A line doubler is a device used to de-interlace video signals prior to display. It is usually an accessory to high end home entertainment systems, especially in the context of home cinema, though most display devices now have equivalent functionality built-in. A line doubler may also perform other tasks such as sharpening the image, doubling the frame rate, or scaling to a higher resolution, and a device that performs all these functions is often known instead as a scaler.

The main function of a line doubler is to take an interlaced video source and create a progressive scan output. 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. Some line doublers are capable of using the former technique in moving areas and the latter in static areas, which improves overall sharpness.

A common feature in line doublers is pulldown correction. This is a special operation mode used when the interlaced signal was created using 3:2 pulldown, a technique used to convert 24 frame/s film into 60 frame/s television. The line doubler detects which fields contain data from a particular film frame and merges them together, effectively recreating the original frames. This technique produces superior results to conventional line doubling when working with a 3:2 source, and it is generally possible for the line doubler to switch techniques automatically.