File:Telephony echo canceler with hybrid transformer and switch.png

English: The function of H, the hybrid transformer is to route x_k, incoming speech (white) from the far end to the near end telephone switch and route speech from the near end telephone to the far end. However, if the hybrid is imperfect or the impedane on the left side of the hybrid transform is different from the impedance on the right side, some of the farend speech will leah through tho the output, d_k, so that it contains both the desired speech (red) from the near end telephone plus leaked speech from the far end. The adaptive filter f_k attempts to minimize the error signal ε_k by filtering the incoming far end speech into a replica, y_k of the far end speech that leaks through the hybrid. The replica is subtracted from the output of H to create ε_k, the signal sent back toward the far end. Once the adaption is complete, the error signal consists almost entirely of speech from the near end telephone.