Patent GB-639954-A (GB)
IPC: H04W84/02
Abstract: 639,954. Radio signaling. GENERAL ELECTRIC CO., Ltd., EVANS, J. I. G., and FAIRBAIRN, E. P. Oct. 10, 1947, No. 27264. [Class 40 (v)] In a radio transmission system, in which the signals are prefaced by a plurality of impulse trains characteristic of the required station, the receiver comprises a single motion step-by-step electromagnetic switch operated by the received impulses, with indicating means which operate if the switch contact engages a plurality of predetermined fixed contacts at the end of successive trains of impulses. The arrangement is especially applicable to a system involving a fixed transmitter, and a plurality of mobile receiving stations, such as taxi-cabs. A suitable form of transmitter is shown in Fig. 1. Removal of a telephone handset closes contacts 1 and operates relay A, energy being supplied from oscillator 3 via rectifier 4. Operation of a telephone dial closes off normal contacts 5 and 6 to operate relay C. Dial contacts 2 then provide the desired number of impulses to control relay A, which results in impulses of modulated carrier wave being emitted from transmitter 7. At the mobile receiver station (Fig. 2) the receipt of a carrier operates a muting relay (not shown) which closes contacts 9 causing relay B to operate and busy lamp 11 to light. The operation of relay B, (i) prevents the operation of the mobile transmitter, (ii) connects uniselector magnet 12 to a stepping circuit via contacts Y2 and D1, and (iii) operates relay Y. The operation of relay Y breaks the earth connection to the wiper of uniselector bank ZA, and prepares the stepping circuit for the uniselector. The circuit is now ready to receive the impulses of modulated carrier signals, which are fed to transformer 15, the secondary of which is tuned to the modulation frequency. The receipt of an impulse causes the anode current of valve 17 to increase and operate relay D which operates uniselector magnet 12, wipers 13 and 21 moving at the end of the impulse when D releases. The calling identification number of the station is assumed to be 38. At the end of impulsing slow-release relay Y releases, and if the wipers 13 and 21 are at the appropriate position for the first digit of the called station, relay W operates. The operation of relay W, (i) locks relay W to earth in parallel with relay B, (ii) prepares the circuit for operating relay X, and (iii) re-operates relay Y. The receipt of the second train of impulses causes similar stepping of the uniselector. After the required eight impulses, relay Y releases and relay X operates. Contacts X1 establish a holding contact for relay X via telephone cradle contacts 10, release relays B and W, and extinguish the busy lamp; contacts X2 connect the receiver output to the telephone unit; and contacts X3 operate the calling lamp 22. The uniselector is then stepped by self-interruption to a home position on the first contact of the banks ZA and ZB. When the operator answers, removal of the telephone operates contacts 10 and 14 which causes relay X to release but maintains the connection of the receiver to the telephone unit. Other mobile stations within range of the transmitter will have a busy condition indicated due to the operation of the appropriate muting relay which is of the slow-release type. A modification is described in which all the mobile stations may be called simultaneously by dialling the digit "O," the eleventh contact on bank ZA being connected direct to relay X at all the stations. Alternatively, a selected group of mobile stations may be called simultaneously by allocating a particular initial digit to each group and appropriately connecting relay X. It may also be arranged that the busy lamp is lit only if the operator attempts to transmit while carrier signals are being received. An arrangement involving three digits is also described. Each mobile station may be provided with impulsing and transmitting apparatus, and may also be associated with automatic telephone exchange equipment.