Time Division Conferencing of Four-Wire Telephone Circuits

This paper describes a system which can conference up to several hundred 4-wire telephone circuits with sufficient return loss to prevent objectionable far-end echo effects. The causes and problems of echo in such connections are discussed, as well as the advantages and limitations of other methods used for correcting this situation. The method described uses a time-division echo treatment scheme with a simple common-bus conferencing technique. Each transmit line is sampled at the Nyquist rate during which time its corresponding receive line is disconnected from the bus for the duration of the sample time, thus eliminating the return path through the conference bus. To demonstrate feasibility, a working system is described which provides a return loss of at least 50 dB below transmission level when three out of a possible 20 4-wire lines are conferenced.