Loss-Noise-Echo Study of the Direct Distance Dialing Network

In any telephone connection, energy can be reflected back to the talking customer at any impedance discontinuity or at any junction between four-wire and two-wire circuits. This energy manifests itself to the talking customer as an echo of his own voice. If the reflected energy has sufficient amplitude and delay, it can be annoying and can interfere with the talker's normal speech process. Through the years, considerable effort has been made to reduce the amplitude of the talker echo by using impedance-matching procedures. However, it is not feasible to completely eliminate echo. Loss is therefore introduced into the connection to further attenuate the echo energy. The amount of loss needed increases with the delay of the echo signal relative to the talker's voice signal.1 This loss also attenuates the talker's voice signal received at the far end of the connection. When the amount of loss needed for talker echo control causes undue degradation of the received voice signal, echo suppressors2 are inserted into the connection. Echo 1