[sdiy] Control voltages for digital modules

[Tom Wiltshire]

> Here's the problem: how do I get a pitch CV from a sequencer or other module converted to the 0-5 volt input necessary for the microcontroller ADC?

I've dealt with this part myself doing the design of the VCDO. It's a problem. The fundamental issue I had was that the micro controller's ADC didn't really have enough resolution to cover a full audio range. For example: Assume the 0-5V covers five octaves (logical, right?). In fact, it makes more sense to cover 0-63 notes, rather than 61, but we'll call it five octaves nonetheless. So the notes use 0-63, which is the upper six bits of your ADC value. If you've got a ten bit value, that gives you four bits per semitone, which is 100/16 = 6.25 cent steps. That's *just about* acceptable, and that's assuming you've got *no* jitter at all in your ten bit value, which is unlikely.
So a 12-bit ADC would be much better.

I got around this by quantising the ADC value for the Note CV, and then adding a Modulation CV on a separate input. This allowed me to limit the range of the modulation and thereby make best use of the resolution.

> And what sort of DAC should be used to make proper CVs out of the digital signals?

What kind of DACs can you get? What have you used before? Is SMD acceptable? Parallel or serial input?

I often use the MCP4822 SPI input 12-bit dual DAC from Microchip. It's not the greatest, but it's easy to breadboard and use. Produces an acceptable CV.

HTH,
Tom

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