Video Production Interlaced Scan

Interlaced Scan

Interlace is a technique of improving the picture quality of a video signal primarily on CRT devices without consuming extra bandwidth. Interlacing causes problems on certain display devices such as LCDs[1]. It was invented by RCA engineer Randall C. Ballard in 1932,[2][3] and first demonstrated in 1934, as cathode ray tube screens became brighter, increasing the level of flicker caused by progressive (sequential) scanning.[4] It was ubiquitous in television until the 1970s, when the needs of computer monitors resulted in the reintroduction of progressive scan. Interlace is still used for most standard definition TVs, and the 1080i HDTV broadcast standard, but not for LCD, micro mirror (DLP), or plasma displays; these displays do not use a raster scan to create an image, and so cannot benefit from interlacing: in practice, they have to be driven with a progressive scan signal. The deinterlacing circuitry to get progressive scan from a normal interlaced broadcast television signal can add to the cost of a television set using such displays. Currently, progressive displays dominate the HDTV market.

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.
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 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.