Polyphase: do the work at the low rate

Set taps and a decimation factor. The taps are dealt into R short filters that each run at the output rate, and the work per output drops by R.

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Where the numbers come from

gHashTag/t27 specs/fpga/dsp/signals.t27 (t27#8985): every value on the page is its fn bodies, compiled to wasm (logic.js)
R. E. Crochiere, L. R. Rabiner, Multirate Digital Signal Processing, Prentice Hall, 1983: decimation and polyphase structures