Patent GB-341480-A (GB)
Title: Improvements in or relating to wave signalling systems
IPC: H04B1/46
Filed: 1929-10-16
Abstract: 341,480. Combined telegraphy and telephony systems. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London.-(Western Electric Co., Inc. ; 195, Broadway, New York, U.S.A.) Oct. 16, 1929, No. 31403. [Class 40 (v).] In a two-way transmission system, automatic means are provided for transmitting code signals whenever the channels are not being used for speech currents. As speech starts or finishes, the voice currents are utilized to stop or start the transmission of printer telegraph signals, so that each available channel is continuously in action for one or the other form of signalling. Fig. 1 shows the invention applied to a combined radio and linewire system. When outgoing speech is initiated from the line LL, part of the speech current is shunted through an amplifier-detector 18 and operates relays 20, 21, and 19. Relay 20 breaks the line from the receiving aerial 5, so as to prevent any incoming signals whether speech or code from passing beyond the shunt containing the telegraph printer receiver 17. Relay 21 stops the operation of the code printer sender 8. Finally relay 19 removes a short-circuit across the line to the transmitting aerial 4. The station is now in condition for voice transmission outwards and to receive code signals from the opposite direction. At the opposite terminal station the receipt of code signals is effected on the shunt line through the network 14 to the recorder 17. The switch 28 is closed at that station, but the switch 33 is also closed, so that code signals do not reach the telephone receiver in the line LL, the impedance 34 serving to prevent reflection effects. In the shunt line 36, an antiresonant circuit 22 prevents code signals from reaching and operating the switch 24. When telephone signals are received, however, they do not operate the recorder 17 because of the network 14, but they do pass along the shunt circuit 36 to operate, first, the relay 25 to allow the passage of speech through to the phones in the line LL, and, secondly, the relay 24 to break the line to the aerial 4 and prevent the passage of outgoing speech. The code-transmitter 8 is now in action and is transmitting code outwards whilst speech is on the incoming channel. Since the speech sending and speech receiving circuits are never in operation simultaneously, " singing " around the loop circuit is impossible. Reflection of the telegraph currents is prevented by making the frequency of the oscillator 11 at the sending station different from that at the receiving station. In a modified arrangement, suitable for use with a short-wave singlesidebands radio link, there are two separate modulators. One modulates the printer telegraph signals, and transmits the upper sideband, whilst the other modulates the telephone signals and transmits the lower sideband. At the receiving station the two sideband are separated and passed to different detectors connected to the same oscillator. In this system the received printer signals are filtered away from the telephone circuit. Specifications 142,115 and 206,715, [both in Class 40 (iv), Telephones &c.] are referred to.