GB1178694A - Improvements in or relating to a Carrier-Telephony System - Google Patents

Abstract

1,178,694. Carrier telephony systems; radio signalling. GENERAL ELECTRIC & ENGLISH ELECTRIC COMPANIES Ltd. 20 July, 1967 [13 June, 1966], No. 26278/66. Headings H4K, H4L and H4R. In a carrier telephony system such as that of Specification 1,178,693, in which a number of subscribers sets operate on individual send and receiver carrier frequencies and in which in order to make a call the calling subscriber operates on the frequencies allotted to the subscriber he wishes to call, each subscribers set has a device which responds to the absence of the transmitting carrier frequency of a subscriber being called to provide an indication that the called subscriber is already engaged as the calling subscriber in an existing call. The arrangement enables the caller to distinguish between absence of a reply due to the called subscriber being absent, from absence of reply due to the called subscriber being engaged as a calling subscriber in an existing call, and therefore transmitting on a different frequency from that allotted to him. As described a relay is provided operated by the received carrier signal and having contacts which complete a positive feedback path over the subscribers receive amplifier in the unoperated condition of the relay, so that when no carrier of the appropriate frequency is received the amplifier oscillates at audio frequency to reproduce a busy tone in the telephone receiver.