The matrix
It is also possible to express FDNs as special cases of digital
waveguide networks, in which case stability depends on the network
being passive.
Smith and Rocchesso 1994
This analysis reveals that the FDN is lossless if and only if the
feedback matrix
has unit-modulus eigenvalues and
linearly independent eigenvectors (see the
Rocchesso and Smith
1996
for details).