VoIP Voice and Fax Signal Processing

6.9 ECHO CANCELLATION IN MULTIPLE VOIP TERMINALS

As explained in Chapter 2, VoIP deployment makes use of several user interface devices such as IP phones, WiFi phones, personal computers (PCs) as soft IP phones, residential gateways with FXO interfaces, and VoIP-to-PSTN gateways. The echo cancellation for these VoIP systems is given in this section.

6.9.1 Echo Cancellation in IP and WiFi Handsets

The IP phone works as a four-wire interface without using a two-to-four wire hybrid. The microphone and speaker are used as electrically separate units. In a normal analog phone, sidetone is generated in the design of hybrids and coupling at two-to-four-wire interfaces. In the IP phone, sidetone has to be created as an internal electrical signal provided from the microphone path to the speaker path, and this function will be limited as sidetone and will not be continued as electrical echo. In summary, the IP phone works as a four-wire digital phone in regular handset mode. In digital phones, TCLw from the handset is 45 dB. This level is sufficient for VoIP calls up to delays of 400 ms. IP phone acoustics are used as a normal headset, speakerphone, and headset interface. The normal headset mode works as a four-wire digital phone. In speakerphone mode, acoustic echo is created and an acoustic echo canceller is used to cancel it. IP phone acoustic interfaces are extended through headsets and Bluetooth terminals. In general, these interfaces work as four-wire interfaces. A small acoustic echo ...