Line doubler
A line doubler is a device used to enhance video signals prior to display. It is usually an acessory to high end home entertainment systems, especially in the context of home cinema. A line doubler improves the quality of the video signal by making the picture smoother and/or more detailed. A line doubler is not required for the operation of these systems, but is rather an upgrade used to increase the viewer's enjoyment.
A line doubler's input may come from any video source, such as a TV cable, DVD player, video game console, Laserdisc player or VCR. The upgraded output of the line doubler is sent to a display such asa Television, a video projector, HDTV or big-screen display. Most normal televisions are incapable of handling the high quality output of a line doubler, so line doublers are only used with modern, high end displays.
Some line doublers are standalone devices, while some modern TVs or other video gear (such as some DVD players) may have line doubling features built in, such as the "digital reality creation" feature found in some high end Sony televisions. Digital reality creation is a proprietary video processing scheme capable of performing (among other things) basic line doubling. Other manufacturers have similar technology. Another device featuring a built-in line doubler is the Progressive Scan DVD Player.
Line doublers perform two basic functions. Either one may be considered "line doubling," and a device that performs either is considered a "line doubler". A device that can perform both functions simultaneously is called a "line quadrupler". The first (and most significant) function of line doubling technology is deinterlacing, which improves the smoothness of moving video. Deinterlacing is the conversion of video from interlaced to progressive scan format.
The name "line doubler" is derived from how the device actually deinterlaces the video, it simply takes the lines from an interlaced frame and doubles them, filling in the missing lines.
The second function of line doubling technology is interpolation, also sometimes called upsampling, used to improve the clarity and detail of the video. Interpolation is a mathematical process applied to the pixels of each video frame to increases the resolution of the video frame. It is sometimes used in order to allow a low-quality video source, such as a VCR, to be connected to a high quality display, such as an HDTV.
Examples of "line doubling":
- 480i to 480p is a common form of deinterlacing line doubling. This feature is found on some modern TV sets as well as some DVD players. All stand-alone line doublers can perform this conversion.
- 480i to 768i is an interpolation line doubling, where the signal remains interlaced but its resolution is increased. Again, this feature is found on some high-end modern TV sets and other home theater gear.
Sony's "digital reality creation" technology can at present perform either of these two features but cannot perform both simultaneously. This technology may therefore be considered a line doubler but not a line quadrupler.
- 480i to 768p is technically considered line quadrupling as it requires both deinterlacing and resolution enhancing to achieve. This requires a significant amount of signal processing, and so 480i to 768p conversion is generally restricted to dedicated and expensive units.
Other forms of line doubling/quadrupling exist as well, including:
- 768i to 1080i
- 768i to 1080p
- 480p to 1080i
Some types of line doublers are capable of performing "pulldown correction," a process performed only with deinterlacing. In addition to the normal deinterlacing process coverting the signal to progressive scan, pulldown correction removes the frame rate error that results when a film is displayed on a video system. A film, such as a major motion picture, is shot using a film camera capturing twenty-four frames per second. However, a normal television display in the U.S. refreshes sixty times a second. Since the average television display is interlaced, that corresponds to thirty distinct frames every second. Therefore, when a film is converted to a video media, such as a VHStape, laserdisc, or DVD, the twenty-four frames in a second of film must be converted to thirty frames of video. This conversion is usually performed in a complex process known as stutter frame that repeats the display of every fourth frame of the film. The net result is that the film's proper frame rate is not maintained and the video is alternately speeding up and slowing down constantly. A pulldown correcting line doubler can detect this error and undo it, restoring a consistent rate of frame display. Pulldown correction is becoming a common feature in many line doublers.
