Patent GB-819907-A (GB)
Title: Improvements in or relating to electric selective calling systems
IPC: H04B5/04
Filed: 1957-09-10
Abstract: 819,907. Electric signalling systems. BULL, R. A., COATES, C., HUNTRISS, H. C., and LARDNER, W. E. Sept. 10, 1957, No. 28538/57. Class 40 (5). A central transmitter can selectively call one or more of a group of mobile receivers, and during transmission of speech also transmits a control signal which automatically reduces the gain of the receiver from a maximum to a value dependent upon the level of the received control signal. Transmitter. Selection of one or a group of receivers is achieved by sequential transmission of two audio-frequency tones derived from an oscillator 10, Fig. 1, whose tuned circuit capacitance consists of preselected values of capacitor Ca, Cb ... Cn. The capacitors are selected by throwing keys Sa, Sb ... Sn one up and one down, and the sequential switching is carried out by contact RYb during the operation of relay RS. Depression of switch SW operates relay RS via SW-1, and contact RSa operates the slugged relay RX via contact RYa normal. Change of contact RXa operates slugged relay RY whose contact RYa disconnects relay RX so that relays RX and RY alternate in operation. The alternate operation and release of relay RY changes contact RYb and connects preselected capacitors Ca, Cb ... Cn to inductor L to produce an alternating audio frequency which, after amplification, 11, 12 and 13 modulates the transmitter 14 for transmission as a calling signal. Relay RS also operates contact RSd so that the control signal from 18 is disconnected while the calling signal is transmitted. RSc, when operated, removes the short circuit from the tuned circuit of oscillators 10. RSb in conjunction with RXb controls the signal lamps N1, N2 and N3 so that N1. lights to indicate that the transmitter is ready for use, N2 flashes to show that calling is taking place and N3 glows at the end of the calling signal. When. C1 is discharged RS releases and RSd connects the control tone from 18 and the speech from 16 and 17 to the amplifier 12 for transmission to the antenna which is tuned by distributed series capacity around the loop and a variable capacitor 28. Receiver. The calling signals are received by a ferrite rod aerial and detected before passing through a three-stage transistor amplifier to the calling selector 56, Fig. 2, which is a vibratory reed device. The first calling frequency charges capacitor 57 and the second frequency transfers this charge to 58 and triggers the oscillator 60 to produce an audio tone in the sound reproducer 63. On receipt of the calling signal the switch 62 is operated so that the sound reproducer is connected directly to the output of the amplifier 55 for the receipt of speech. The speech and control signals are amplified by 53, 54 and 55 and at the output of 55 the control tone is fed back via, capacitor 65 and detector 64 to form a gain control of the receiver. The value of capacitor 65 is such that it passes the control tone but has a higher attenuation to signalling tones and speech. Thus the receiver is working at maximum gain during calling when no control signal is received and the gain is automatically adjusted during speech by the level of the received control tone. Specifications 806,233, 813,672 and 819,908 are referred to.