GB729541A - Improvements in or relating to television film scanners - Google Patents

Abstract

729,541. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. April 2, 1951 [April 1, 1950], No. 8205/50. Class 40 (3). A television film scanner employing a light spot for scanning the film is arranged to scan the bar separating adjacent frames of the film in order to generate a datum level signal which is employed for D.C. re-inserting purposes. In both positive and negative films the bar corresponds to absolute black, and during the interval following the termination of the blackout pulse for the cathode-ray tube of the scanner until the termination of the frame suppression pulse, the light spot scans the bar and serves to generate in the photo-cell of the scanner a true black-signal. Preferably, a "clamp" type D.C. re-insertion circuit as described in Specification 449,242 is employed, the circuit being switched into operation during the occurrence of the black level signals by pulses derived from and occurring after the black-out pulses. The D.C. re-inserting circuit may be arranged in the feed-back path of a " gamma " correction circuit. The " gamma " correction circuit described in Specification 702,325, [Group XL (c)], may be employed. During the period of the black-out pulse, when the cathode-ray beam is extinguished, the photo-cell output is blacker-than-black and a limiter circuit may be employed to reduce the signal. When the " gamma " correction circuit described in Fig. 3 of Specification 702,325 is employed, the rectifier shown therein can be arranged to function as the limiter. Alternatively, pulses, the amplitude of which are approximately equal to the difference between the black-out signals and the true black signals, may be added to the black-out signals. Specification 729,621 also is referred to.