Patent GB-903478-A (GB)
Title: Improvements in or relating to signal transmission systems
Inventor: CHERRY EDWARD COLIN
IPC: H04B1/66
Filed: 1958-10-10
Abstract: 903,478. Secret transmissions. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Sept. 30, 1959 [Oct. 10, 1958], No. 32376/58. Class 40 (4). [Also in Groups XL (b) and XL (c)] The bandwidth required for transmitting a signal is reduced by identifying periods of high, medium and low information content in the signal, and expanding, leaving unchanged, or compressing the signal in time so as to obtain a more uniform distribution of information in time, a train of pulses indicating the rearrangement of the signal also being transmitted to the receiver. In the described embodiment, a television signal, Fig. 2 (a) from a picture tube 1, Fig. 1, is supplied to a detail unit 7 which identifies the level of information content, an output on both leads 9, 9<SP>1</SP> signifying high picture detail, on lead 9<SP>1</SP> only signifying medium detail, and no output signifying low detail. Gating unit 11 receives clock pulses (b) and is controlled by the output (c) of unit 7 so as to pass all clock pulses during periods of high detail, every third pulse during periods of medium detail, and every ninth pulse during periods of low detail. Switch 40 receives clock pulses (b) and is controlled by the output (e) of unit 11 so as to step in one direction during periods of high detail, to step in the other direction during periods of low detail, and to remain stationary during periods of low detail, a corresponding lead 19 being energized in each position of switch 40. The signal (a) is supplied to all modulators in a bank 20 where it is sampled periodically by the modulators selected by switch 40, the sampling rate corresponding to the identified level of picture detail. The a.m. pulses (f) on leads 51 are supplied to respective taps on delay line 50 so that the resulting output signal (h) at terminal 52 comprises a train of equidistant pulses which may be supplied to a pulse coder or a low-pass filter, the output of which is used to modulate a carrier. Switch 40 also controls a position information generator 21 which provides, in each position of switch 40, a pulse on a corresponding lead 22 whose amplitude also represents the position of the switch. The output at terminal 53 is thus a train of equidistant pulses representing, by their amplitudes, the successive positions of switch 40. Alternatively, generator 21 may merely produce signals indicating the direction in which switch 40 has stepped. The position information signals are transmitted on a different channel to the rearranged signal. At the receiver, Fig. 3, the position information pulses control the setting of switch 60 and hence the particular sampling circuit in bank 70 to which a pulse from generator 107, operating at the medium frequency, is applied over lines 109. The continuously varying rearranged picture signal is thus sampled and supplied to the appropriate tapping on delay line 100 so as to be subjected to a delay which is compelmentary to that introduced in the transmitter. The output (f) of delay line 100 is supplied to a linear interpolator 90 which inserts pulses of appropriate heights between successive sampled pulses, the original signal (a) being derived from low-pass filter 101. Synchronizing pulses are derived from signal (a) and control linear timebase 104. In an alternative receiver, Fig. 9 (not shown), the position information pulses control a three-speed time base, the original picture being retrieved by line scanning at the appropriate one of three different rates.