Patent GB-1506559-A (GB)
Title: Data transmission systems
IPC: H04N7/167
Filed: 1972-01-17
Abstract: 1506559 Data transmission systems SIEMENS AG 17 Jan 1972 [5 March 1971] 2208/72 Headings H4L and H4P In a data transmission system, intentional or unintentional electromagnetic interference is suppressed by rendering the bandwidth of the RF transmission very much wider than the bandwidth of the signal to be transmitted. Thus, at the transmitter, an analogue signal to be transmitted is converted either to a frequency-modulated signal or to a digitally-coded signal which is then phasemodulated by or mixed with a pseudo-random code signal prior to its transmission; at the receiver, the received phase-modulated signal is detected and subjected, once again, to the pseudo-random code signal with the result that the frequencymodulated signal or digitally coded signal is regained and subsequently re-converted to the analogue signal which is reproduced. At the transmitter (Fig. 1) a TV camera 1 and waveform generator circuit 2 form a complete video signal which is frequency-modulated in an oscillator 3 before being applied to a phasemodulator 6 controlled by a pseudo-random code signal from a generator 4. The output from phasemodulator 6 passes to a frequency converter 9 to produce a modulated RF carrier for transmission and thence to an amplifier 11 and directional antenna 12. A constant-frequency oscillator 10 may be used for frequency conversion in the converter 9, although the oscillator 10 may also be a frequency-hopping device, operated in accordance with a specific programme. In an alternative construction, the units 3 and 6 are replaced by a circuit-arrangement shown in broken-line fashion. Here, the video signal from generator 2 passes to an A/D converter 13 to form a binary signal (PCM, #-modulated or PCM-#- modulated). Subsequently, code words are formed in a coder 15 and a pseudo-random code signal from generator 4 is added to the output from coder 15 in a half-adder 16. Thereafter, the signal is phase-modulated in a modulator 17 coupled to an oscillator 18 which produces an output signal from unit 17 to correspond, in its frequency state, to the output from the phase-modulator 6 provided in the first construction. At the receiver (Fig. 2) the received phasemodulated signal is filtered and amplified in an RF stage 20 before passing to a frequency converter 21 coupled to an oscillator 22 which can be a constantfrequency device, or frequency-hopping device if such an oscillator 10 is employed in the transmitter. The emergent IF signal is fed via an amplifier 23 to a frequency converter 24 and a synchronizing device of a pseudo-random code signal generator 25 which produces a code signal of the same form as that used in the transmitter. The original frequency-modulated signal is regained at the output of converter 24, the video signal being regained from a frequency demodulator 27 and reproduced on a CRT 28. In an alternative construction corresponding to the alternative construction of transmitter, the frequency demodulator 27 is replaced by a circuitarrangement shown in broken lines. Here, the output from converter 24 is applied to a phase jump demodulator 29 coupled to a code word recognition circuit 30 containing either correlators or matched filters. The recognition circuit 30 also receives an input from a synchronizing signal separator 36. Analysis of the correlator or matched filter outputs occurs in a decision device 31 from which the output is stored, in a store 32, in digital form. Individual image element information in store 32 can be called up by CRT 28 via line 40; the signals called up are passed through a D/A converter 33 which outputs respectively luminance, frame sync. and line sync. signals.