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The main function of a line doubler is to take an interlaced video source which consists of a two-field frame and create a progressive scan (single non-interlaced frame) output. This can produce a brighter, smoother, higher-resolution picture. The additional lines provide greater light output and makes for an image that is smoother and more film like. CRT-based displays (both direct-view and projection) are capable of directly displaying both interlaced and progressive video, and therefore the line-doubling process is an optional step to enhance picture quality. Most other types of display, including LCD, plasma and DLP, generally cannot accept an interlaced signal directly, and therefore some kind of line doubler must be used. This is usually built in and transparent to the user. Progressive scan DVD players also feature a line doubler.
The most basic and literal way to double lines is to repeat each scanline, though the results of this are generally very crude. Most line doublers use digital interpolation to recreate the missing lines in an interlaced signal, and the resulting quality depends on the technique used. Generally a line doubler will only interpolate within a single field, rather than merging information from adjacent fields, to preserve the smoothness of motion, resulting in a frame rate equal to the field rate (i.e. a 60i signal would be converted to 60p.) Some line doublers are capable of using the former technique in moving areas and the latter in static areas (to avoid bob effect), which improves overall sharpness. It is worth noting that even if a line doubler employs the merging method it cannot be considered an inverse telecine device if a frame rate of 60p other than the original 24p is obtained. From this aspect of view some hyped progressive scan technologies (including Pioneer's PureCinema Progressive Scan) bearing an inverse telecine insignia are thus overstated.
Line doublers have been replaced recently by video scalers which incorporate 3:2 pulldown removal and the ability to scale the image to the various screen resolutions used on LCD, plasma display and DLP projectors and displays.
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