1 Network Echo Cancelers

Figure 2.14: Block diagram of the network echo canceler.

Network echo cancelers are essential in telephone networks, especially long-distance calls. Echos in telephone lines are generated mostly at the hybrid devices located in central switching offices. The function of the hybrid is to convert the end of the two-wire subscriber link which connects the subscriber's telephone to the central office (at the right side of Fig. 2.14) to the four-wire inter-office trunk lines (at the left side of Fig. 2.14). A perfect hybrid would pass all incoming voice signals (Rin) through to the two-wire side without any leakage. Such a perfect hybrid, however, does not exist in practice and usually a portion of the incoming voice signal is leaked to the transmit terminal (Sin). The result is that the remote user will hear his own voice delayed by the network round trip (network delay). The longer the network delay, the more annoying the echo becomes, a phenomenon well observed in long-distance calls. When the telephone network is used for data communication (such as in modem and fax applications), both short and long delayed echo have severe consequences. This problem can be solved by employing a network echo canceler at the four-wire side of the hybrid as shown in Fig. 2.14. The network echo canceler is usually an adaptive transversal filter the coefficients of which are adjusted to minimize the error signal $e(n)$ at the Sout terminal. By minimizing the error, the adaptive filter coefficients converge to a FIR model of the hybrid circuit since the hybrid and the adaptive filter share the input and the error is formed by taking the difference between the hybrid output and the adaptive filter output signals. Filtering the incoming signal through the hybrid model results in an estimate of the echo, shown as $y(n)$ in Fig. 2.14. Subtracting this echo estimate from the signal at the Sin terminal reduces the echo at the Sout terminal. For sufficient echo reduction, the number of coefficients of the adaptive transversal filter should be chosen so that an accurate estimate of the hybrid impulse response can be estimated. Usually, the hybrid impulse response is assumed to last for 20 to 30 milliseconds. Assuming that the echo canceler is working at sampling frequency of 8 kHz, the number of coefficients of the adaptive filters are usually in the range of 240 coefficients. Such filter can be readily implemented on any general purpose DSP using the Normalized Least Mean Squares (see Section 4.11) or the Leaky NLMS algorithms (see Section 4.7). Several echo canceler application scripts are also included with ASPT such as echo_nlms (Section 10.16), echo_leakynlms (Section 10.15), echo_bfdaf (Section 10.14), and echo_pbfdaf (Section 10.17).