DTV tradeoffs

[Gregg Lengling]

One of my favorite sayings is: "There are no solutions, only tradeoffs." This is because a true solution to a problem would solve every aspect of the problem, leaving no loose ends. It would make everyone satisfied. It is very rare that such a solution is found. Usually, we have to settle for a partial solution, one that leaves some aspects of the problem unsatisfied. That's where we come up with the notion of a tradeoff. We choose optimizing one aspect of the solution at the expense of a non-optimal solution to other aspects. The goal of the technologist is to optimize as much of the solution as possible and maximize satisfaction.

I've written many times before about the interface between cable and consumer electronics products, both analog and digital. This is a prime example of where the best we can hope for are reasonable tradeoffs. There are no complete solutions! And, in fact, with digital television (DTV), it only gets worse.

In recent columns, I've discussed some of the problems with HDTV and DTV and the consumer. I've mentioned my reluctance to buy an HDTV display before the copy protection issues are resolved. My primary concern covered the disposal of a large HDTV receiver that might be crippled by the eventual copy protection settlement(s). If the "analog hole" is plugged and the analog connection between an HDTV receiver and its display becomes unavailable for significant programming, I would be faced with a big, heavy, expensive display that was no longer very useful. I couldn't even give it away because the recipient would be stuck with the same problem. A secondary concern is cost. It's a lot of money to risk.

So I was delighted to think up a tradeoff. I needed a projector for PowerPoint slides for some of the consulting I do. These devices have dramatically come down in price and increased in capability over the last few years. I bought a projector based on Texas Instruments' DLP (Digital Light Processing) technology. While not inexpensive ($2,250), it wasn't horribly expensive either. The technology is truly amazing. The device listed 38 different computer display modes from VGA at a resolution of 640 x 350 to UXGA at 1280 x 1050. Since the number of pixels on the chip is 1024 x 768, resolutions higher than this are not possible. Higher inputs are downconverted. In addition, the device accepts video inputs in NTSC, NTSC wide, PAL and its M and N flavors, SECAM and HDTV. The HDTV display includes 480 i/ p, 576p, 720p, and 1080i. Again, those formats requiring more than 1024 x 768 pixels are downconverted.

The beauty of this tradeoff is that even if the copy protection rules make my projector unusable for video programming, it still will do fine for its primary business purpose. Any videos I make will be usable because they won't have copy protection. And, in any case, my biggest concern (mentioned above) is covered. The unit is very small (9.4 x 7.4 x 2.9 inches) and weighs only 3.5 pounds. If I had to get rid of it, only a small volume would be taken up in the Dumpster.

From a positive perspective, the picture is very bright and large. The video from a DVD player is superb. Even NTSC is very nice. Lack of "burn-in" is a major benefit of the DLP technology. Burn-in is the phenomenon of creating a semi-permanent ghost image on the screen from stationary video. The primary culprit is incorrect aspect ratio.

Aspect ratio is the height of the picture divided by the width. The two primary television aspect ratios are 4:3 and 16:9. But some "letter box" movies are even more severe. Most DTV and HDTV displays are wide screen. If an ordinary NTSC picture is displayed most of the time, the center area of the screen will develop a different brightness.

This unpleasant effect is not present in the DLP chip.

But there are tradeoffs. Nothing is perfect. The video needs to be watched in a darkened room for best results. Even at that, the contrast ratio is less than I would like. Contrast ratio is the quotient of the brightest pixel divided by the darkest pixel. Ideally, it should be infinite (divide by zero light in a black spot). Practically, it is very difficult to make black spots. They are almost always gray. Some of the light that makes the spots that should be black turn out gray comes from the ambient, ordinary room light. Even if all of the room lights are out and the windows sealed (or it's after dark outside), light from the bright spots in the picture bounces back to the screen and makes the areas that should be black, gray. This is a fundamental limitation of front screen projection. And subjectively, high contrast ratio gives a picture "snap" and apparent "sharpness." A poor contrast ratio makes the picture seem washed out. These pictures are not washed out, but they could do with a bit more snap.

A problem common to all front projectors is that the space between the projector and the screen must be unobstructed. The design of the unit for its primary purpose has it projecting upwards. This means that for a large screen, the projector has to be on the floor, putting it in harm's way.

Given these few negatives, the net sum is a big positive. To quote the title of a current book, it is "perfect enough" for my purposes at the present time.

E-mail: Walter S. Ciciora