Project Summary

The CV Quantizer is intended to accept a control voltage signal from 0 to 4 volts (a subset of the standard -15 to +15 volts) where every 1 volt represents an increase of one octave in pitch. It will then convert that pitch to the closest neighboring pitch that matches a pre-configured scale and/or key. Adjustments will allow the user to select any key from A to G (including sharps / flats). For the normal 12 tone scale, key changes could be implemented using a summing node on the output op-amp. Another adjustment will select either the chromatic, major, minor, 14 note, 19 note or 24 note scale. Each switch will contribute to the E-PROM address. Since we will be restricting ourselves to a 4 octave scale, with no more than 24 notes per octave, we will need 4 x 24 = 96 notes, with 2 bytes per note, for 192 bytes of ROM per scale. To simplify calculations, we will allocate 256 bytes of ROM to each scale. This will allow the various options to toggle bits in the upper portion of the address.

Operation

I'd like to do a fixed 0 to 4 Volt A/D and D/A conversion, so that the top 3 bits are scale selection, then use the next 9 bits to select a note on the current scale. This means 12 bits per scale, with another bit to select low / high portion of the note, so that would be a total of 13 bits per scale. This gives 3 more bits to select one of eight scales. This all assumes a 64K ROM (16 bits).

An alternative plan would be to use a 4 volt reference on a 12 bit A/D, pass the top 2 bits through, decode the ROM using the next 10 bits and extract an 8 bit value that is fed to a 10 bit D/A.

A clock signal (1 MHz) will clock and latch even and odd bytes from the ROM by toggling the least significant address bit, and after each pair is latched, fire off the D/A process. This approach has not been officially adopted yet.

Design Options
1 or 4 or 8 multiplexed CV inputs / outputs.
12 position rotary switch to select the key.
6 position switch to select the key.
CV adjustment potentiometer to shift the output CV up by some amount for tuning / transposing.

Possible Scales / Progressions

Chromatic	Major	Minor
Balinese	Javanese	Werkmeister
Pentatonic	Dorian	Mixolydian
Blues	Inverted	Flatten all sharps
Sharpen all flats	Compress octaves	Expand octaves

Components
12 bit A/D Chip - LTC1415 from Linear Technology
A 28 pin EPROM able to hold at least 8 scales (64K) - 28C256-25 is a 64K EEPROM
12 bit D/A Chip - LTC7541A
Voltage reference, latch chips

Block Diagram



Recent Stuff

I've played with the ADC0804, and for a Chromatic Quantizer, All I'd need to do is use a 5.33 volt Vcc, a 2.67 volt reference (half of 5.33) and drop the lowest 2 bits. I already have a MC1408 (similar to a DAC0807). All I need to do is Force the low 2 bits to logical one. By the way, the ADC output bits use a low voltage for a 1. This is referred to as negative or inverted logic. It is commonly used with TTL and CMOS logic devices.

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Last Modified: Sunday, June 15, 1997