Patent GB-766801-A (GB)
Title: Improvements in or relating to radio communication systems and to apparatus therefor
IPC: H04M11/00
Filed: 1953-09-28
Abstract: 766,801. Automatic exchange systems. GENERAL ELECTRIC CO., Ltd., FORBES, W. A., and SUMNER, J. Oct. 4, 1954 [Sept. 28, 1953], No. 26603/53. Class 40 (4). An automatic telephone exchange is linked by radio to one or more mobile outstations. The outstations are restricted to simplex operation and an exchange line involved in a call to an outstation is held until a clearing down signal is received at a main radio station in the exchange network. A background signal comprising pips of tone is transmitted from the main station to the exchange whenever the outstation transmits. The carrier from the main station is at all times modulated with pip tone. Dialled impulses are transmitted as pulses of 10 kc/s. modulated on the carrier. The clearing down signal is a two-second pulse of 10 kc/s. Exchange calls outstation. When the exchange calls, relay R in the main station, Fig. 1, responds to ringing current. Relay RR pulls up over winding 21 and holds over winding 22 as R falls back. The ringing is tripped as the loop is closed. The main station transmitter starts when lead 4 is grounded at rr5. At the outstation, a loud-speaker 23 draws attention to the call, being connected to the radio receiver by lead 16. Cradle switch contacts 24, 25 are closed and relay AA pulls up when the receiver 11 is raised. Transmission starts when wire 18 is grounded by a press-to-talk switch 14, the microphone 12 being linked to the outstation transmitter through a low-pass filter 29. The outstation carrier operates relay CA at the main station, the only effect at this stage being to connect pip tone to the exchange line 1, 2 over contacts ca3, ca4, &c. If the outstation clears first, relay CA falls back and when the exchange clears, relay RR falls back and the circuits return to normal. If the exchange clears first the line 1, 2 is held pending a clearing down signal from the outstation, to be described. Outstation calls exchange. The operator lifts the receiver 11 controlling cradle switch contacts 24, 25 and presses switch 14 to start the transmitter if no pip tone is heard from the main station. Relay AA pulls up. The switch 14 is then released to await dialling tone from the exchange. On dialling relays D, VA pull up over off-normal contacts 26, 27, and D locks. Relay AA responds to impulses by applying pulses of 10 kc/s. from generator 28 to the transmitter over wires 17. When the receiver is replaced at the end of a call, relays A, D fall back but relays T, VA pull up during the release period of D to transmit a two-seconds pulse of 10 kc/s. as a clearing signal. At the main station, relay CA responds to carrier and A to tone pulses. Relay CA is followed by CB, DR and RL. The exchange line 1, 2 is seized by application of ground to wire 1 and is held over the loop when RR operates under the control of CB. The transmitter starts as wire 4 is grounded. Relay CA falls back when the outstation operator listens for dialling tone but the exchange connection is held by CB, DR, so long as CA is not back for more than five seconds. Relay A follows the dialled impulses, relay C pulls up during impulse train but falls back between them and relays B, BA remain up for the duration of the call, CB now being locked to BA. The connection is held by CB until B, BA fall back when A responds to a two-second clearing signal or until CB is unlocked by a key 31. Two or more spaced main stations may be connected to a common exchange outlet. They may use slightly different transmitting frequencies within the range to which the outstations are tuned. A warning may be given at the exchange if the handset is left off its rest at an outstation after a call has been terminated or if a mobile outstation moves out of range.