MIDI-CV converters (was: Building my first synth...)

gstopp at fibermux.com gstopp at fibermux.com
Tue Feb 6 16:57:35 CET 1996


     Good point about the DAC's - I'll take a look in all the DAC 
     databooks.
     
     Regarding exponential CV's - this is a long-standing debate - well in 
     a nutshell the reason is that if you take a CV and run it into the 
     inputs of two linear VCO's in parallel, if you set them to the same 
     frequency they will be modulated identically ("track" in tune) no 
     matter what value the CV is. But, if you set them to an interval, like 
     a fifth, and change the CV, they will no longer track and go out of 
     diatonic tune with each other.  So you need a dedicated CV for each 
     linear VCO if you want them to track each other, whereas you could 
     control the frequency of a dozen VCO's from one CV and they will 
     always track each other perfectly (or at least within some small 
     margin).
     
     But linear VCO's are so much cheaper. But exponential VCO's have a CV 
     sweep range of a thousand times the range of a linear VCO. But linear 
     VCO's are temperature-stable without exotic components. But 
     exponential VCO's follow the natural musical frequency doubling for 
     octaves. The arguments go on...
     
     Probably the biggest reason to build MIDI-CV converters that have 
     linear voltage outputs (for controlling exponential VCO's) is that 
     there are so many exponential synthesizers laying around to control.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: RE: MIDI-CV converters (was: Building my first synth...)
Author:  "Mikko H." <MHELIN at tne01.tele.nokia.fi> at ccrelayout
Date:    2/6/96 6:12 AM

How about serial D/A convertor on printer port, there are some 4 channel 
12 bit DACs made by Analog Devices (and even octal ones), but it will be 
cheaper and better to use single DAC with analog switches and S/H's. 
Optoisolators between parallel port and DAC thing would reduce noise and
help with grounding problems. Paia Fatman uses voltage dividers and multiplexor 
to select an octave and the DAC08 for the notes inside octave, but it generates 
exponential CV for linear CV inputs.
     
Btw. I haven't ever understood why exponential CV inputs are so important, 
what is the point, as CV's are now mostly generated by computers. Is it
so difficult to build an analog exponential EG, or what is the reason?
     
-Mikko




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