https://www.youtube.com/watch?v=gTOKDwQ97wI
It evolved from simple observations such as:
arcsin(sin(x))*(2/pi) = triangle wave which swings in range (-1,1)
arctan(cot(x))*(2/pi) = sawtooth wave which swings in range (-1,1)
which I then conceptually extended to compound (nested) expressions like these:
sin(atan(2*sin(a*tanh(b*tan(x)))))
sin(8*atan(1/sinh(a*tan(x))))
asin(cos(a*tanh(b*tan((x)))))*(2/pi)
tanh((a*cos(a*atan(b*sin(x)))))
which produce a wide range of interesting, evolving sounds both IMO 'FMish' in character, and otherwise. You can see one of the comments to the video re this technique:
hmm...am interested in anything you can offer to reasonably explain from a mathematical standpoint the similarity AND the differences between 'traditional FM', phase distortion/modulation, and the above techniqueEffectively this IS FM synthesis with operators sequentially modulating the phase of each other. Just you have access to other functions than sin