A stable and distortion-free echo and howling canceller [telephone networks]

IIR (infinite-impulse-response) echo cancellers (EC) and howling cancellers (HC) have to freeze adaptation during `double talk´ (DT). However, a fixed series-parallel structure induces signal distortion. Current methods, using a fixed parallel structure or equalizer, need a stability test which costs O(N²) multiplications per sample period. A configuration of IIR EC and HC, consisting of a maximum-entropy two-channel lattice filter and its inverse filter, is proposed. It can cancel echoes and during DT eliminate the signal distortion by the fixed inverse lattice. Its stability is automatically guaranteed without the necessity of tests. The computational cost is O( N)