Saturday, June 04, 2005

Aspect Ratios & Film Transfers

16:9 image
4:3 image
HDTV uses a "widescreen" 16:9 aspect ratio: a screen 9 units tall has 16 units of width. Traditional TV programming is 4:3 — 4 units tall by 3 wide.

My two 16:9 hi-tech TV displays consequently can have problems displaying old-style 4:3 material without stretching the image horizontally to fill the screen's full width, making everybody look short and fat.

I say I "can have problems" because in many situations there is a quick remedy. I can use the Aspect button on the remote to select the "Normal" mode, rather than "Wide." Black "pillarbox" bars magically appear at either side of a properly proportioned 4:3 image.

Only problem is, that doesn't work quite the way I want it to when I'm using the Samsung's DVI (Digital Video Interface) input from my Motorola cable box. In the DVI input's "Normal" mode, the TV not only squeezes the image horizontally, it squeezes it vertically as well. I end up with a very small, properly proportioned, 4:3 picture in the center of the screen.

What's going on is that — for DVI only, not for component-video input — the Samsung in "Normal" mode uses a 640-pixel by 480-pixel area of the whole 1280x720 screen, so as to do an "ultra-correct" one-to-one, pixel-by-pixel mapping of input pixels to output pixels.

That may be "ultra-correct," but it's not smart. I'd prefer the entire height of the screen to be used, for 4:3 material. What to do?

One option is to switch to the Samsung's component-video input, using "Normal" mode, to view 4:3 images. But that's a pain.

A better solution is to twiddle the user settings of the Motorola DCT6412 cable box. If I set TV Type to 16:9 (the default setting) and DVI/YPbPr Output to 720p or 1080i rather than 480p or 480i, I can choose Off, 480i, 480p, or Stretch for 4:3 Override. It turns out that Off does exactly what I want: "pillarboxes" 4:3 material between black bars at either side.

That works only if the cable box can tell the material is 4:3, though. Today I tuned in the TNT HD feed (on cable channel 249) while it was playing the Sylvester Stallone action thriller Assassins. The screen's width was filled by fat-faced characters, as before. I tuned to the regular TNT channel (30) and found a properly proportioned 4:3 image of the same movie there — meaning that the 4:3 "pan and scan" version of the film was being digitally stretched by TNT for the HD feed. My gear couldn't do much about that (unless I tediously set it up to watch channel 249 over the component-video input, using "Normal" mode).

A bit later, Assassins ended and TNT began its telecast of Clint Eastwood's Blood Work. That one looked just fine, over DVI, via channel 249 ... and also just fine via channel 30! For this movie, TNT was showing a widescreen HD version on 249 and chopping its sides off to make a 4:3 image on 30. Over "Wide" mode DVI input on my Samsung, the latter showed up at a correct 4:3, while the former came in at the intended 16:9.

The moral: TNT ought to use the second method exclusively, never the first. It ought to chop a 16:9 image's sides for standard-def 4:3 feeds, not stretch 4:3 to make 16:9.

But that assumes TNT has access to 16:9 "transfers" of all its widescreen movies.

Every TV movie has to undergo a film-to-video transfer. The original celluloid is scanned, frame by frame, on a machine that is generally called a telecine. In modern telecines, an optical sensor follows a sharply focused beam of light and turns minute spots on the film into pixels, one pixel at a time. The output pixels are recorded on, say, a computer hard drive. From there, they can be manipulated in various ways and sent out to storage media such as magnetic tape.

The telecine operator, sometimes called a colorist, often has choices to make. One choice may be what digital video format to use for the output pixels. Commonly used today are "2K" and "4K" formats. 2K scans a typical 35mm film frame at 2048 pixels across by 1556 vertically. 4K does 4096 x 3112 scans.

See this web page for more on 2K and 4K. One poster there says,

It's generally taken that 4K is the resolution needed to fully capture all the detail down to the level of individual grains, on 35mm film. 2K has been used more often though, as it's half the resolution for 1/4 the data and storage space.

Notice that both 2K and 4K give better pixel resolution than (1920 x) 1080i, much less (1280 x) 720p. A 2K or 4K scan of a film has to be downconverted for HDTV ... and possibly again for 480i SDTV.

2K and 4K telecines are relatively recent technology. Earlier telecines produced lower-resolution — often analog — video scans: HDTV-quality digital, only fairly recently; EDTV-quality digital, less recently; or even plain old 480i analog, in the good old days. Those older transfers or video "masters" abound in the vaults today. They were used not only for telecasts, but also for mastering DVDs, VHS tapes, and laserdiscs.

Another decision the colorist makes is which aspect ratio to use: 4:3 or 16:9.

For 4:3 transfers, when the source film is widescreen, there has to be an accomodation:

"Pan and scan" 4:3 transfers zoom in on the part of the scene where the action is, then hop to a different part of the image for the next scene. Sometimes there are artificial pans from one talking head to another in the same scene, ones the director of the movie wouldn't approve. Yuck!

"Letterbox" 4:3 transfers put black bars above and below the entire widescreen image, with no cropping or zooming in. Some people say yuck! to that, as much of the screen isn't used.

"Anamorphic" 4:3 transfers compress the widescreen image horizontally, with the understanding that playback gear can stretch it back out for display on a 16:9 screen. In some cases, there may still be minimal black bars at top and bottom.

Most aficionados think anamorphic is the way to go. It puts the greatest possible amount of the original image into the greatest possible portion of the 16:9 TV screen. For a 4:3 screen, the playback hardware simply simulates letterboxing.

Then there are the true 16:9 transfers. If the source movie also happens to use the 16:9 (aka 1.78:1) aspect ratio, then there's no problem. But few movies do. Most widescreen movies use either 1.85:1 or 2:35:1. Then there are basically two options:

"Matted" or "cropped" 16:9 transfers just chop off enough of the two sides of the widescreen (say, 2:35:1) film to make the picture fill an entire 16:9 aspect ratio frame from top to bottom. This is roughly equivalent to "pan and scan" for 4:3 transfers. It destroys the original framing of the scene, a.k.a. the OAR, which stands for "original aspect ratio."

(Actually, I made the names "matted" and "cropped" up. For all I know, the industry calls this sort of transfer "full screen," or something like that. It might also be well described as "zoomed," since the Zoom mode of an HDTV produces a similar effect.)

"OAR" 16:9 transfers, like those for 4:3, put black bars above and below the image, allowing the entire image to be seen, exactly as framed by the filmmaker, in OAR. Naturally, if the movie was originally filmed with a 16:9 or 1.78:1 frame, there are no black bars in an OAR transfer.

(There is no such thing, by the way, as an anamorphically squeezed 16:9 transfer. Anamorphic squeezing is done only to shoehorn 16:9 material into a nominally 4:3 frame.)

TNT's copy of the originally 1.85:1 Assassins is, clearly, 4:3 pan and scan. There are boatloads of pan and scan and letterbox 4:3 transfers in the vaults, from the days before DVD players and other digital devices could unsqueeze anamorphic transfers.

TNT's copy of the originally 2:35:1 Blood Work (since it fills a 16:9 screen side-to-side and top-to-bottom) looks like a matte/crop/zoom transfer. Given the choice, I'll take it over stretching a 4:3 pan and scan transfer to fill a 16:9 frame. But, best of all in my book, give me a true OAR 16:9 presentation, even if it leaves the top and bottom of the screen black.

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