Gerzon Nested MIMO AllpassAn interesting generalization of the single-input, single-output Schroeder allpass filter (defined in §2.8.1) was proposed by Gerzon  for use in artificial reverberation systems.
The starting point can be the first-order allpass of Fig.2.31a on page , or the allpass made from two comb-filters depicted in Fig.2.30 on page .3.15In either case,
- all signal paths are converted from scalars to vectors of dimension ,
- the delay element (or delay line) is replaced by an arbitrary unitary matrix frequency response.3.16
is a diagonal matrix of pure delays, with the lengths chosen to be mutually prime (as suggested by Schroeder  for a series combination of Schroeder allpass sections). This structure is very close to the that of typical feedback delay networks (FDN), but unlike FDNs, which are ``vector feedback comb filters,'' the vectorized Schroeder allpass is a true multi-input, multi-output (MIMO) allpass filter. Gerzon further suggested replacing the feedback and feedforward gains by digital filters having an amplitude response bounded by 1. In principle, this allows the network to be arbitrarily different at each frequency. Gerzon's vector Schroeder allpass is used in the IRCAM Spatialisateur .
Example Allpass Filters