The purpose of a CV Quantizer is to take any input voltage and output the same voltage locked to some fixed voltage interval - usually a 1/12 of a volt. This fixes the output voltage to a chromatic scale and the quantizer is often used with analog sequencers to lock the output to notes.
This quantizer uses a counter and 7bits of a 12 bit flash D/A converter to generate a stepped reference voltage with 128 steps. This output is scaled (and inverted) so that each step has a "height" of 1/12 of a volt, and the range is about +5 to -5v (actually, the range is -5v to 127/12 - 5 = 5.83v. By connecting or disconecting the lower bits of the counter to the DAC, you can lock scales other than a chromatic scale and still have all of your notes fit into the equal-tempered scale.
The input voltage runs through a comparator that goes high the moment the falling stepped voltage goes lower than the input voltage. This switching to high is delayed slightly via a simple RC lowpass to give the DAC a chance to settle, then it is converted to a pulse via an RC differentiator, and this goes to the trigger input of a LF398 sample & hold IC. The S&H samples the stepped voltage and outputs it. Because the counter is clocked at 25Hz, and goes through 128 steps, the voltage is resampled every 5ms. Another RC filter on the output keeps it smooth.
Be sure to check the Vectorboard Building Tips before you start.
Copyright 1998, C.G. List