GB1374134A - Position control for a raster scanning a free flowing film - Google Patents

Abstract

1374134 Television SINGER CO 7 Aug 1972 [23 Aug 1971 (2)] 36765/72 Heading H4F A system for scanning a continuously moving film to obtain a video signal for display, e.g. in a - flight simulator, includes an integrator 45, Fig. 4, to which a signal controlling the film drive rate is applied, a summing amplifier 59 which sums the output of the integrator (which represents the instantaneous vertical centre of the film frame being scanned) with the vertical scan signal (V scan) and applies the resulting sum to the film flying spot scanner (or television camera), and a detector arrangement which indicates when the scanned frame begins to move out of the flying spot scanner's raster and resets the integrator 45 to index its output to the vertical centre of the next film frame. The detector arrangement includes a threshold detector 47 connected to the output of the integrator 45 and preset to give an output when its input reaches a value corresponding to the point when a scanned frame begins to move out of range of the scanner's raster, this output is fed together with the vertical blanking signal to an AND gate 49 whose output sets a bi-stable multivibrator 51 which activates switches 55 and 57 to remove the normal scan signals (V scan and H scan) from the scanner and replace them with offset voltages (V off and H off) to drive the scanning spot to the sprocket hole area of the film. In addition when set multivibrator 51, via reset driver 53, causes integrator 45 to integrate towards zero, and the output of a photomultiplier 63 which detects the sprocket holes is counted by bi-stable multivibrators 65 and 67 so that upon reaching a count (i.e. two) equivalent to the number of sprocket holes associated with a film frame provides via OR gate 69 a reset input to bi-stable multivibrator 51 whereby the offsets are removed and a normal scan starts on the next film frame. A second threshold detector 71 is connected to the output of integrator 45 and is set to detect zero voltage and provide a reset input to the bi-stable multivibrator 51, this operates if for some reason sprocket holes are not detected. At slow rates of film movement the raster is susceptible to drift and to compensate for this the raster may be reindexed for each field (Fig. 6, not shown) by detecting during vertical retrace (the scanning spot being offset) the voltage at which a sprocket hole is crossed, and sampling and holding this voltage for use as a correction.