Study of echo cancelling algorithms for full duplex telephone networks with vocoders

In the cancellation of telephone echo an algorithm with fast convergence and relatively simple implementation is desired. For narrow band digital voice transmission, it is also of interest to know what effects vocoders have on telephone echo and vice versa [1]. From our experiments, it has been shown that the frequency domain LMS [2] algorithm provides faster convergence than the Widrow LMS algorithm [3], whereas the gradient lattice algorithm [4] produced inferior results. It is also found that when the echo is strong, its effects are more perceptually annoying with an LPC vocoder in the loop. However, when the echo is weak, an LPC vocoder helps to suppress the echo.