Speech immunity of push-button tone signalling systems

Two methods for protection of tone signalling against interference from noise of all descriptions within the frequency band received are feasible. The tone can be accompanied by a signal immune to noise, such as a modification of the feed current that switches the tone receiver into operative state only while the push-button is actuated; or the tone receiver can be designed to distinguish a calling signal from noise and to be made immune from noise by guard circuits. The paper studies the measures for best design of a push-button calling system with guard circuits, and the collateral question of how many signal immitations can be tolerated. The problem is set out and examined in detail. 18 illustrations, 9 tables and 25 references are provided.