GB607227A - Two-voice frequency signalling equipment - Google Patents

Abstract

607,227. Voice-frequency signalling. STANDARD TELEPHONES & CABLES, Ltd. Dec. 7, 1945, No. 33146. Convention date, Dec. 26, 1944. [Class 40 (iv)] In a signalling system in which signals are sent by two voice-frequencies transmitted simultaneously, a protective means is provided which operates at frequencies approximately one-half and one-third of the signalling frequencies. Thus, not only must both signalling currents be present at the correct strength for positive working, but improper operation by frequencies whose second or third harmonics approximate to the signalling frequencies is prevented. In one embodiment, incoming signals are amplified in valves L1, L2 and passed across a voltage limiter K to tuned circuits C1 ... C4. C1 and C2 are tuned to the signal frequencies which they pass through rectifiers R1, R2 to operate relays r1, r2 which jointly control the signalling circuit. Circuits C3 and C4 are tuned to approximately onehalf and one-third of the signalling frequencies and if these exist relay r3 operates and prevents operation of r2. Alternatively, relay r3 may have a break contact in the final signal circuit or rectifiers R3, R4 may feed opposing windings of relays r1, r2. In one modification, Fig. 2 (not shown), the signalling frequencies are sufficiently close to pass 'through a single tuned circuit, after which they are fed to a modulator and their difference frequency is selected and rectified to operate a single signalling relay. A further modification, Fig. 3 (not shown), uses a magnetic amplifier having two opposing control windings, one energized by currents of signal frequency and the other by those of frequencies passed by the protective circuits. Specification 605,408 is referred to.