FIR filters that implement resampling of a data stream at a fixed fractional rate P/Q, where P and Q are integers up to 64, are available for the Systolic Multiply-Accumulate architecture. In This Figure , the operation of an interpolation filter with interpolation rate P=5 is contrasted conceptually with the operation of a fixed fractional rate filter with rate P/Q=5/3.
The normal (integer rate) interpolator passes the input sample to all P phases and then produces an output from each of the phase arms of the polyphase filter structure. In the fractional rate version, the output is taken from a phase arm which varies according to a stepping sequence with step size Q.
This Figure illustrates how the fractional rate decimator structure is implemented for a rate change of P/Q = 3/5. Input samples are fed into the data vector until enough samples are received to generate the current output given the interpolation and decimation rate. In this example, two new input samples are required to generate the first two outputs, with the third output requiring only a single input. To generate each output, the data vector samples are applied to the corresponding coefficients.
Symmetry is not currently exploited when using the fractional rate structures.