Design of 2D IIR digital filters with approximately flat group delay characteristic

The procedure for designing two-dimensional recursive digital filters with controllable amplitude characteristic and flat group delay characteristic is presented. It is necessary first to make a maximally flat group delay one-dimensional IIR filter with the proposed amplitude characteristic. A stable two-dimensional IIR transfer function is obtained using a digital spectral transformation. The way of obtaining the parameters for the digital spectral transformation is also given. This method does not need nonlinear optimization techniques. The obtained two-dimensional filter is guaranteed to be stable, the error between the specified passband contour is very small and the amplitude and group delay characteristic of the one-dimensional prototype are well preserved by the two-dimensional filter.