TL;DR: Here, have a SysEx file with a slightly randomized tuning.Although I was initially looking at the alternate tuning capabilities of the Rev2 for use in microtonal composition and performance, it seems to have generated some interest here as a way to emulate the behavior of vintage synths.
I wanted to see how control voltages are derived from keyboard input in a vintage monophonic synth, so I took a look at the schematic for the
Moog/Realistic MG-11. See attached clip from the service manual.
This generates 32 different voltages (one for each key), ideally all uniformly spaced from zero up. The “Hi-End Trim” sets the voltage for the highest key. (The nominal value shown of 2.58 volts is about 31÷12, or the number of semitones spanned by the keyboard, scaled to 1 volt per octave.)
In reality, since the resistors have a specified tolerance of ±1%, there’s potentially that much deviation in the width of a semitone between any two keys. The Hi-End Trim can only correct the overall span of the keyboard, and would need to be accurate to less than 1 mV to get the span tuned within 1¢.
This is very easy to model with a randomly-generated tuning table.I have attached a SysEx
2 that will load into alternate tuning slot 4. Scale steps were randomly generated in
Google Sheets so that each semitone (across a 61-key range) is 100±1¢. I didn’t rescale the overall to duplicate the “Hi-End Trim” adjustment. The total span ended up a bit flat-- about 5¢. That will emulate an older instrument that hasn’t had its High-End Trim readjusted in a while.
1I chose this model mainly because my family had one-- I was in high school when Radio Shack sold them and Dad got one for us. It was also relatively inexpensive, so I expected to see some corners cut in the circuit design/engineering. Yes this synth had “polyphony” (which was more properly paraphony since there was one envelope and filter section) but that was a separate circuit from what I showed.
2If you're interested, here's the head and tail of the .scl file used to generate the SysEx:
! 61-Rando 1 pct.scl
!
Randomized +/- 1% over 61-key keyboard
60
!
100.07755
199.45729
298.68053
399.62779
498.88812
598.46807
698.61219
... 46 more in this pattern ...
5393.75303
5494.70841
5595.09756
5695.59127
5795.96337
5895.48586
5994.74663