Patent GB-787331-A (GB)
Title: Improvements in or relating to the generation of image signals
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N11/22
Filed: 1955-08-12
Abstract: 787,331. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Aug. 12, 1955 [Aug. 18, 1954], No. 23991/54. Class 40 (3). In signal storage apparatus comprising a cathode-ray tube with a charge storage electrode, information is stored on the electrode in a pattern of discrete lines extending in a one direction whilst further information is stored subsequently without a real or apparent line structure, or in a pattern of lines with a different line spacing, the electrode being scanned in lines in a direction transverse the first lines so as to reproduce the stored informations simultaneously. The transverse scanning of a pattern of discrete lines produces a carrier modulated by the stored information and having a frequency determined by the line spacing, whilst the scanning of information stored without a line pattern produces a normal video signal. By frequency selection two simultaneously produced signals may readily be separated. In a first embodiment, Fig. 1, the invention is described as applied to an apparatus for converting frame sequential colour signals produced by camera 1 operating with filter disc 3 at 150 fields per second into simultaneous signals at 50 fields per second. The camera signals are applied to display tubes 14 and 19 scanned synchronously with the camera under the control of synchronizing generator 2, and the tubes are viewed by storage-type pick-up tubes 16 and 20 operating a 50 fields per second. Camera 1 and display tubes 14 and 19 are scanned with the line direction vertical whilst pick-up tubes 16 and 20 are scanned normally with the line direction horizontal. The first channel comprising tubes 14 and 16 functions to produce a luminance signal Y in accordance with the American N.T.S.C. standards and receives appropriate portions of the green, red and blue camera signals as determined by potentiometer 6 via gate stages 8, 9 and 10 opened during the corresponding fields by a three-stage ring circuit 11, 12, 13, such as that described in Specification 766,987, [Group XIX], synchronized from generator 2. The second channel comprising tubes 19 and 20 produces simultaneous red and blue signals and functions in accordance with the invention. The camera signals are applied to this channel through a gate stage 17 controlled by a connection from stage 12 in the ring circuit so as to pass only the red and blue signals. During the " blue " field the scanning beam in tube 19 is focused normally so that the image appears in a pattern of discrete lines, but during the " red " field a signal from stage 11 in the ring circuit is arranged to defocus the beam whereby the lines are caused to merge and virtually to eliminate the line structure of the image. In the output of pick-up tube 20, the simultaneously produced red and blue signals are thereby rendered separable in that the blue signal appears as modulation of a carrier and may be derived by means of a band-pass filter 21 and detector 23, whilst the red signal appears as a normal video signals and may be derived directly by a low-pass filter 22. Undesired components of the blue signal appearing with the red signal are removed by a subtractor 24. Instead of defocusing the beam in tube 19 during " red " fields, the effective width of the lines may be increased by other means, such as spot wobbling or spot broadening, or by optical defocusing. The frequency of the carrier produced during " blue " fields may be reduced by blanking out alternate lines or alternate pairs of lines. Single tubes using reading and writing beams may replace the combinations of display and pick-up tubes. In a second embodiment, Figs. 2 and 3, the invention is applied to the production of simultaneous green, red and blue signals directly from a storage-type camera tube 30 exposed to the corresponding colour components of an image in succession by means of a rotating filter disc 31. The camera tube is scanned at 50 fields per second, but the disc is rotated at a speed such that each component impinges the camera for 1/150th of a second. The disc is constructed as shown in Fig. 3 with conventional green, red and blue filter sections 33, 34, 35, the red and blue sections, however, bearing in addition large numbers of thin opaque arcuate lines, the number of lines being greater in the case of the blue filter 35. By this means, as the filter is rotated, green, red and blue component images are stored on the camera target, the green image being a normal image without a line structure, but the red and blue images being broken up into patterns of lines, the directions of which are arranged to be vertical. On scanning in the normal horizontal direction, the camera output then comprises simultaneous green, red and blue signals with the red and blue signals appearing as modulations of carriers, the frequencies of which differ due to the different line spacing on the filters. Filter networks 44, 45, 46 pass the signals into separate channels. To prevent overlapping of the frequency spectra of the three signals, diffusing or defocusing means may be used in conjunction with the red and blue filters to reduce the corresponding image definitions. The filter disc may be replaced by a loop of film bearing filters. In a third embodiment, not illustrated, the invention is applied to measure and compensate for transmission or sensitivity variations in a converting arrangement employing a display cathode-ray tube viewed by a pick-up tube, such as 19 and 20 in Fig. 1. For example, in, say, the red channel of a three-channel sequential to simultaneous signal converter, the red signals are displayed with the beam defocused, and during the subsequent green and blue fields, when signals appear in other channels, the tube is scanned with a sharply focused beam of constant intensity. The output of the pick-up tube then comprises the red signal together with a carrier modulated by transmission variations introduced by the display and pick-up tubes, lens, &c. The carrier is separated on a frequency basis and demodulated to obtain a signal which is applied to operate a correcting gain control in the channel either before or after the converter. As an alternative, the red signal may be caused to appear on the carrier and the correcting signal as a normal signal. Reference is made to the control of the tube focus in accordance with deflection to maintain good focus overall, and also an arrangement for varying the focus cyclically at a high rate so as to ensure that the tube passes through a condition of precise focus. The signal derived for correction purposes is then peak rectified to obtain a signal indicating the condition at precise focus. Reference is also made to storing the correcting signal to improve the signal-to-noise ratio by integration. Specifications 778,179 and 778,182 also are referred to.