The New York Times gets it wrong

Bloggers have great fun fisking articles from the regular media. Since my stuff is published in the regular media, I ought to be cautious, but this piece from The New York Times has a few mistakes in it.

And those wanting to see DVD movies in their full glory need a television capable of displaying, at a minimum, what are known as 480p images.

Wrong. Entirely wrong. To see a DVD movie (as opposed to a video-sourced TV show) in its full glory, all you need is a TV capable of displaying 480 lines (for NTSC). Whether they are shown progressively or interlaced makes no difference to resolution. However a progressive scan TV can reduce the combing that is inherent in the NTSC (but not PAL) TV system. This comes at a cost, though: a subtle jerkiness can appear in pans because half the film frames are shown twice, and the other half are shown three times, in the pattern 2:3:2:3:2:3 etc. This is inherent in translating 24 frames per second to a 60 hertz system. Some computer based systems (and, potentially in the future, DVD players) can do their progressive scan at 48 or 72 hertz which will truly eliminate this.

DVD movies have the same 480 lines of resolution as television, but on a DVD all 480 lines are refreshed in every frame. This is known as progressive (as opposed to interlaced) scanning — hence 480p.

This appears to be implying that the picture is stored in progressive format on DVDs. But generally it isn’t. While I don’t agree with everything on the DVD FAQ I do agree with this:

Progressive-source video (such as from film) is usually encoded on DVD as interlaced field pairs that can be reinterleaved by a progressive player to recreate the original progressive video

The confusion arises because filmed material (as opposed to video material) is indeed progressive scan, since each frame captures a single instant in time. Nevertheless it is stored on the DVD interlaced, and the fields have to be woven together again on way or the other. This can be in electronics by a progressive scan DVD player or the processing circuitry on a digital projector. But even an old-fashioned interlaced CRT TV still performs the weaving. It’s just that it’s done in the time domain by the rapid alternation of odd and even fields.

The first wave of rear-projection units used cathode-ray tubes. While in a direct-view television, electrons are beamed directly onto a glass tube, a rear-projection set is more like a self-contained movie theater, where images are beamed as visible light onto a screen (in this case translucent). The imprecision of that process explains why analog rear-projection units do not look so great.

The problems with CRT rear projection TVs have nothing to do with ‘imprecision’ of any process, at least in recent years. They have to do with the fairly low light output of the tubes contained therein. CRT front projectors also have low output, but no one expects to use one of these anywhere but in a darkened room. RPTVs, though, are expected to be used in normal TV viewing environments just like a regular TV. So various strategies have been employed to improve their brightness. One of these could be thought of as ‘excessive precision’. Most CRT RPTVs focus their light beams a bit too well and consequently leave a visible horizontal line structure which I, for one, detest. You can get rid of this by sitting further back, but then the effective size of the screen (in terms of the angle of your vision that it occupies) is reduced to not much bigger than a regular CRT TV. The reason for this is because the perceived level of brightness is not an average across an area, but judged from the peaks. Spreading the picture lines a little to eliminate the between-line gaps would, even though the average remains the same, make the picture seem duller.

The other problem with RPTVs is that in order to optimise brightness, the translucent screen is very directional, making the bulk of the light appear within a fairly narrow vertical angle. Stand up while you’re watching an RPTV and the picture gets dull.

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