In its simplest form it involves a sine wave carrier whose instantaneous frequency is varied ... A trickier example is 8:5. First we reduce 8-5=3 and get 3:5 (not NF). Reducing again, we get 3-5=-2, ...

Sine-wave speech can be thought of as a sort of auditory illusion, a sensory edge case in which one’s experience has a clear “before” and “after” moment, like going through a one-way door.