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A line doubler, in signal processing, is a device used to deinterlace a video signal before it is displayed on the screen. It is used in LCD and plasma TVs in which interlaced digital video can not be directly displayed, and hence needs intermediate processing.
A line doubler is also known as a progressive converter.
The simplest line doubler delivers each scan line to the TV twice, hence the name. A line doubler itself does not improve resolution, rather it just changes the two-field frame video from the source into a single-field frame. A line doubler uses interpolation to recreate and fill in the missing lines from interlaced video to present one progressive scan frame. The process is usually not noticeable to the user.
Line doublers should not be confused with line splitters, which merge two frames into one or splits one frame into two.