Patent US 4,031,338

An echo suppressor for use in the send path of the four-wire part of a long distance telephone circuit employs frequency-selective center clipping. The bandwidth of the channel in which echo occurs is subdivided in contiguous bands or subchannels by appropriate filtering, and each resulting signal is center clipped by a center clipper. Threshold control voltages are predicted from knowledge of the average speech amplitude distribution characteristic as a function of frequency. This characteristic is subdivided into the same frequency bands as that used by the center clippers, and the ratios of energies in the bands are used to equalize the ratios of the control voltages that determine the center clipper thresholds. This technique is implemented through the use of a peak rectifier, the input of which is coupled to the receive path of the echo suppressor and the output of which is connected to each of the filtered subchannels by a resistive dividing network. The values of the resistances in each dividing network are determined so that the ratios of the various voltages appearing at these networks are proportional to the ratios of the average energies of the telephone speech in the respective frequency bands. These voltages are then applied to the center clippers to regulate each clipper threshold so that it is proportional to the applied voltages.