GB576845A - Improvements in or relating to electrical signalling systems - Google Patents

Abstract

576,845. Electric selective signalling. DRYSDALE, L. H., and SQUIRES, M. Nov. 8, 1943, No. 18522. [Class 40 (i)] In a system in which a signal of variable duration causes an impulse generator at the receiver to step a selecting switch for the duration of the signal, in order to counteract the effect of fading and at the same time to guard against spurious signals, an operation takes place after the signal has been received for a certain time by which a subsequent break in the transmission does not disable the impulse generator unless it lasts for longer than a predetermined time. A selective system operating over a radio link is shown in which the radio transmitter TT is modulated on a frequency X for a period dependent on the operated one of a set of keys KA... KK, and on a frequency Y momentarily at the beginning of the period and shortly after its termination. Relay SA controlled by a start key KS connects up the X and Y frequency oscillators and starts an impulse-generating relay PU to step the switch AW, the Y frequency being cut off when the switch moves off-normal. Relays XA, YA at the receiver respond to the X and Y frequencies and their simultaneous operation starts an impulse-generating relay IP to step the switch BW, the circuit being maintained by relay XB until the switch reaches position 3, and then by relay ST. If the X frequency stops before the switch reaches position 3, relay HA pulls up, disconnects IP and makes a homing circuit for the switch. If the X frequency stops after this time, impulses are delivered to counting relays U... M as long as XA is back, but the operation of the switch BW is unaffected unless the break persists long enough for relay M to energise. At the transmitter, assuming for example that key KE is operated, relay T pulls up in position 7 of the switch AW and cuts off the X frequency, and relay TA pulls up in position 9 and the Y frequency is sent during the release period of relay SA. The switch AW then goes home. On the cessation of the X frequency signal, relay M energises after two more impulses from relay IP, and since relay YA is operated by the Y frequency at this time, ground is connected to the selected wire of the set SD 1 ... 10. The release of relay ST sends the switch BW home. Instead of the self-interrupting relays PU, IP, thermionic oscillators may be used for generating impulses.