DJB-013 Quantizer and Lag Module |
I often use my sub-octave divider to create sequences and wanted to be able to quantize the outputs (I had not yet built the Blacet MiniWave nor the PSIM, both of which will quantize). I also wanted a lag circuit so I combined them both in this design.
I started with an Arp-style quantizer that I found on Chris List's CV Quantizer site. I enhanced this design with a better clock oscillator and a parallel counter. I also improved the sample clock generation from the rising edge of the comparator. I added ~1/2 semitone and hysteresis to the input so that quantized notes would quantize to the same value (e.g. 0 volt input quantizes to 0 volts).
For the lag circuit I wanted both a linear and exponential lag. I changed the time constant between the two responses so that they sound about the same duration. I used 1M linear potentiometers since I couldn't find a log taper in this style (Oakley/Krisp1 now sells a BTT 1M log taper potentiometer in this style).
I shortened a MOTM-000 prototype board for building this module.
DJB-013 FrontPanelExpress design file
DJB-013 schematics page 3 updated
Circuit Operation
The seven bit counter and DAC form a 128 step ramp waveform with each step equal to 78.1 mV. This ramp is inverted, offset, and scaled through the two op amps to form a saw (e.g. ramp down) waveform with each step equal to 83.3 mV (middle trace). The input (top trace) is compared to the saw waveform and the comparator output (bottom trace) goes high when the saw is lower than the input. The input to the comparator is increased by ~1/2 semitone and hysteresis is added from the comparator output which can be seen in the small steps in the input waveform. Adding ~1/2 semitone assures that notes that are already quantized will be quantized to the same value. The hysteresis assures a clean comparator output for any noise on the input.
When the reference saw goes below the input, the comparator switches state. The value of the reference saw is the quantized value of the input.
The rising edge of the comparator is processed by a transistor circuit to generate ~5 µS pulse to sample the reference saw to provide the output. The input comparator, pulse generator, sample & hold, and output buffer are duplicated for the second quantizer channel.
This image is the quantizer operating on a LFO sine wave input. You can clearly see the quantized steps in the output waveform.
Calibration
Remove the offset jumper and adjust the voltage at the op-amp pin 1 output/square pin to -5.333 volts.
Move the gain jumper to the alternate position and adjust the voltage at the op-amp pin 14 to -10.583 volts (gain of -1.066).
Replace the offset jumper and move the gain jumper back. Adjust the offset control very slightly to get as close to 0 volts on the quantizer module output with no input.