| 摘要 |
A raster, for example in magenta, generated on a phosphor screen of a color picture tube is picked up by a television camera through an optical filter which allows a magenta component to pass therethrough predominantly. The television camera scans the displayed raster in the horizontal direction on the line base thereby to produce a video signal representing luminances at individual points on the raster lines. A sample and hold circuit is provided to extract paired video signal values at paired points on the raster lines located symmetrically to each other relative to a vertical center line of the display screen. One of the paired video signal values extracted through the timed sampling operation is subtracted from the other at a processing circuit.
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