Echo suppressors for long telephone circuits

A device has been developed by the Bell System for suppressing “echo” effects which may be encountered under certain conditions in telephone circuits which are electrically very long. This device has been given the name “echo suppressor” and consists of relays in combination with vacuum tubes, which are operated by the voice currents so as to block the echoes without disturbing the main transmission. This paper gives a brief description of this device, together with a discussion of its possibilities and limitations. A number of echo suppressors have been operated on commercial telephone circuits for a considerable period so that their practicability has been demonstrated.