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- For the method of incrementally displaying raster
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- For the decorative motif used in ancient European and Celtic art, see Migration Period art and Celtic knot.
Interlaced scan refers to one of two common methods for "painting" a video image on an electronic display screen (the second is progressive scan) by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image. A PAL based television display, for example, scans 50 fields every second (25 odd and 25 even). The two sets of 25 fields work together to create a full frame every 1/25th of a second, resulting in a display of 25 frames per second.
DescriptionWith progressive scan, an image is captured, transmitted and displayed in a path similar to text on a page: line by line, from top to bottom.
The interlaced scan pattern in a CRT (cathode ray tube) display completes such a scan too, but only for every second line. This is carried out from the top left corner to the bottom right corner of a CRT display. This process is repeated again, only this time starting at the second row, in order to fill in those particular gaps left behind while performing the first progressive scan on alternate rows only.
Such scan of every second line is called interlacing. A field is an image that contains only half of the lines needed to make a complete picture. The afterglow of the phosphor of CRTs, in combination with the persistence of vision results in two fields being perceived as a continuous image which allows the viewing of full horizontal detail with half the bandwidth that would be required for a full progressive scan while maintaining the necessary CRT refresh rate to prevent flicker.
Only CRTs can display interlaced video directly – other display technologies require some form of deinterlacing.
HistoryWhen motion picture film was developed, it was observed that the movie screen had to be illuminated at a high rate to prevent visible flicker. The exact rate necessary varies by brightness, with 40 Hz being acceptable in dimly lit rooms, while up to 80 Hz may be necessary for bright displays that extend into peripheral vision. The film solution was to project each frame of film three times using a three bladed shutter: a movie shot at 16 frames per second would thus illuminate the screen 48 times per second. Later when sound film became available, the higher projection speed of 24 frames per second enabled a two bladed shutter to be used maintaining the 48 times per second illumination — but only in projectors that were incapable of projecting at the lower speed.
But this solution could not be used for television — storing a full video frame and scanning it twice would require a frame buffer, a method that did not become feasible until the late 1980s. In addition, avoiding on-screen interference patterns caused by studio lighting and the limits of vacuum tube technology required that CRTs for TV be scanned at AC line frequency. (This was 60 Hz in the US, 50 Hz Europe.) In 1936 when the analog standards were being set in the UK, CRTs could only scan at around 200 lines in 1/50th of a second. By using interlace, a pair of 202.5-line fields could be superimposed to become a sharper 405 line frame. The vertical scan frequency remained 50 Hz, so flicker was not a problem, but visible detail was noticeably improved. As a result, this system was able to supplant John Logie Baird's 240 line mechanical progressive scan system that was also being used at the time.
From the 1940s onward, improvements in technology allowed the US and the rest of Europe to adopt systems using progressively more bandwidth to scan higher line counts, and achieve better pictures. However the fundamentals of interlaced scanning were at the heart of all of these systems. The US adopted the 525 line system known as NTSC, Europe adopted the 625 line system, and the UK switched from its 405 line system to 625 in order to avoid having to develop a unique method of color TV. France switched from its unique 819 line system to the more European standard of 625. It should be noted that although the term PAL is often used to describe the line and frame standard of the TV system, this is in fact incorrect and refers only to the method of superimposing the colour information on the standard 625 line broadcast. The French adopted their own SECAM system which was also adopted by some other countries, notably Russia and its satellites. PAL has been used on some otherwise NTSC broadcasts notably in Brazil.
ApplicationInterlacing is used by all the analogue TV broadcast systems in current use:
Benefits of interlacingWith any video system there are trade-offs. One of the most important factors is bandwidth, measured in megahertz (for analog video), or bit rate (for digital video). The greater the bandwidth, the more expensive and complex the entire system (camera, storage systems such as tape recorders or hard disks, transmission systems such as cable television systems, and displays such as television monitors).
Interlaced video reduces the signal bandwidth by a factor of two, for a given line count and refresh rate.
Alternatively, a given bandwidth can be used to provide an interlaced video signal with twice the display refresh rate for a given line count (versus progressive scan video). A higher refresh rate reduces flicker on CRT monitors. The higher refresh rate improves the portrayal of motion, because objects in motion are captured and their position is updated on the display more often. The human visual system averages the rapidly displayed still pictures into a moving picture image, and so interlace artifacts aren't usually objectionable when viewed at the intended field rate, on an interlaced video display.
For a given bandwidth and refresh rate, interlaced video can be used to provide a higher spatial resolution than progressive scan. For instance, 1920x1080 pixel resolution interlaced HDTV with a 60 Hz field rate (known as 1080i60) has a similar bandwidth to 1280x720 pixel progressive scan HDTV with a 60 Hz frame rate (720p60), but approximately 50% more spatial resolution.
Note that this is assuming an analog or uncompressed digital video signal. With digital video compression, as used in all current digital TV standards, interlacing introduces some additional inefficiencies over fully progressive video, and so the bandwidth savings are significantly less than half.
Problems caused by interlacingInterlaced video is designed to be captured, transmitted or stored and displayed in the same interlaced format. Because each frame of interlaced video is composed of two fields that are captured at different moments in time, interlaced video frames will exhibit motion artifacts if the recorded objects are moving fast enough to be in different positions when each individual field is captured. These artifacts may be more visible when interlaced video is displayed at a slower speed than it was captured or when still frames are presented.
Because modern computer video displays are progressive scan systems, interlaced video will have visible artifacts when it is displayed on computer systems. Computer systems are frequently used to edit video and this disparity between computer video display systems and television signal formats means that the video content being edited cannot be viewed properly unless separate video display hardware is utilized.
To minimize the artifacts caused by interlaced video display on a progressive scan monitor, a process called deinterlacing can be utilized. This process is not perfect, and it generally results in a lower resolution, particularly in areas with objects in motion. Deinterlacing systems are integrated into progressive scan television displays in order to provide the best possible picture quality for interlaced video signals.
Interlace introduces a potential problem called interline twitter. This aliasing effect only shows up under certain circumstances, when the subject being shot contains vertical detail that approaches the horizontal resolution of the video format. For instance, a person on television wearing a shirt with fine dark and light stripes may appear on a video monitor as if the stripes on the shirt are "twittering". Television professionals are trained to avoid wearing clothing with fine striped patterns to avoid this problem. High-end video cameras or Computer Generated Imagery systems apply a low-pass filter to the vertical resolution of the signal in order to prevent possible problems with interline twitter.