Patent GB-673173-A (GB)
IPC: H04W88/18
Abstract: 673,173. Selective signalling. AUTOMATIC ELECTRIC LABORATORIES, Inc. Jan. 9, 1950. [Apri1 2, 1949] No. 532/50. Class 40 (i). [Also in Group XL (c)] A selective signalling system, e.g. for calling individually the cars of a police radio system, involves the transmission of a series of digits each of which consists of a burst of tone frequency current which may have a single one of a number of possible frequencies or may have two or more of the frequencies. During the reception of each digit, a condenser is charged and when the digit ends, the condenser is discharged over a path dependent on the digit. If the characteristic digits of the receiving station are received, a calling signal is given. In the system described, there are three frequencies and three digits each of which can then have any value from 1 to 7. Digit 7 is reserved for calls to all cars simultaneouslv. Numbers likely to give rise to false calls are not used. Transmitter. The transmitter may merely involve the direct connection of the appropriate tone frequency generator(s) to the line or modulator by means of a key strip, Fig. 1 (not shown). As shown in Fig. 2, however, dialled digits received by impulse relay 200 are registered on three minor switches 120, 121, 122 in known manner with the aid of a dialling relay 210 which remains operated during each train and relays 220, 230 which are operated over offnormal springs ON and respectively transfer the impulse circuit to the driving magnets 121m, 122m at the termination of the first and second digits. Relay 230 also connects the tone frequency generators 110 ... 112 to the modulator 162 or line in accordance with the setting of the first digit-register 120. On the termination of the final (3rd) digit a circuit is made via off-normal springs and a thermal resistor 291 for a relay 250 the operation of which is thus delayed by two seconds and this relay is followed after similarly contrived two-second intervals by relays 260, 270, 280. 250 on operation terminates the first transmission and 260 connects up the generators in accordance with the setting of the second register. Similarly, 270 terminates this transmission and 280 initiates the 3rd and final transmission and completes a circuit via a thermal resistor for the release magnets of the minor switches which are thus restored. after a further two seconds, releasing the relays 250 ... 280 and terminating the transmission. 220 and 230 are released at 230 by a switch or other means (not shown). Receiver. Each receiver comprises three vibratory relays A ... C, Fig. 3, responsive respectively to the three tone frequencies used in the system, a relay 70, which is operated when any-tuned relay is operated and charges a condenser 60, and three slow-to-release relays 20, 40, 50 operated respectively by the tuned relays and having their contacts connected together and to terminals 1 ... 7 in a manner characteristic of the station. At the termination of each signal,' 70 falls back and the condenser 60 is discharged either via a resistor, if the number received does not correspond to the station number, or via relays 80, 90 and 100 for the successive digits if it does. Relay 100 locks up and sounds a call bell and is unlocked by the hook switch. As shown, the single digit, 7 (all frequencies simultaneously) gives the call at all stations.