Recall the reconstruction equation for the two-channel, critically
sampled, perfect-reconstruction filter-bank:
This can be written in matrix form as
where the above
where
It turns out orthogonal filter banks give perfect reconstruction filter banks for any number of channels.
Orthogonal filter banks are also called paraunitary filter banks, which we'll study shortly in polyphase form.
The AC matrix is paraunitary if and only if the polyphase matrix is paraunitary. (See Vaidyanathan.)