article

For the method of progressively displaying raster graphics, see Interlace (bitmaps).
For the decorative motif used in Celtic art, see Celtic knot.

Interlace is a technique of improving the picture quality of a video transmission without consuming any extra bandwidth. It was invented by RCA engineer Randall Ballard in the late 1920s.. It was ubiquitous in television until the 1970s, when the needs of computer monitors, resulted in the reintroduction of progressive scan. While interlace can improve the resolution of still images, it can cause flicker and various kinds of distortion. Interlace is still used for all standard definition TVs, and the 1080i HDTV broadcast standard, but not for LCD, micromirror, or plasma displays. These devices require some form of deinterlacing which can add to the cost of the set.

Description

With 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 would complete such a scan too, but only for every second line and then the next set of video scan lines would be drawn within the gaps between the lines of the previous scan.

Such scan of every second line is called a field. The afterglow of the phosphor of CRT tubes, 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 which would be required for a full progressive scan while maintaining the necessary CRT refresh rate to prevent flicker.

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Odd field
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Even field
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Only CRTs can display interlaced video directly – other display technologies require some form of deinterlacing.

History

When 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 twice: a movie shot at 24 frames per second would thus illuminate the screen 48 times per second.

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, the limits of vacuum tube technology required that CRTs for TV be scanned at AC line frequency in order to prevent interference. (This was 60 Hz in the US, 50 Hz Europe.) In the late 1940s when the current analog standards were being set, CRTs could only scan 240 on-screen lines in 1/60th of a second. By using interlace, a pair of 240-line fields became a sharper 480 line frame. While this wasn't as sharp as scanning all 480 lines progressively, it was much more practical.

Modern monitors and television sets use active-matrix liquid crystal displays or other display technologies which do not have the afterglow characteristics of CRT displays. They do not have any flicker, and can display DVD material which is progressively scanned without flicker and smooth motion because of the motion-blur effect used by film.

Interlace and computers

In the 1970s, computers and home video game systems began using TV sets as display devices. At this point, a 480-line NTSC signal was well beyond the graphics abilities of low cost computers, so these systems used a simplified video signal in which caused each video field to scan directly on top of the previous one, rather than each line between two lines of the previous field. This marked the return of progressive scanning not seen since the 1920s. Since each field became a complete frame on its own, modern terminology would call this 240p on NTSC sets, and 288p on PAL. While consumer devices were permitted to create such signals, broadcast regulations prohibited TV stations from transmitting video like this. Computer monitor standard such as CGA were further simplifications to NTSC, which improved picture quality by omitting modulation of color, and allowing a more direct connection between the computer's graphics system and the CRT.

By the 1980s computers had outgrown these video systems and needed better displays. Solutions from various companies varied widely. Because PC monitor signals did not need to be broadcast, they could consume far more than the 6-8 MHz of bandwidth that NTSC and PAL signals were confined to. Apple Macintoshes built a custom 342p display into their case, and EGA for DOS PSs was 350p. The Commodore Amiga created a true properly interlaced NTSC signal (as well as RGB variations). This ability resulted in the Amiga dominating the video production field until the mid 1990s, but the interlaced display mode caused flicker problems for more traditional PC applications. 1987 saw the introduction of VGA, which Macs and PCs soon standardized on.

In the early 1990s, monitor and graphics card manufacturers introduced newer high resolution standards that once again included interlace. These monitors ran at very high refresh rates, intending that this would alleviate flicker problems. Such monitors proved very unpopular. While flicker was not obvious on them at first, eyestrain and lack of focus nevertheless became a serious problem. The industry quickly abandoned this practice, and for the rest of the decade all monitors included the assurance that their stated resolutions were "non-interlace". This experience is why the PC industry today remains against interlace in HDTV, and lobbied for the 720p standard.

Application

Interlacing is used by all the analogue TV broadcast systems in current use:
  • PAL: 50 fields per second, 625 lines, odd field drawn first
  • SECAM: 50 fields per second, 625 lines
  • NTSC: 59.94 fields per second, 525 lines, even field drawn first

Problems caused by interlacing

For the purpose of reducing the bandwidth necessary to transmit the video-based material, interlacing is inferior to the modern digital block-based compression techniques for the following reasons:

  • Interlacing performs poorly on moving images, leading to saw tooth or combing distortion
  • Fine horizontal detail is subject to twice as much flicker as the rest of the picture
  • Progressive MPEG is flexible and adaptive about which details of the image it compresses and how much, while the compression by interlacing does not discriminate about the perceptual complexity of the element of the image being compressed.
  • The quality of consumer-grade deinterlacers varies, while the MPEG decoder is absolutely deterministic in regard to the decompression of the progressively compressed stream.

The combination of interlacing with block-based compression technique inherits all the drawbacks of the interlacing, while also reducing the efficiency of block-based compression. Because interlacing samples every other line without prefiltering, it increases the amount of high-frequency components in the signal fed to the block transformation. This leads to lower efficiency of block transformation (i.e. DCT), or alternatively increases the amount of artifacts after decompression. This also decreases the effectivness of the motion compensation technique, used in the interframe compression formats like MPEG.

When vertical color compression (also called decimation or color subsampling) is included to the combined compression system, it is further effectively compressed by the interlacing. And vertical color subsampling is almost always included into digital and analog television systems (with the exception of broadcast NTSC, and of controversy broadcast PAL), all over the world. Thus with 4:2:0 color compression technique (i.e. half horizontal and half vertical resolution) the vertical colour resolution drops from 1:2 to 1:4, and overall color resolution from 1:4 to 1:8.

It is sometimes claimed that combining MPEG compression with interlacing reduces the amount of processing power required from the MPEG decoder almost in half. However, this argument does not stand when faced with the immense processing power needed for unobjectionable deinterlacing of the image after MPEG decompressor; and all modern displays but the (gradually disappearing) CRTs require progressive image as its input.

Another argument is that combining interlacing with MPEG drives up the overall sweet spot of the compression system. (Note though, that the sweet spot does not get close to doubling, due to the inefficiencies described above.) Specifically, it makes it possible to transmit 1920x1080 60 Hz video over the broadcasting bit pipe chosen for the ATSC system. However, essentially the same effect on the sweet spot without the drawbacks of interlacing could be achieved by simply prefiltering high frequencies out before applying a progressive MPEG compression; or, less efficiently, by filtering out high-frequency components from the compressed MPEG stream right before injecting it into the broadcasting pipe. On the other hand, most DVB flavours (T, S) offer a suitable bit pipe already today, and a better terrestrial broadcasting technology could have been selected for ATSC too.

Yet another argument is the concern about the 2 times higher technological complexity of the camera and production equipment in case of progressive video, due to twice the uncompressed bitrate at the moment of capture, and twice the computational power needed for processing and compression. This argument will become less relevant in the future with the progress of science and technology, and the TV producers of today or tomorrow may not be very sensitive to the capital costs. However, with interlacing still in use today, many works of art and television recordings of important events will continue utilizing it.

Despite arguments against it and the calls by many prominent technological companies, such as Microsoft, to leave interlacing to history, interlacing maintains a strong grip on the television standard setting bodies, still being included in new digital video transmission formats, such as DV, DVB (including its HD modifications), and ATSC for the purpose of compressing video-based material.

See also

External links

Film and video technology

Exploració entrellaçada | Zeilensprungverfahren | Exploración entrelazada | Entrelacement | 비월 주사 방식 | Przeplot | Lomitettu kuva | 隔行扫描

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Interlace".

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