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Automatic letterbox detection

Image Number 4 for United States Patent #5486871.

A video control system, comprises a deflection system having a dimensionally adjustable raster, a circuit for detecting a letterbox video signal source and a circuit for dimensionally controlling the raster of the deflection system responsive to the detecting circuit. The detecting circuit and the control circuit are operable automatically. The detection circuit can comprise a circuit for measuring video luma levels of the video signal source in at least two regions of each video field and a circuit for comparing the luma levels from each of the regions to respective threshold levels. In an alternative, the detection circuit comprises a circuit for comparing respective minimum and maximum luminance values for a plurality of successive video lines, a circuit for storing minimum and maximum luminance values for the plurality of video lines, a circuit for generating gradients indicative of the stored values and a circuit for comparing the gradients to threshold values.

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