Implementing high performance, low computation, IIR filters with 2-path recursive all-pass filters and the harris-sharpening filter

We present here two techniques to form high performance, low cost digital filters from a set of low cost, low performance, prototype filters. Here cost is related to number of arithmetic operations and performance is measured by depth of out-of-band spectral attenuation and shallowness of in-band spectral ripple. The two mechanisms that support this process are the cascading of two-path recursive all-pass filter which form high order prototype filters with a small number of coefficients and application of the harris sharpening filter which uses the prototype filter and an all-pass companion filter to compensate for the increase of in-band ripple levels due to the cascade of prototype filters.