Just what is 1080i? Part 4

Alert: 720p editors need to read Part 4, as do those shooting 1080i interlaced video that will be transferred to film. The moment legacy interlaced video is dropped into a 720p timeline, the editor will have to decide how it should be converted to progressive. Likewise, when editors complete a 1080i interlaced production, they face the issue of how they will convert interlaced to progressive — whether they do it or the film lab does it.

In the last Newsletter, I promised to explain why 48 percent of current HDTVs cannot display more than 540 lines of 1080i video. This startling fact was revealed by Gary Merson''s tests on 54 current 2005 model “HD resolution” HDTVs of all types (Home Theater, March 2006). These HDTVs were progressive displays with 720 rows, 768 rows, or 1080 rows.

Merson used a moving 1080i SMPTE 133 test pattern. If 1080 lines of vertical resolution were seen, the HDTV passed the test. If not, the HDTV set failed.

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If you have an earlier model HDTV, the odds are very high that it would fail the test, whereas if you have a 2006 model the probability is higher that would pass. (Obviously, 720p video need not be de-interlaced.)

You might assume that manufacturers who promote the virtues of high-resolution of HDTVs — especially those with 1920x1080 panels — would build sets whose circuitry supplied video with adequate vertical resolution to fill these panels. Unfortunately, it is common practice for manufactures to promote certain specifications — 1080i and 1080p come to mind — while building these products with components that do not support these specifications.

The inferior part inside the failing HDTVs is the de-interlacer — a critical circuit that does the very hard job of converting interlaced video to progressive video. There are multiple ways to accomplish de-interlacing, these methods fall into two categories: non-adaptive (bob, weave, and 2D FIR) and adaptive (frame-adaptive, region-adaptive, and vector-adaptive).

Why is de-interlacing using an interpolator called bob? To some, the constructed image seems to vibrate up and down, hence the term “bob.” An interpolator generates each target line from pixels in the lines directly above and below target lines. Diagonals are not handled well because an interpolator cannot create pixels in the lines that would yield smooth diagonal lines since there are no pixels above and below the locations (yellow cells) where they should be placed.

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