Interlacing Versus Progressive Scan
Analog television systems use interlaced video scanning with two sequential scans called fields (50 PAL or 60 NTSC fields per second), one with the odd numbered scan lines, the other with the even numbered scan lines to give a complete picture or frame (25 or 30 frames per second). This is done to save transmission bandwidth but a consequence is that in picture tube (CRT) displays, the full vertical resolution cannot be realized. For example, the maximum detail in the vertical direction would be for adjacent lines to be alternately black then white. This is not as great a problem in a progressive video display but an interlace display will have an unacceptable flicker at the slower frame rate. This is why interlace is unacceptable for fine detail such as computer word processing or spreadsheets. For television it means that if the picture is intended for interlace displays the picture must be vertically filtered to remove this objectionable flicker with a reduction of vertical resolution. According to the Kell factor the reduction is to about 85%, so a 576 line PAL interlace display only has about 480 lines vertical resolution, and a 486 line NTSC interlace display has a resolution of approximately 410 lines vertical. Similarly, 1080i digital interlaced video (the "i" in 1080i refers to "interlaced") would need to be filtered to about 910 lines for an interlaced display, although a fixed pixel display (such as LCD television) eliminates the inaccuracies of scanning, and thus can achieve Kell factors as high as 95% or 1020 lines. It should be noted that the Kell Factor equally applies to progressive scan. Using a Kell factor of 0.9, a 1080p HDTV video system using a CCD camera and an LCD or plasma display will only have 1728 × 972 lines of resolution.
Fixed pixel array displays such as LCDs, plasmas, DLPs, LCoS, etc. need a "video scaling" processor with frame memory, which, depending on the processing system, effectively converts an incoming interlaced video signal into a progressive video signal. A similar process occurs in a PC and its display with interlaced video (e.g., from a TV tuner card). The downside is that interlace motion artifacts are almost impossible to remove resulting in horizontal "toothed" edges on moving objects.
In analog connected picture displays such as CRT TV sets, the horizontal scanlines are not divided into pixels, but by the sampling theorem, the bandwidth of the luma and chroma signals implies a horizontal resolution. For television, the analog bandwidth for luminance in standard definition can vary from 3 MHz (approximately 330 lines edge-to-edge; VHS) to 4.2 MHz (440 lines; live analog) up to 7 MHz (660 lines; DVD). In high definition the bandwidth is 37 MHz (720p/1080i) or 74 MHz (1080p/60).
Read more about this topic: Display Resolution
Famous quotes containing the word progressive:
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—Woodrow Wilson (1856–1924)