Patent GB-867362-A (GB)
Title: Improvements in or relating to the electrical transmission and storage of information represented by direct voltages
Inventor: MATTHEWS ROBERT
IPC: G01S13/66
Filed: 1956-11-13
Abstract: 867,362. Frequency modulations; frequency and phase discriminators; radar. DECCA RECORD CO. Ltd. Dec. 12, 1957 [Nov. 13, 1956; Dec. 12, 1957], Nos. 34748/56 and 3684/57. Classes 40(5) and 40(7) Apparatus for converting information represented by a direct data voltage of variable magnitude into an alternating voltage having a frequency representative of the amplitude of the direct voltage comprises a sawtooth generator which is cut-off and restarted each time the instantaneous amplitude of the sawtooth output becomes equal to the amplitude of the direct -voltage. The sawtooth output is then converted into a sinusoidal signal of related, frequency, e.g. a subharmonic, which is recorded and/or transmitted to a remote receiver and a direct voltage of amplitude proportional to the frequency of the recorded or transmitted sinusoidal signal is produced by generating a sawtooth waveform synchronized by pulses derived from the sinusoidal signal and sampling the peak amplitude of the sawtooth waveform by means of a pulse sampling demodulator. Multiplex transmission or recording may be effected on a frequency-sharing or time-sharing basis. When reproducing a recorded signal, the speed of the record medium, e.g. magnetic tape, past the reproducing head may be controlled by a synchronizing signal recorded on the medium. This synchronizing signal may comprise a standard reference frequency recorded on a separate recording channel or a readily identifiable signal in a multiplex system as described below. The invention is particularly applicable to the transmission and recording of intercom markers in a radar display system in which the location and direction of an adjustable electronic marker trace is defined by two pairs of direct voltages. Modulation, Figs. 1 and 2. As shown the sawtooth 30, Fig. 2, is generated by an integrator 13, Fig. 1, of the type disclosed in Specification 818,477 which comprises (1) a high-gain amplifier 14 having a feedback capacitor 15 and a feedback resistor 16 associated respectively with input resistors 17 and 18 and (2) ganged electronic switches 20, 21 such that when the switches are in their upper positions as shown the amplifier 14 operates as a Miller integrator to generate the linear sawtooth voltage 30 whose slope is determined by a reference voltage applied to resistor 17. When the instantaneous amplitude of the sawtooth becomes equal to that of the input data direct voltage applied at 10, a comparator 11 produces an output pulse which triggers a monostable flip-flop 26 whose output pulse changes over the switches 20, 21 for the duration of the pulse thereby terminating the sawtooth and charging the capacitor 15 to the starting level of the next sawtooth as determined by a reference voltage applied to resistor 18. The internal 32, Fig. 2, between adjacent sawteeth depends upon the duration of the output pulses from the flip-flop 26. The recurrence frequency f of the output pulses from the flip-flop 26 is inversely proportional to the amplitude of the data direct voltage and these pulses trigger a bi-stable multivibrator 33 whose output is applied to a low-pass filter 34 giving a sinusoidal signal of frequency f12 which is applied to the transmission line 36 and/or magnetic tape recorder 38. The reference voltage applied to the resistor 17 is adjusted so that the frequencies of the output signals are within the band 300- 3,000 c/s. Demodulation, Fig. 3 and Fig. 4 (not shown). As shown in Fig. 3 the sinusoidal signal from the transmission line 42 or reproducer 43 is applied through a low-pass filter 44, variable gain amplifier 45 and squarer 46, e.g. a "long-tail pair", Fig. 4 (not shown), to trigger a monostable flipflop 60. One set of output pulses from the flipflop 60 controls the electronic switches of a sawtooth generator 76 similar to the generator 13 in Fig. 1 to produce a sawtooth waveform synchronized with the input sinusoidal signal which is applied to the input resistor 69 of the pulse sampling demodulator 63 which comprises a high-gain amplifier 64, feedback capacitor 65, feedback resistor 66 and ganged electronic switches 68, 71 controlled by