[sdiy] 1v /oct with ADC question

Mike HEQX mike at heqx.com
Wed Nov 30 23:24:48 CET 2016

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. 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.

Not sure I can solve that, but maybe there is a way to roll those notes 
down to a lower octave or something, which would also be very strange.


On 11/30/2016 5:01 PM, mskala at ansuz.sooke.bc.ca wrote:
> On Wed, 30 Nov 2016, Vinicius Brazil wrote:
>> However, in order for the ADC deviation not to be perceived as detuning,
>> this deviation has to be less than 1%.
>> (1/12) / 100 = 0.0008333volts
> Bear in mind that this is an ADC, measuring the voltage from a pot.  It's
> just supposed to know which semitone the pot has been turned to.  I don't
> think the original poster really needs the boundaries between those
> semitones to have single-cent accuracy.  Even if it were a DAC generating
> a control voltage, single-cent accuracy seems excessive; very few VCOs are
> capable of tracking anywhere near that closely, and making the voltage
> input much better than the VCO is wasted effort.  Sub-millivolt precision
> like that will also be lost at the first ordinary (not expensive special
> low-offset) op amp it goes through, anyway.
> However:
>> 5 Volts / 0.000833 Volts = 6000 steps, so you need a 13-bit converter.
>> 12 bits is already satisfactory.
> If you *do* need single-cent accuracy, 12 or 13 bits in your converter
> isn't going to be good enough by itself, because ADCs and DACs are
> frequently not perfectly linear.  A voltage that ought to correspond to
> the number 5000 might read out as 4997 or 5003.  The specification for how
> many counts the value might be off depends on the individual ADC chip and
> often gets worse when there are more bits.  Not all 12-bit converters are
> equally linear.  If it matters to you, there's no solution except reading
> the fine print on the data sheet carefully and making sure you know its
> consequences in your application.

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