Design of Recursive Digital Filters for Subband Coding

Direct synthesis of the recursive lowpass prototype filter of quadrature mirror filter bank with nearly perfect reconstruction is described in this paper. Passband attenuation characteristic of this prototype digital filter is placed in the area between Butterworth and Elliptic attenuation characteristics. By complementary filter decomposition technique this filter is decomposed into parallel connection of two allpass filters. Then lowpass/highpass filters are obtained by adding/subtracting of these allpass filters, respectively. The main advantages of this kind of filters are hardware reduction, cost savings and smaller sensitivity to digital word length changing.