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MillimeterJust what is 1080i? Part 5
Apr 10, 2006 2:51 PM, Steve Mullen
In the last newsletter, I began to explain why an inadequate de-interlacer results in HDTVs that cannot display more than 540 lines of 1080i video. CRT monitors, of course, do not need a de-interlacer, so one might expect they would have no problem fully displaying 1080i.
Unfortunately, CRT monitors have three points where resolution may be lost. First, a monitor’s analog amplifiers may not have an adequate bandwidth to carry a 1080i signal. Second, even when an HDMI connection is implemented, some manufacturers simply use A/D converters to convert digital data to analog signals. With such monitors, the A/D converters can both limit video bandwidth and add noise. (This is why HDMI is not always the best connection to use.)
There are also structural reasons why CRTs are not capable of displaying 1920x1080 video. First, shadow mask tubes typically can pass only about 1200 TVL/ph. And often, this resolution is only achieved at the center and not at the edges of a screen.
Rear- and front-projection HDTVs (and direct-view monitors designed to deliver a very bright picture) suffer from a different problem. In order to obtain a bright image, beam current must be high. As current is increased, beam spot size naturally increases — which causes both horizontal and resolution to be decreased.
The inability to build CRT displays that display more than about 600 vertical lines led some manufacturers simply to display each 540-line field on top of each other. This trick also allowed manufacturers to build less precise deflection circuits. Therefore, the 540-line limit encountered in the attempt to display 1080i is neither new nor confined to progressive displays.
Since CRT monitors are rapidly being phased-out, the key to properly displaying 1080i video is to confirm the monitor you use does not exclusively use bob de-interlacing. If a non-adaptive de-interlacer is used, it should use weaving, which will provide maximum vertical resolution, although the edges of moving objects may either display combing or be excessively blurred from the removal of combing. The better option is to confirm that adaptive de-interlacers are employed. These maximize resolution for both static and motion video.
Unfortunately, manufacturers typically give company-unique names to their de-interlacers, so you learn little by examining their marketing literature. These names include “DNIe” and “DRC-MF v2.” Some companies do provide the name of the de-interlacer chip’s manufacturer. In theory, you can now search out information on these chips. Regrettably, once again you are likely to run into a wall of marketing jargon. For example, Faroudja calls their technology “DCDi” that stands for Directional Correlation De-interlacing. Is this better or worse than ATI’s Vector Adaptive scheme?
In case you think you can get the right de-interlace technology by buying certain brands—you are partially correct. Only four brands (Hitachi, JVC, Pioneer, and Toshiba) had all of their tested HDTVs pass. (This refers to Gary Merson's tests on 54 current 2005 model "HD resolution" HDTVs of all types in Home Theater, March 2006. See Part 4).
Nor can you assume price is a reliable indicator. At least one manufacturer had both its cheapest and most expensive units pass, while the mid-price unit failed.
De-interlacing, of course, isn’t confined to display devices. Both 480i and 1080i DVD players that output 480p and 1080p also incorporate de-interlacers. For an excellent discussion of this topic, follow this link.
The lesson of this series is that when it comes to the capture, processing, recording/transmitting, and display of HD, only two things are reasonably certain. First, 720p HDV always has a resolution of 1280x720. Second, 1080i almost never has the 1920x1080 resolution that would be necessary to meet marketing claims of 2-million-pixel, high-definition video.
Perhaps the proper name for this series should have been “What are 1080i?”
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For those buying HDTVs, you’ll encounter another problematic technology: cadence detection. Cadence detection is used to determine whether incoming 1080i video is video- or film-based. For film-sourced video, pulldown must be correctly removed and the resulting odd and even lines combined into frames.
Alas, it is exceedingly difficult to reliably detect film cadence because movies shown on television often have their cadences modified to add or delete pulldown “fields” in order to alter running time to fit a time schedule. Likewise, MPEG-2 field flags are often incorrect. Worse, some 3:2 detectors function only with 480i video. Hopefully, movies released on Blu-ray and HD-DVD will have correct 3:2 pulldown and correct cadence metadata.
