polyphase allpass filters for oversampling
well, that
DESCRIPTION
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.
There are two sets of filters, with minimum and quasi linear phase responses. The minimum phase filters give more reduction, but are unsuitable for applications where a constant group delay is required, such as peak detection. For the quasi linear phase filters, a latency is specified, which can be used to delay the non-oversampled signal before mixing it with the oversampled signal in some way. Each set has separate subsets for use with and without processing between stages, and each subset comes in three levels of quality (in terms of max reduction).
The resampling is done in x2 stages. The fractions of sample rate (SR/3, /4, /8) are the filters' corner frequencies. The passband remains almost flat up to that point, then falls to -3 dB at SR/2, and keeps falling. For quasi linear phase, the group delay remains almost constant up to the corner frequency.
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.
v2.0
- removed the 3/7 SR set.
- added a complete set of quasi linear phase filters.
- x2/x4/x8 inputs and outputs are now bundled.
- the /2 gain compensation is now integrated in the downsamplers.
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