A line doubler is a device which deinterlaces video signals. This involves converting an interlaced video source into one with a progressive scan. This can be done in two main ways, with differing results.
An interlaced video source is one where each new frame only updates half of the lines of pixels. With each new frame, alternate lines are updated: in effect, odd numbered lines update with one frame and even number lines with the next. This system was developed as an efficient way to cut down the information that needs to be stored and transmitted with a video signal. It works because the frames are updated somewhere in the range of 25 to 30 times per second depending on the broadcast system. This means the human eye cannot usually detect the fact that not all lines of pixels change at once.
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The problem with interlaced sources today is that they can only be displayed optimally by cathode ray televisions, the older-style, non-flatscreen type of TV set. Plasma and LCD screens instead use progressive scanning, meaning the entire picture refreshes at once. When such screens play an interlaced source, the mismatch can cause visible flaws in the picture, known as artifacts. This means some form of deinterlacing is needed for any footage shot and broadcast in interlaced form, which includes most non-HD footage and some forms of HD footage.
A line doubler is one way to do this. It can exist as a standalone device, but is commonly built into both DVD players designed for use with progressive scan video displays, and the television sets themselves. It is possible to view interlaced signals without a line doubler or other deinterlacing method, but the results would be poor. They might also be particularly notable on the larger screens which are more common with plasma and LCD technology.
The simplest form of line doubler simply works by taking the refreshed lines in each new frame and then repeating them immediately below, in place of the previous line which was not refreshed in the new frame. This means that each new frame produces an entirely new picture, unlike with an interlaced format. The major disadvantage is that because of the repetition, the amount of detail in the picture is effectively halved and the picture will likely be visibly less sharp.
A more advanced form of line doubler will use digital interpolation. In effect, this creates a new frame by taking the lines which have been refreshed and then artificially creating content for the lines in between. As a generalized and simplified explanation of this process, if a particular pixel to be created falls above and below refreshed pixels of the same color, the created pixel will also be that color. If the pixels above and below are different, the created pixel's content will take other surrounding pixels into account.
Many video display devices today have replaced the line doubler with a video scaler. This performs the same deinterlacing function, but can also scale the video source to fit a different screen resolution to that used for the original source. This is more important today as HD screens come in a variety of resolutions, whereas old-style CRT screens tended to have the same number of pixels, with larger screens simply having each pixel be larger. Because a video scaler includes a deinterlacer, using a separate line doubler would be redundant.