 0 451 2269
--------------------------------------------------------------------------------




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