[sdiy] 1v /oct with ADC question

Tom Wiltshire tom at electricdruid.net
Fri Dec 2 12:05:20 CET 2016


It's a bizarre way of doing it. If you've got a microprocessor, why not just scan the keyboard, and then output the voltage to the DAC? (Like the Pro-One or SH-101 does) Maybe they ran out of pins.

On 2 Dec 2016, at 05:41, Michael E Caloroso <mec.forumreader at gmail.com> wrote:

> The Moog Source has no ADC.
> 
> The keyboard is analog, but the system is using a comparator with the
> keyboard CV at one input and a successive approximation voltage
> generated from the DAC at the other input to derive the key note.  The
> comparator output is simply a logic input to the CPU.
> 
> That's how Moog got away without using an $$$ ADC.
> 
> MC
> 
> On 12/1/16, Tony K <weplar at gmail.com> wrote:
>> Have a look at the Moog Source keyboard circuit. As far as I am aware it is
>> the only analog keyboard to ADC to DAC again. I love that synth.
>> 
>> Tony K
>> 
>>> On Dec 1, 2016, at 1:46 PM, Mike HEQX <mike at heqx.com> wrote:
>>> 
>>> This makes a lot of sense. Never thought of using a DAC after the ADC to
>>> generate the CV.
>>> 
>>> I would never have been able to make it work otherwise. So now I know for
>>> the next time.
>>> 
>>> I can't wait to make it play notes for the first time.
>>> 
>>> Thanks to all who responded. Everyone had a different view, and all were
>>> very valuable.
>>> 
>>> Mike
>>> 
>>> 
>>> 
>>> 
>>>> On 12/1/2016 6:51 AM, Tom Wiltshire wrote:
>>>>> 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.
>>>> 
>>>> HTH,
>>>> Tom
>>> 
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