Polyphase filter banks using wave digital filters

The branch filters in a digital polyphase network can be designed either as FIR filters by decomposing the impulse response of an FIR low-pass prototype filter, or as ordinary IIR filters by the synthesis method of Bellanger. The use of all-pass networks has hitherto been considered unfeasible because of the associated computational difficulties involved in the design of filter banks with many branches. The purpose of this paper is to demonstrate that it is indeed possible to design the branch filters as all-pass-low-pass sections without the need of a prototype filter. Moreover, these sections can be realized as wave digital filters, which give improved properties over the other designs with respect to hardware requirements, group delay, sensitivity, dynamics and limit cycles. Examples, including the design of the practically important 60-channel filter bank for the transmultiplexer, are given.