Suzanne Ciani created the Xenon pinball soundtrack (1980)

I think the way it works from the way I read it, is that there's only a single bit of output for each step of the waveform being shaped that can specify up or down. For a female/higher frequency waveform the crests/troughs occur more rapidly so the bits have to be output faster thus needing more bits per unit time. There might be more to it than that as there might also have been something there about variable time steps, but that was the gist of it that I got.