Patent GB-697825-A (GB)
Title: Signal-translating system for a television receiver
IPC: H04N5/208
Filed: 1951-08-03
Abstract: 697,825. Television. HAZELTINE COR. PORATION. Aug. 3, 1951 [Aug. 14, 1950] No. 18442/51. Class 40 (iii). [Also in Group XL (c)] Relates to transient-modifying circuits for use in television receivers for improving the sharpness of transient video signals which are distorted due to the attenuation of H.F. components during transmission. According to the present invention the video signal channel is interrupted during the occurrence of a transient and the output of the channel during said period is maintained at an amplitude level equal to that of the signal immediately preceding or succeeding the transient, the interruption being effected by means responsive to the rate of change of the input signal. The invention may be applied to improve the picture sharpness of television receivers of normal bandwidth or it may be applied to receivers having a reduced bandwidth to produce pictures having a sharpness comparable to those produced by conventional receivers. As shown in Fig. 1 the distorted signal A, Fig. 2, from the cathode-follower 16 is applied to the grid of the cathode-ray tube 18 through a normally conductive balanced bridge rectifier 27, 28, 32, 33 and is also differentiated at 41, curve B, and rectified (full wave) at 42 to produce a control signal C equal to the absolute magnitude of the first derivative which is applied through amplifier 43 to block the rectifiers. When the rectifiers are conductive, capacitor 34 connected between the grid of the cathode-ray tube and earth, is charged and the charge is maintained during the blocking periods, i.e. the negative portions of curve D, so that the potential at the grid of the tube varies as in curve E, the output waveform E being delayed with respect to the original waveform. The blocking threshold level L, Fig. 2, C, determined by bias battery 35 is selected so that only those transients having a derivative greater than a predetermined value are modified by the circuit. The balanced bridge rectifier effectively prevents the appearance of the control signal at the output terminal. In a modification, Fig. 3, section 45 of the video channel includes a parallel storage capacitor 57 and a normally conductive series rectifier 47 so that when the input video signal has a substantially constant amplitude capacitor 57 is charged to a corresponding voltage but when a negative-going transient occurs the potential across the capacitor cannot follow the rapid change of input signal and the rectifier is cut-off thereby inducing oscillations in the circuit comprising inductor 63, capacitors 57, 62 and 49, resistor 58 and the low impedance source coupled to the input terminals, the parameters being such that the rectifier becomes conductive after a half cycle of the oscillation; section 46 of the channel operates in a corresponding manner for positive-going transients. In a further embodiment, Fig. 5, and Fig. 4 (not shown), the distorted signal A, Fig. 6, is applied to a delay line 70, 72 (delay time D) whose input, output and centre tap are connected respectively to the signal grids of pentode gates 78, 79 and 77 having a common anode load 80, 81. The input signal is also applied to a differentiating circuit 73<SP>1</SP> having a time delay D/2 and two push-pull outputs from 73<SP>1</SP> are applied respectively via capacitors 96 and 97 to the suppressors of V78 and V79 and to rectifiers 93, 94 having a common load resistor 95 connected to the suppressor of V77. In the absence of transients, V77 is conductive and V78 and V79 are cut-off by the common bias 100 so that the output signal is delayed D/2 but when a positive-going transient occurs the negative derivative of the transient produces a negative voltage across resistor 95 which cutsoff V77 and the positive derivative is applied via capacitor 97 to render V79 conductive so that for the duration of the transient the output signal has traversed the total length of the delay line and is therefore the same as the input signal immediately preceding the transsient provided the delay D is at least equal to the duration of the transient. The negative derivative of the transient applied via capacitor 96 to V78 has no effect since the valve is already cut off. When a negative-going transient occurs, the negative derivative again cuts off V77 and the positive derivative renders V78 conductive so that for the duration of the transient the output signal is not delayed by the line and is the same as the input signal immediately succeeding the transient. In this embodiment the positive transient of the output B, Fig. 6, is delayed but the negative transient is not delayed, i.e. the positive going portion of the wave is contracted; this small geometric distortion may be reduced by generating more complex derivatives in circuit 731. In any of the embodiments the rectifiers may be contact, crystal or thermionic.