a second set of output pulses from the flip-flop 60, each pulse commencing just before the termination of each sawtooth sweep, such that during a switching pulse (switches 68 and 71 in their lower positions) the capacitor 65 is charged to the peak value of the sawtooth and in the interval between switching pulses the amplifier 64 operates as a Miller integrator with zero rate of change of output voltage thereby holding the charge on the capacitor 65 until the next sampling pulse. The relative durations of the switching pulses from the flip-flops 26, Fig. 1 and 75, Fig. 3, are adjusted to cancel the effect of the intervals 32, Fig. 2, in the sawtooth waveform. The output direct voltage from the capacitor 65 then corresponds to the input data voltage at the transmitter and the amplitude ratio of these voltages may be adjusted by adjusting the reference slope-control voltages applied to the sawtooth generators 13 and 76. This adjustment is facilitated by the periodic transmission of a fixed datum input signal. Positive and negative voltages may be distinguished by altering the reference voltages. Frequency multiplex, Fig. 5 (not shown). A separate transmitter is used for each channel and the reference voltages of the separate sawtooth generators are adjusted so that the output sinusoidal signals are in separate frequency bands. Time-division multiplex, Figs. 6A and 6B. This is described as applied to the transmission from a local radar P.P.I. C.R.T. display 117, Fig. 6A, to a remote C.R.T. display 192, Fig. 6B, of the four direct voltages controlling the location and orientation of an electronic interscan marker trace generated and controlled as disclosed in Specifications 744,640 and 867,361. As shown both the radar P.P.I, scan and the marker trace for the local display 117 are produced by a single pair of sweep generators 118, 119 coupled to fixed deflection members. The four control direct voltages are applied sequentially by means of an electronic switch 142, Fig. 6A, to a single transmitter unit comprising components 144-148 corresponding to components 11, 13, 26, 33, 34 in Fig. 1 to give an output comprising four sequential sinusoidal wave trains which are transmitted via a line 160 to a remote station Fig. 6B including a receiver 161 comprising components 162-167 and a fast pulse sampling demodulator 184 corresponding respectively to components 44-46, 60, 75, 76 and 63 in Fig. 3 so that the output from the demodulator 184 comprises a direct voltage which changes its level in accordance with the changes in frequency of the input sinusoidal signal. This direct voltage is applied in parallel to four further pulse sampling demodulators 180-183 similar to the demodulator 63 in Fig. 3 and sampling pulses from the flip-flop 165 are applied sequentially to the demodulators 180- 183 by means of a distributer 168 synchronized with the switch 142, Fig. 6A, so that each demodulator 180-183 samples the output from the fast demodulator 184 only during the middle of the corresponding switching period. The direct outputs from the demodulators 180-183 are then applied to interscan drive units 190, 191 to produce on the C.R.T. display 192 a marker trace corresponding to the marker trace on the radar display 117, Fig. 6A. Alternatively the fast demodulator 184 may be omitted in which case the sawtooth output from the generator 167 is sampled directly by the demodulators 180-183. The switch 142, Fig. 6A, and distributer 168, Fig. 6B, are each actuated by sequential switching pulses from four bi-stable multivibrators triggered by a counting tube synchronized by a common A.C. source, e.g. a mains supply. In a modification, Figs. 7-9 (not shown), an additional reference direct voltage of amplitude greater than the data voltages is applied to the switch 142 and is transmitted once per cycle as a synchronizing signal. At the receiver the output from the fast demodulator 184 is applied together with a local reference direct voltage to an amplitude comparator which produces a synchronizing pulse during reception of the synchronizing signal; the synchronizing pulses may be applied to reset the counting tube 175 or to synchronize an R.C. oscillator which constitutes the A.C. source at the receiver. Alternatively the received reference sinusoidal signals may be separated from the data signals by means of a filter.