Patent GB-890858-A (GB)
Title: Electric signal shaping equipment
IPC: G11C13/04
Filed: 1959-12-21
Abstract: 890,858. Automatic control systems. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 21, 1959 [Dec. 23, 1958], No. 43382/59. Class 38 (4). [Also in Group XL (c)] Apparatus for re-shaping pulse signals (such as those shown at 3A), which oscillate about a varying mean level to provide binary data, comprises a clipping circuit operating about a variable reference level controlled by a specified sampling system. In the system described, the signals 3A remain positive during binary ones and becomes less positive during binary zeros. A film 14 carrying binary information is scanned by light from a swept cathode-ray tube 10, a photo-tube 17 being provided to convert variations in light reflected from the film 14 into electric signals of the type referred to above. The mean voltage level is liable to vary for a number of specified reasons, as shown in Fig. 4A. The output waveform 3A of the photo-tube 17 is fed to a clipping circuit 20 in which the mean clipping voltage is automatically controlled to follow the mean voltage level as shown in Fig. 4B. Clipping circuit 20 comprises a cathode coupled pair of valves 21, 30, valve 21 being cut-off by signals appreciably below the clipping control voltage fed to the grid of valve 30. Signals appreciably above this level are limited by overloading. The clipped waves at the anode of valve 21 are amplified and fed to the timing signal generator 40, the waveform being shown at 3B. The timing signal generator 40.-The synchronized clock pulse generator 42 provides constant width pulses 3D, the timing of which is corrected at each negative-going edge of waveform 3B. The AND gate 45 provides an output only when both waveform 3D and the inverted waveform 3B (shown at 3C) are positive. The output is differentiated and pulses corresponding to the trailing edges are used to change the state of circuit 46 so as to give a positive output. The AND gate 44 provides an output only when both waveforms 3D and 3B are positive. This is differentiated and the trailing edge used to reset the bi-stable circuit 46. The negative-going edge of waveform 3E from circuit 46 thus occurs at an integral multiple of the clock pulse duration after the start of each binary zero of waveform 3A. Single cycle generator 50.-Waveform 3C is differentiated by capacitor 53 and positive halfcycles removed by diode 52 to give waveform 3H. The oscillator 51 generates a single cycle (3J) each time a pulse 3H occurs. The period of waveform 3J is equal to the duration of a standard single binary digit. Waveform 3J is squared at 57 to give waveform 3K. Pulse phase comparator 60.-An OR gate 61 is supplied with waveform 3E, D.C. restored to give waveform 3F and is also fed with waveform 3K. The OR gate 61 provides an output waveform 3L corresponding to its highest input. The AND gate 70 provides an output (waveform 3N) whenever waveforms 3L and 3M are both positive or both negative. The mid-point of each cycle of waveform 3K occurs at a fixed interval after the end of each binary zero of waveform 3A, hence the clipping level correction depends on whether waveform 3K is positive or negative at the time of the negativegoing edge of waveform 3F. If the clipping lever is too high, as at W in waveform 3A, the binary zeros will be too wide, resulting in a negative pulse output from the AND gate 70. Too low a clipping level, as at X, results in a positive pulse output. Positive and negative correction pulses are respectively passed by diodes 81, 82 to charge capacitors 86, 88 to provide waveforms 3P, 3Q. The net output (waveform 3R) is fed via capacitor 37 to correct the clipping level in circuit 20.