VoIP Voice and Fax Signal Processing

6.2 NAMING CONVENTIONS IN ECHO CANCELLER

Naming conventions of the echo canceller are marked in Fig. 6.2. The echo canceller is represented as a four-terminal device connected in the receive and send voice path. Echo cancellation blocks and literature uses the names of R_in, S_in, L_RES, S_out, and S_gen for representing the signals [ITU-T-G.168 (2004), ITU-T-G.161 (2004)]. Signal R_in is the reference signal coming from the far end. In the receive path, no modification for this signal R_in occurs. The signal coming out of EC in the receive path is marked as R_out. This signal is the same as R_in. The signal R_out is played on the speaker. Echo is created based on the R_out signal. The near-end signal is S_gen. During the near-end signal, the echo canceller will work as simply pass-through. The combined signal of echo and S_gen is the input S_in to the echo canceller. The echo canceller uses R_in as a reference and cancels echo present in S_in. The echo canceller first level removes the linear part at the summing junction and gives the nonlinear part and small uncancelled linear part as residue represented as L_RES. The suffix “RES” denotes residue. In this book, S_RES is used in place of L_RES for easy identification of the send path. This residue is processed in nonlinear processor (NLP) to obtain a nonlinear echo removed signal denoted as S_out or L_RET. The signal at S_out is expected to be combination of imperceptible echo and S_gen without any significant distortions. The returned echo level at ...