VoIP Voice and Fax Signal Processing

6.6 ECHO PATH

Voice is an analog signal transmitted from the telephone to the VoIP adapter's ADC/DAC interface. Digital samples enter DAC and give analog signals from the SLIC hybrid. The SLIC hybrid introduces certain leaks and sends back part of the signal to the ADC path. The analog signal entering the TIP-RING line goes through distortions. Multiple telephones connected on the TIP-RING interface and their telephone hybrids and impedance mismatches create additional echo. The combined influence of SLIC hybrid, TIP-RING, and telephone is treated as a filter. It is also called by different names as echo impulse response, echo path, echo span, echo model, echo spread, and echo tail. The maximum spread of the significant response of the echo path filter is the tail length. G.168 provides eight echo path models to conduct tests as per G.168 recommendation.

For analog phones used with VoIP adapters, an echo canceller with a 12- to 16-ms span or tail length is sufficient. On the commercial side, many boxes are marketed as supporting 32 ms. For FXO side interfaces, echo span varies based on the termination point of the destination phone. A typical span of 48 ms would be sufficient for more regions. Some PSTN-to-VoIP gateways use an echo canceller span of 128 ms mainly to cater to the local long-distance PSTN side influences. Independent echo canceller systems are built up to a 512-ms echo span as given in [URL (Orion-EC), URL (Ditech)].

6.6.1 Delay Offset and Tail-Free Operations to ...