- The simplest allpass filter is a unit-modulus gain
- A lossless FIR filter can consist only of a single nonzero tap:
- The transfer function of every finite-order, causal,
lossless IIR digital filter (recursive allpass filter) can be written
We may think of as the flip of . For example, if , we have . Thus, is obtained from by simply reversing the order of the coefficients and conjugating them when they are complex.
- For analog filters, the general finite-order allpass
transfer function is
Elementary Filter Problems