VoIP Voice and Fax Signal Processing

6.3 LINE AND ACOUSTIC ECHO CANCELLER

Echo cancellers for voice processing are classified as LECs and AECs based on the source of echo generation. Several techniques and requirements overlap between these two echo cancellers. In general, acoustic echo canceller can be reused as line echo cancellers. Line echo canceller may not cater to all requirements of the acoustic echo cancellation. The main differences of the line and the acoustic echo canceller are listed in Table. 6.1.

Table 6.1. AEC and LEC Functional Differences

As shown in Fig. 6.3(a), acoustic echo is the coupling/leakage of voice through acoustic media, usually through air as media in most telephone applications. Acoustic echo is created while using phones in speakerphone mode. Several phones (mobile phones, IP phones, WiFi phones, car phone adapters) are supporting speakerphone mode. With speakerphone, acoustic waves are stronger in strength to reach several feet distance. The strong acoustic waves from the speaker will also be reaching the microphone creating acoustic echoes. The signal level caused by acoustic coupling is much stronger than the usual speech level reaching the microphone. The phones with a speakerphone option come with a built-in acoustic echo canceller. The acoustic echo canceller is mainly governed by P.340 [ITU-T-P.340 (2000)] recommendations. Many proprietary acoustic echo cancellation techniques ...