New class of 2-D digital filters composed of allpass subfilters

We have proposed a new synthesis method for 2-D circularly symmetric digital filters by using allpass subfilters. The advantage of the proposed filter is to have approximately linear phase and small amplitude deviation in the passband. With our method, three filters are needed to realize the circularly symmetric characteristic, but two 2-D filter sections have the identical parameters and fewer independent parameters are required than conventional filters. The structure has very low sensitivity, because of the allpass property.