A ripple reduction technique for digital filters

A post-filtering technique is proposed for reducing the frequency domain error (ripple) of FIR (finite impulse response) and FFT (fast Fourier transform) based digital filters. The post-filter is of low complexity and acts to cancel the frequency ripple of the main filter. By assuming that the frequency ripple of the main filter is approximately sinusoidal, the post-filter can be constructed from a single shift-adder and unit delays. This post-filter resembles a simple echo canceller and typically reduces the passband ripple (in dB) of optimum digital filters by a factor of 2-6. The reduced passband ripple permits lower-order filters to be used with more weight given towards increasing the stopband attenuation or narrowing the transition bandwidth.<
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