Abstract: A device used in conjunction with key telephone systems, for example, intercom systems. The device consists of a circuit for decoding the tone output from touch-tone telephones, indicating which digit has been operated and selectively signalling the called party. Special features include a minimum latcing time for received signals and a time-out circuit. A noise, or "talk-down" protection circuit is included; if a signal of any type is present in the slot between the low group and the high group touch tone signals, the input to the decoders is shunted out and the time-out circuit is disabled. Also included is a second phase lock loop connected to the lowest frequency decoder in order to decrease the recognition time. A relay decoding matrix is also provided, arranged to accommodate either single or two-digit codes. The device may be used in key telephone systems which include rotary dial equipment as well as touch tone equipment.

Abstract: The present invention relates to a device which is to be used in conjunction with key telephone systems, as for example intercom systems. More particularly, the invention consists of a device for decoding the tone output from touch-tone telephones, indicating which digit has been operated and selectively signalling the called party. Special features include a minimum latching time for received signals and a time-out circuit. A relay decoding matrix is also covered with provision for accommodating either single or two digit codes. The device may be used in key telephone systems which include rotary dial equipment as well as touch-tone.