Analysis of even order IIR digital filters implemented as a cascade of first order complex allpass sections

A low-coefficient-sensitivity filter structure based on identical two-input complex allpass sections has been introduced. This structure is suitable for implementing even-order transfer functions such as bandpass and bandstop filters derived from lowpass prototypes. A scaling procedure for the cascade realization of identical second-order sections has been developed. The roundoff noise characteristics are calculated and compared with the roundoff noise for the original transfer function implemented as a cascade of second-order direct form sections and a cascade of single input complex allpass sections. The inband signal-to-noise ratio of the complex structure shows some improvement over the other implementations, with the signal-to-noise ratio remaining essentially constant irrespective of section order. The signal-to-noise ratio remains constant over the complete frequency band because of the complementary nature of the original implementation.<>