[sdiy] Odp: Re: 1v /oct with ADC question

Roman modular at go2.pl
Thu Dec 1 08:44:41 CET 2016


Set your input to 5V, that is 61st note, and use trimmer at ADC input to reach 0xF0 at ADC output, not full scale 0xFF. Then you will have exact note number at highest 6 bits of that 8-bit ADC, 00 being lowest note and 0xF0 as top C. Divided by 4 (which is equivalent of skipping 2 LSBs) gives 60 in decimal, so 61st note.   You will also need some hysteresis, because when you set the pot at boundary of notes, it may flip constantly between the two, so add DAC which is hooked directly to ADC output, and a feedback resistor from there to ADC input. Make the resistor about 200 times bigger than the resistance of ADC input circuit. This will add sligtly more than 1LSB (8-bit) of hysteresis, that should get rid of all unstable situations.   Roman  Dnia 30 listopada 2016 23:06 Mike HEQX <mike at heqx.com> napisał(a):  Gordon,   I understand what you're saying, however the adc will be quantizing the  voltage to 8 bit resolution so i'll get too many digital steps for the  true 1v / oct   i'm basically doing a player piano so the binary values are simply  addressing key switches from 1 to 61   the CV output is a good side effect since the engine of this thing is  basically an analog sequencer.   So maybe the problem is already solved? I just let the ADC have 64 steps  even though the player piano will be missing a few notes on the top end  that the cv out will actually produce.   That may get strange, but I can't see another way to do it.   Mike     On 11/30/2016 3:41 PM, Gordonjcp wrote:  On Wed, Nov 30, 2016 at 02:10:30PM -0500, Mike HEQX wrote:  Hi folks,    I want to use an ADC to create discreet note values so I am looking  at an 8bit ADC.   I only need 6 bits to get to 64 values. ( I really only need 61 of  those values to make 5 octaves plus 1 )   I want to scale the voltage across my pot so that I get 1v / oct  output for cv usage at full scale, and I also need to produce 61  discreet values from the adc at the same full scale. So it looks  like I have to do something mathematical, but I know not what do do.  You need to run your 6-bit DAC off 5.333V because that way you will automatically get 1V/octave.   5.333V / 64 = 0.08333V/semitone  0.08333 * 12 = 1V/octave   Build it just like in the DAC section of the TB303.    ______________________________  Synth-diy mailing list   Synth-diy at synth-diy.org  synth-diy.org synth-diy.org
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