The Digitone 2 adds key tracking as a modulation source, as well as Sample Rate Reduction on the FX page. I generally find sample rate reduction to be a bit hard to use on a synthesizer, since the effect changes drastically depending on the pitch of the incoming material, so you lose the ability to have a consistent tone across the keyboard range. I use a lot of Noise Engineering oscillators, and they adjust the sample rate along with the pitch of the oscillator so that the waveform is consistent (but still aliased) no matter the pitch. So the obvious conclusion is that we should be able to get similar consistency from the DN2, by modulating the sample rate reduction with key tracking.
The challenge is that the numbers on SRR have no obvious audible relation to the actual underlying sample rate, so it isn’t obvious what the modulation depth should be to get a consistent effect. I used my oscilloscope to take some measurements to characterize what the actual sample rate is when using the sample rate reduction. I used machine learning to write me an equation which would match those measurements. I then graphed that equation in Desmos. In the below link, the X axis is the number of the SRR parameter on the device, and the Y axis gives the sampling frequency.
Please take this with a few grains of salt. I was not able to accurately measure below a value of 47, due to the sample rate increasing to a point where my oscilloscope could not easily tell where one sample starts and ends. Also this curve is made using polynomial regression with coefficients a computer came up with, so I can’t say how absolutely accurate it is, but above 47, it is pretty decent.
To wrap it up, it ends up that a keytracking modulation depth of -22 to -23 gives the most consistent sounding sample rate reduction. Unfortunately it is not exactly perfect. SRR only having 128 distinct values means that it cannot really track the waveform perfectly. But it does get a much more consistent SRR sound than without.
P.S. here are the raw measurements I took if anyone else is interested.
127 = 2.1 ms = 476.19hz
111 = 1.6 ms = 625hz
95 = 1.2 ms = 833hz
79 = 840 us = 1190.47hz
63 = 520 us = 1923hz
47 = 320 us = 3125hz