Patent GB-556081-A (GB)
Title: Improvements relating to multichannel carrier communication systems
IPC: H04B7/26
Filed: 1942-03-13
Abstract: 556,081. Multiplex wireless signalling. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., and HOUGHTON, L. W. March 13, 1942, No. 3375. [Class 40 (v)] In a multiplex system, each of a plurality of signal channels is arranged to modulate its respective sub-carrier frequency and all the subcarrier frequencies are combined together to modulate a principal carrier frequency at a central station, one side band with or without the principal carrier frequency being radiated, while at each sub-station the side band with or without the principal carrier frequency is or are received and one or more of the modulated subcarrier frequencies is or are obtained therefrom. Fig. 1 shows an arrangement for two-way signalling between a central station and two substations in which the output from a pulse generator 1 rich in harmonics is used to modulate the output from an oscillator 2 to produce a series of equally spaced frequencies for use as subcarriers which are respectively selected by filter amplifiers 3, 3<SP>1</SP>, modulated at 5, 51 by signals from microphones 20, 20<SP>1</SP> and combined at 6 to modulate the output of a principal carrier frequency oscillator 8. The upper side-bands are selected and amplified at 7 and radiated at aerial TA7. For transmission, switches 11, 13, 15, 17 and 12, 14, 16 and 18 are set to position T. At the substation, say No. 1, the appropriate side band is received on aerial RA21 and after amplification at 21 is heterodyned at 22 by oscillator 23 to produce the modulated sub-carrier which is selected by filter 24, demodulated at 25 and fed to headphones 30 after low frequency amplification at 26. While receiving, switches 31 ... 34 are at position R. For transmitting back to the central station switches 31 ... 34 are set to position T, and a signal from microphone 29 after amplification at 26 is used to modulate an oscillator 27 to produce a frequency which, when heterodyned by oscillator 23, produces a frequency equal to the lower sideband of the signal transmitted from the central station, this being the frequency allotted to the return channel from substation No. 1. This signal is fed via amplifier 28 to aerial TA 28 for radiation and is received on aerial RA10 at the central station. After amplification at 10 it is heterodyned at 9 by oscillator 8 to produce a modulated sub-carrier which is selected at 3, demodulated at 4 and fed to headphones 19 after low frequency amplification at 5. For receiving, switches 11, 13, 15, 17 and 12, 14, 16, 18 are set to position R. Figs. 5 and 6 show respectively a central station and a substation in a system using only a single aerial at each station. As before, oscillators 45, 46, Fig. 5, generate the subcarriers for selection at 47, modulation at 49, after which all the modulated sub-carriers modulate the output of a principal carrier frequency oscillator 50 which supplies to aerial TA50 for radiation one side band and the principal carrier frequency. Switches 53, 54, 55 and 56 are at T for transmit and R for receive. Substation aerial RT58, Fig. 6, receives the signal which is demodulated at 59 to give all the modulated subcarriers, the appropriate one being selected at 60, demodulated at 61 and fed to earphone 72. Switches 65-71 are at T for transmit and R for receive. To transmit from the substation, the output from local sub-carrier frequency oscillator 63 is modulated at 62 by a signal from 73 and caused to modulate a principal carrier frequency oscillator 64, the resulting signal being radiated by aerial RT58. This signal is received on the central station aerial TA50, Fig. 5, where a demodulator 57, which is permanently coupled to the oscillator 50 in such a way as to absorb little power, extracts the modulated sub-carriers for selection at 47 &c. with demodulation at 48 and utilization at 52. A modified substation arrangement is described, Fig. 7 (not shown), in which the principal carrier frequency oscillator 64 of Fig. 6 is replaced by a filter and amplifier arrangement which together function as an oscillator at the principal carrier frequency. An arrangement for preventing undesired effects due to mutual cancellation of carrier signals at the same field strength from stations whose distances differ by an odd number of half wave lengths is also described, Fig. 9 (not shown). Intercommunication between the substations may be arranged by means of preset frequency adjustments of the filter and oscillators.