Patent GB-824248-A (GB)
Title: Improvements relating to circuits embodying television pick-up tubes
Inventor: PERKINS DENIS GORDON
IPC: H04N9/09
Filed: 1955-03-02
Abstract: 824,248. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Feb. 16, 1956 [March 2, 1955], No. 6119/55. Class 40 (3). In order to compensate for distortion of the signals derived from a television pick-up tube, the photo-sensitive surface of the tube is scanned by a periodically-fluctuating light spot and means are provided for deriving a signal from the pick-up tube corresponding to said fluctuating light spot and employing it to effect a correction to remove the distortion. The correction may be effected by modifying the image video signal generated by the tube, or, where the image to be picked up is capable of being controlled, as in standards conversion systems, by modifying the signal forming the image. The distortions to which the invention is particularly applied are those arising due to non-uniform sensitivity over the tube photosensitive surface and the addition of photo-pulse signals to the video signals, although the invention also corrects for non-uniform response of the phosphor of the display tube in a standard converter and for defects in the optical system. Fig. 6 illustrates the invention as applied to a standard converter where signals according to one scanning standard received at A are applied to produce an image on a tube 14 which is viewed by a pick-up tube 1 operating according to a different scanning standard to produce an output at B. In accordance with the invention, distortion in the video signal generated by tube 1 due to non-uniform sensitivity of the photosensitive surface is corrected by illuminating the surface with light from a flying-spot scanner 4 scanned synchronously with tube 1. The flying spot is of constant intensity over the raster but is subject to a high-frequency modulation, e.g. 3.5 mc/s., due to a source 6. The fluorescent screen of the flying-spot is of short persistence so as to produce a significant brightness modulation at 3.5 mc/s. The output of tube 1 across load 9 then comprises a normal video signal component due to electron scanning and corresponding to the displayed image, this signal being selected by low-pass filter 10, and a photo-pulse component at 3.5 mc/s. due to the flying-spot illumination. This latter component, which is modulated in accordance with the sensitivity variations, is selected by band-pass filter 11, demodulated at 12 to obtain a signal representation of the variations, and applied to control the gain of an amplifier 13 through which the main video component passes. In accordance with a further feature of the invention, distortion introduced into the signals generated by tube 1 due to photo-pulse components produced by the image on tube 14 are compensated by causing tube 14 to display a 9 mc/s. signal modulated by the input-signal A and utilizing the component at this frequency in the output to effect compensation. A low-pass filter 23 selects components of signal A and applies them to modulate a 9 mc/s. signal in stage 24, the resultant signal being combined with the original signal A in stage 25 and applied to tube 14. The 9 mc/s. component from tube 1 is then selected by filter 27, demodulated at 28 and applied to a bucking circuit 29 in the main video signal to offset the photo-pulse components. The two correction features described above may be applied separately, Figs. 2 and 5 (not shown). Tube 4 may be scanned in synchronism with tube 14 and the output of demodulator 12 applied to control the gain of an amplifier in the channel transmitting signals A to tube 14, Fig. 3 (not shown). In another arrangement, Fig. 4 (not shown), tube 14 is modulated with a high-frequency signal to effect the sensitivity correction, as well as signal A, and tube 4 is dispensed with. The output of demodulator 12 is applied to a comparison circuit receiving a reference level signal and any difference applied to control a variable gain amplifier in the channel for signal A. This arrangement has the advantage of also correcting for non-uniformity in phosphor of tube 14 and the optical system imaging tube 14 on pick-up tube 1. Correction for photopulse distortion is effected by applying part of the A signal to a bucking circuit in the B signal channel. Fig. 1 (not shown) illustrates the invention as applied to a direct pick-up system. The Specification discusses a number of subordinate features including (1) compensating for photo-pulse distortion by a photo-cell observing the mean output of tube 14, (2) correcting for non-uniformity of the phosphor of tube 4 by a negative feed-back loop including a photo-cell observing the screen, (3) restricting the video signal band-width optically, as by use of a lenticular screen, to prevent interference with the correction signals, (4) preventing the highfrequency correction signal, when applied to tube 14, from producing a video output-signal (where the resolving power of tube 1 is sufficient) by making the frequency of the correction signal a multiple of line scan frequency and subjecting it to a 180 degree phase change between fields, and (5) using the output from demodulator 12 for " gamma " and black-level correction. The invention may be applied to colour television and Figs. 7-11 (not shown) illustrate optical arrangements for scanning three colour-component pick-up tubes simultaneously with the correction light source. This may be provided by one flying-spot scanner where it is desired to scan each tube with the same resolution, or by two scanners where one pick-up tube, for example the Y or green component tube, requires a different resolution from the others. Fig. 12 (not shown) shows the invention applied to an arrangement for converting field-sequentials colour signals to simultaneous signals. The invention may also be applied to pick-up tubes employing light spot scanning as described in Specifications 426,505, [Group XL], and 760,950. The cathode-ray tube providing the scanning spot may be used to provide the correcting light stimulus. It is necessary for the correcting light spot to be displaced from the scanning light spot and this is achieved either by the use of a two-gun tube, or a single-gun tube with spot wobble and synchronized brightness modulation to produce effectively two displaced spots. Reference is made to the use of the invention with tubes having an image section, the correcting signals being derived from the photo-pulse current in the mesh or photo-cathode of the image section.