Multirate Filtering

Multirate filtering is a technique for reducing the calculation rate needed in digital filters and, in particular, the number of multiplications to be performed per second. Sample rate reduction, or decimation, modifies the Z‐transform of the signal, as does sample rate increase, or interpolation. Multirate filtering will be introduced first for finite impulse response (FIR) filters, where it appears naturally. The principle of decomposition can be extended to the sampling rate reduction subunit and to the interpolation subunit, which introduces a further advantage. The FIR half‐band filter is a particularly efficient element for implementing these subunits. A discrete Fourier transform computer is a bank of filters which are suitable for multirate filtering. The polyphase network has the effect of correcting, in the useful part of the elementary band, for the interleaving of the numbers at the output of the discrete Fourier transform computer.