[sdiy] 1v /oct with ADC question

Tom Wiltshire tom at electricdruid.net
Thu Dec 1 12:51:52 CET 2016

On 30 Nov 2016, at 22:24, Mike HEQX <mike at heqx.com> wrote:

> Yeah, it's more like a player piano so I really only need 61 actual binary values. I also need the CV out and it just seems so convenient to pick that off of the pot.

If you do that, the CV output is continuous, but the note value is discrete. That's likely to mean that the CV output is a semitone out wrt the note value, since you're unlikely to have got the pot in a "perfectly tuned" position.

Better would be to use the ADC to read the pot, and then output the note value to a DAC. That way, the CV value follows the note value. It might seem so convenient to use the pot as a CV directly, but it throws up as many problems as it solves, and for the cost of one DAC, it isn't worth the saving.

> It looks like the CV will have to generate 64 values at full scale, but there will be 3 extra notes on the binary side. That's not ideal in this case.

Another advantage of doing it this way is that having a  few extra values at the top is no problem. You can stick a resistor above the pot to limit the input voltage to give you only 61 of your 64 values and the top three values will simply be unused. The ADC won't read them, and the DAC won't produce them.

I dealt with some of these problems when designing my VCDO PIC oscillator. It too uses an ADC to capture an incoming CV and turn it into a note number. Since I wanted to be able to capture rapidly changing CVs, I wasn't able to put enough filtering on the incoming values, and skipping between notes was a problem.
I used the 8-bit ADC on the PIC, and discarded the bottom two bits. It works fine, but it needs some careful tuning. For your application, I'd heavily filter the incoming values from the ADC using a shift-based IIR filter. Then you could reduce them to 6 bits without so many jitter problems, although you'll still need some sort of hysteresis in this step.


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