polyphase allpass filters for oversampling
Elliptic half-band filters implemented with allpasses, for up/downsampling. They have very low passband ripple, high attenuation and a short transition band. They are relatively light on the cpu. The cost -some variable group delay.
The resampling is done in 2x stages. A 1-input, 2-output macro does 2x upsampling; connecting it to a 2-input, 4-output one does 4x, and so on. The fractions of sample rate (SR/3, /4, etc.) are the corner frequencies of the filters. The passband remains almost flat up to that point, then falls to -3 dB at SR/2, and keeps falling.
v1.1: added a more relaxed, faster set of macros (SR/3) for general use, kept the old one (3/7 SR) in a separate group, reorganized. Notes and tests revised.