Tuesday, December 03, 2002

Hi Def isn't trying hard enough

George Lucas apparently shot most of Star Wars II on a 1920 x 1080 x 3 CCDs x 24 frames/sec. He's been arguing that Hi Def is just as good as film. If you look at 35mm DSLR vs 35mm SLR comparisons, it seems that Hi Def is almost as good as 35mm movie film. But I don't think that's the right thing to compare against.

The DSLR folks like to compare their digital pictures to 35mm film. The consensus: 6 megapixels is close to the resolution achievable with 35mm film, 11 megapixels is maybe a little better. Film has somewhat better dynamic range than the best CCDs.

The Sony F900 (that's George's $80,000 camcorder) has 3 CCDs, so it takes 3 color samples at each pixel location. So does the Foveon sensor in the Sigma SD9 digital SLR, except the Foveon sensor has 3 megapixels. DPreview says the Foveon 3 megapixel is about as high resolution as Canon's 6 megapizel D60. Apply the same scale factor to the F900 and you get a sensor equivalent to 4 megapixels -- about what you have in $700 prosumer digicams today..

35mm SLR film has images which are about 36mm x 24mm, with the short axis aligned to the width of the film strip. 35mm movie film has images which are about 24mm x 16mm, with the longer axis aligned to the width of the film strip. So 35mm movie film has about half the resolution of 35mm SLR film.

Put the two bits together, and the Sony F900 should capture about as much detail as 35mm movie film. But the best movies are shot and printed on 70mm film, with four times that much resolution. Digital sensors with that resolution exist -- the Canon 1Ds has 11 megapixels. The trick, of course, is to get the high-speed readout necessary to take 24 (or more) pictures every second. That requires a differently designed imaging chip.

I think the right spec for Hi Def should be 3840 x 2400 resolution, at 36 frames per second. This will look like 70mm film, and the higher frame rate will eliminate the irritating artifacts that make pans almost unwatchable in American movies. It will downsample to HDTV nicely, but the really interesting display is the PC monitor. 1920 x 1200 resolution 23" CRTs are available today for the price of a modest TV. 3840 x 2400 resolution 24" LCDs are available today for the price of a large HDTV. 36 frames/sec will look good on an LCD, and you can double it to match a CRT's refresh rate.

Curiously, the distribution format for such movies already exists. The pixel stream is about 40 times as fast as broadcast American TV, which MPEG-4 can currently compress into about 1 mb/s. Assuming no improvement in compression, 15 minutes of 3840x2400x36/s would fit on a DVD, and would be playable with an 8x DVD player.

But larger pictures generally compress better, even with no improvement in compression technology. If we suppose that for every factor of 4 pixels we increase the information by 3, then this big stream is about 20 times broadcast American TV, and a standard 4x DVD will supply 30 minutes of it.

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