Algorithm Development of a Telephone Signal Conditioner for the Wideband Integrated Network

摘要

During this program a full-scale testbed simulation of the interfacing of a telephone to the wideband integrated network was completed. This simulation includes the use of different types of echo cancelling algorithms and an LPC vocoder, and allows for other studies to be carried on. Echo cancelling algorithms were studied with a number of interesting conclusions. Because of the non-stationarity of the speech signals, made more so by the vocoder, standard LMS algorithms and lattice techniques are not adequate because of their convergence properties. With the vocoder in the loop, convergence must be faster than without the vocoder, because synthetic speech signals are not so rich in components as are the actual speech signals. An unconstrained frequency domain adaptive filter algorithm was the most effective. The echo cancelling is limited by the nonlinearities of the system. It was found that a nonlinear adaptive filter could reduce the signal-to-noise ratio by a few more dB when used with a stationary system (the hybrid and line characteristics remain fixed). The possible use of an AGC in the system was studied, and it was concluded that the AGC would cause more problems than it would help.