VoIP Voice and Fax Signal Processing

6.5 ECHO CANCELLATION IN VoIP ADAPTERS

Echo canceller functioning between two VoIP adapters is represented in Fig. 6.5. For easy representation, two phones (phone-A and phone-B) connected to two adapters are marked as A and B. These phones are shown to generate two different types of signals. Phone-A is voice from continuous reading of letters “abcd,” simply marked as abcd.., and phone-B voice is also called voice-B or B-voice. In Fig. 6.5, SLIC, hardware CODEC (ADC, DAC), echo canceller, voice encoder, and decoder are shown as part of each VoIP adapter. The other voice functions of dual-tone multifrequency (DTMF), tones, packet loss concealment (PLC), other packetization, and VoIP signaling are not shown. Refer to Chapter 2 for details on complete VoIP adapter blocks and on an end-to-end VoIP voice call.

Figure 6.5. Echo cancellation between two VoIP adapters.

The VoIP adapter will interface with the telephone through an FXS TIP-RING interface. Hybrid inside the phone converts a two-to-four-wire interface for connecting voice to the handset microphone and speaker. In the VoIP adapter, hybrid converts the two-wire TIP-RING interface to the four-wire interface for ADC and DAC signals. In actual implementation, hybrid is part of the SLIC. ADC and DAC are part of the hardware CODEC chip also called the SLAC. In recent devices, SLIC and hardware CODEC are manufactured as a single device ...