where can be any phase value. In the real case
can only be 0 or , in which case
A lossless FIR filter can consist only of a single nonzero tap:
for some fixed integer , where is again some constant phase,
constrained to be 0 or in the real-filter case. Since we are
considering only causal filters here, . As a special case of
this example, a unit delay
is a simple FIR allpass filter.