Patent GB-579600-A (GB)
Title: Television signal-generating system
IPC: H04N5/228
Filed: 1944-03-29
Abstract: 579,600. Television. HAZELTINE CORPORATION. March 29, 1944, No. 5846. Convention date, March 30, 1943. [Class 40 (iii)] A television signal-generating system in which a beam current in a cathode-ray tube scans a photo-sensitive electrode having a charge distribution corresponding to the signals to be generated and in which " shading " components are developed during the scanning incorporates means for periodically and substantially decreasing the current or intensity of the beam from a predetermined value during the.scanning interval at a frequency which is at least as high as the highest video frequency and also includes frequency - responsive means coupled to the tube and having a frequency characteristic related to the frequency at which the beam current is modified for deriving a signal which includes the video-frequency components and which is substantially free of the unwanted shading components. In the arrangement shown in Fig. 1 comprising a cathode-ray tube 15, photo-sensitive electrode 16 and normal transmission and control circuits 21 ... 24 and 27 ... 30, impulses of square or sinusoidal wave form at a frequency preferably at least twice that of the highest picture component frequency are applied from a generator 32 to the control grid 38 of the tube 15. The output from the tube 15 is applied through an amplifier 34 having a flat response-characteristic for frequencies including those of the video signals, the - interruptions and of the shading components, to a diode rectifier 35 whose input includes a stabilizing circuit comprising a condenser 36 and a resistance 37 having a time constant greater than the period of interruption of the beam and short compared with the line-scanning period of the signal generator. The signals applied to the diode 35 are stabilized at their peak voltages, normally corresponding to white in the picture. The output from the circuit 33 as indicated in Fig. 7 (not shown) will be free from the shading component which, as indicated by Figs. 3, 5 (not shown) is represented by an increasing voltage added to the picture signals during the period of line scanning. In a modification, Fig. 8, the amplifier 34 of Fig. 1 is replaced by a valve 40, with additional valves, if necessary, having in its input and output circuits frequency selective filters comprising transformers 44, 45 doubly tuned by condensers 46, 47 and 48, 49 respectively. The video components are constituted by modulation of a carrier frequency determined by the interruption frequency. The shading components do not appear as modulations of the carrier wave since the shading signals have substantially the same values during the periods when the beam is effective and during the succeeding intervals when it is interrupted provided the period of interruption is short compared with the average transit time of the returning secondary electrons. The modulated wave may be applied to the diode 35 and passed through the stages 21 ... 24 to the transmitting antennas 25, 26 or may be transmitted direct or supplied to a frequency changer and heterodyned before transmission. If the generator is operated at a frequency approximating to the highest video frequency, the tuned circuits 44, 45 of the valve 40 are adjusted to select the upper sideband of the modulated carrier since the lower sideband would include frequencies corresponding to the shading components. The band width of the filter may be arranged to eliminate frequencies outside the operational range of the receiver to which the picture signals are supplied.