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| HD Television | Standard resolutions | Comparison to SDTV | Format considerations | Technical details | Advantages of HDTV | Recording and prerecorded media | Early systems |
Standard display resolutions
| Analog television systems use interlace scanning with two sequential scans (50 or 60 fields per second), one with the odd numbered lines, the other with the even numbered lines to give a complete picture (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 a problem in a progressive display but an interlace display will have an unacceptable flicker or twitter 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 would need to be filtered to
about 910 lines for an interlaced display, although a fixed
pixel display (such as LCD) eliminates the inaccuracies of
scanning, and thus can achieve Kell factors as high as 95%
or 1020 lines. Fixed pixel array displays such as LCDs, plasmas, DLPs, LCoS, etc. need a "scaling" processor with frame memory, which, depending on the processing system, effectively converts an incoming interlaced picture into progressive. 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. Also in analog connected picture displays such as CRT TV
sets, the horizontal scanlines are not divided into pixels,
and therefore the horizontal resolution is related to the
bandwidth of the luminance and chroma signals. 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 tv) up
to 7 MHz (660 lines; DVD). In high definition the bandwidth
is 37 MHz (720p/1080i) or 74 MHz (1080p/60). |
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| Television | Standard resolutions | Comparison to SDTV | Format considerations | Technical details | Advantages of HDTV | Recording and prerecorded media | Early systems |