The Effects of Time Delay and Echoes on Telephone Conversations

About, HO to 40 years ago, the effects of time delay and echo became of great concern to engineers planning transcontinental telephone systems. As a result, much effort was devoted to understanding these effects and finding ways to control them. 1 Fortunately, the development of high-speed transmission systems made the problem less severe than originally anticipated, and satisfactory ways were found to handle the amounts of delay that were then involved. As a result, interest in the effects of delay declined and little work has been carried out in this field during the last twenty years. But, as happens so often, the historical cycle is beginning to repeat. With the growth of intercontinental telephony and particularly the proposal of satellite systems involving oneway path lengths of 50,000 miles or more, the problems of delay have again come to the fore. Because of this renewed interest, Bell Laboratories has resumed its studies of echo and delay. The basic problems, first considered 30 or more years ago, have been reviewed to see if improved solutions might result from the technology that has evolved in the intervening period. This work has been addressed not only to technical problems but also to new testing techniques for the evaluation of echo, delay and the control of these factors. Throughout the recent program, emphasis has been 2869