Patent GB-644411-A (GB)
Title: Improvements in radio transmission systems
IPC: H04W88/18
Filed: 1948-02-13
Abstract: 644,411. Electric selective signalling. GENERAL ELECTRIC CO., Ltd., FAIRBAIRN, E. P., and SMART, N. C. Feb. 13, 1948, No. 4258. [Class 40 (i)] [Also in Group XXXIX] Digits are transmitted over a wireless link connecting two automatic exchanges by modulating the wireless carrier with voice frequency code signals, incorrect reception of the signals due to fading being prevented by transmitting each digit code in turn as a continuous signal until a checking code, which may be the same as that transmitted, is received from the distant station. The wireless link is connected to the local exchange through a coder and decoder unit as shown, identical units being provided at either end of the link, the digits being completely registered in the coder before the first voice frequency signal code transmitted. In Figs. 1, 2 and 3 the contacts of relays RW-RZ are shown in their normally operated positions. Transmitting station. When the link is not in use, all the voice frequencies used for signalling are applied to the carrier from oscillators WO-ZO. A call routed to the coder from the local exchange operates relays AD and N over the pulse wire and private wire P respectively. Relay N locks and causes the removal of all the voice frequencies from the carrier by disconnecting earth from the oscillator circuits. Relay BA operates through relay B and disconnects the homing circuits of switches T and U. The checking signal is received from the distant station by voice frequency receivers W-Z and when an unmodulated carrier is returned receiver relays RWRZ fall back, causing relays BU and H to operate. The impulses to be transmitted as code signals are received from the local exchange over the pulse wire and are repeated by relay AD to release and dialling relays B and C, Fig. 3, which function in the usual manner. The first and second digits are registered on switches T and U respectively and the final position of their wipers T2, T3, U3, U4 determine the voice frequency codes to be transmitted. The first impulse train steps switch T over wiper U1 of switch U. The release of the dialling relay C after the completion of the train steps the wipers of switch U over one contact and thereby transfers the impulsing circuit over wiper U1 to switch U, which then registers the second digit. The release of relay C after the second registration completes an earth circuit over wiper U2 to the oscillators selected by wipers T2 and T3 and the first digit code is transmitted over the link. The subsequent receipt of the checking code operates the related receiver relays RW-RZ which cause the corresponding relays in a digit storage relay group TW-TZ to pull up. If the checking frequencies are correct, earth is connected over contacts of relays TW-TZ and wiper T4 to a relay TK, which terminates the transmission of the first digit code. Operation of any of the receiver relays also de-energizes slow-to-release relay BU and if the frequency code test is not completed in its release time then relay H falls back, releasing the digit storage relays and terminating the transmission of the digit code. The release of H may also cause a tone-signal to be connected to the calling line. The operated receiver relays are released when the unmodulated carrier following the first digit code is received and relay TH operates the second windings of the operated storage relays. Relay TH applies earth to the wipers U3, U4 and the second digit code is transmitted, the second digit storage relay group UW-UZ being operated by the checking signal. Relay UK operates over wiper U5 if the frequency code test is correct, terminating the transmission of the second digit code and extending the calling line to the wireless link over a hybrid coil HY. Relay BU is reenergized when the unmodulated checking signal is received and relay UH operates over the second windings of the operated storage relays UW-UZ, disconnecting the storage relays from the signal receiver to prevent inadvertent operation by speech. When the caller releases the connection, relays AD, B and N fall back in sequence. Relay BA homes the switches and operating earth is connected to all the oscillators. The remaining relays are restored to normal when the receiver relays all operate in response to the checking signal. Receiving station. When an unmodulated carrier is received, all the receiver relays fall back and relays BU, H and HA operate. Relay H connects the first digit storage group TW-TZ to the receiver relay contacts and also prevents seizure of the apparatus from the local exchange, while relay HA transfers the oscillator earth circuits to contacts of the receiver relays and enables identical checking signals to be transmitted. The first and second code signals are registered in the digit storage groups TW-TZ, UW-UZ in a similar manner to the registration of the checking signal in the transmitter, an identical checking signal being returned in each case. The storage relay groups apply markings to leads R and V and a start circuit ST is completed for a two-co-ordinate switch (not shown) which moves to the marked positions when relay UH is operated after the second code signal has terminated. The subscriber is then called in known manner and the speech connection extended over the hybrid coil HY. The connection is released and all the relays returned to their normal positions by the receipt of all the signalling frequencies over the link. If relay BU falls back due to prolonged receipt of a code signal, relay H disconnects the storage relays and relay HA disconnects the start circuit ST.