I was asked about the details of sine-width modulation in another thread. The basic idea is you cut up a sine wave into two pieces. One half is “squished” and the other is “stretched” relative to the original wave. This is the same thing as pulse-width modulation of course. However, very few synths do this as far as I know. I have been told that the Casio CZ series from the 1980s can do this and that this is a kind of Phase Distortion synthesis. So it’s cool that the Mono can get it on the fun.
Well, how to do it. I used Wavosaur which is a free WAV editor. I started with a 200 hz sine wave. You can tell Wavosaur to make this in the Tools>Synthesis menu option. You can set the sampling rate to 40000 hz so that it is “cleaner” with regard to a 200 hz wave.
Make one cycle which is 1/200 seconds long (0.005) seconds. This is the base wave. The base wave has an “up” portion (0.0025 seconds) and a “down” portion (0.0025 seconds). Save the base wave to your hard drive. You can call it “S200.wav”.
It is easier for the following waves if you have a spreadsheet program like Excel.
For the next wave, we can make the “up” portion be one octave higher, 400 hz. You can calculate in Excel that a 400 hz wave is 1/400 seconds and that the “up” half of that is 0.00125 seconds. Copy the up portion in Wavosaur and paste it into a new file.
We now want the “down” portion to be long enough to make the whole wave 0.005 seconds. So calculate 0.5/(0.005-0.00125) = 133.33 hz. The 0.5 is in the numerator because we only want the down half. Use Wavosaur to synthesize a 133.33 hz wave and then copy the down half. Paste that behind your up portion from the previous step. You now have a sine-width modulated waveform. Save your file. You can call it “S400.wav”.
From here on out, you can do this as many times as you want. You can do another octave or two and/or you can fill in the gaps between the ones you already have. The more that you do, and the more gradually that you do them in terms of pitch, the smoother your “interpolations” will be. Also, you can consider different “rules” for how to fill in the gaps. I did equal-spacing but this is not required. Depending on how you generate these waveforms, your sounds will come out to have different tonalities.
When you are done, you upload the WAVs to the Digipro. Make sure to send them in the logical order based on the frequency content. For example, make sure the S200 goes in the first slot, then S400 in the next, then etc.
Now program the Mono’s LFO to cycle through the different waveforms at the speed you like as in the video. There is a time parameter on the Mono which smooths out the transitions, but you may also like to leave this off for a “steppy” sound. I used the triangle LFO to make it sound like a classic pulse-width modulation but you can use whatever shape you like. You can also control the depth of the LFO so that it only uses the waveforms closer to the base wave. This will give a very warm sound to your Monomachine. Another thing to try would be to have the LFO reset on every trig. This could give a sharp attack and warm body or vice versa.