HDTV Display Specifications

[Gregg Lengling]

Part I

By Joe Kane


 
Imaging Requirements

In my last article, "HDTV By The Numbers," which appeared in Issue 68, January 2003, I spent a lot of time with the two basic high-definition systems, 720p and 1080i. I didn't mention the numbers of standard-definition (SD) as a reference for how much more information could be contained in HDTV. I also didn't mention that the pixels (picture elements) in HDTV are defined as being square. Both of these points are important to a continued discussion of the future of imaging.

Our SD system uses rectangular shaped pixels and their shape changes depending on the aspect ratio of the image. The number of active pixels in the American SD system is 720 by 486. The count remains the same if the picture is 1.33:1 or 1.78:1. There is an alternate pixel count specified for the 1.78:1 SD format that increases the horizontal resolution. I am not aware that it is being used in production and it's not part of the DVD system which is the major application for 1.78:1 (16:9) SD images.

The pixel shape changes from PAL (720 x 576) to NTSC. Making things slightly more complicated, the 720 x 486 format is not included in the ATSC table of DTV formats, neither is 720 x 480, which is used in DVD. The American Television Standards Committee (ATSC) calls out 704 x 480 and doesn't specify what should be thrown away in the production format in making the transition to the lower pixel count. Image centering is as much of an issue in SD-DTV as it is in DVD. In making the transition from one format to the other, I've seen every possible combination of loosing elements. Even more difficult to follow is pixel shape. Even though the count is different the shape is the same. Think about that if you've ever had to set up geometry and overscan on a TV. For the time being, HD is actually easier to deal with than SD.

The complications of pixel count in SD will be the subject of at least one article after the release of the DVD of Digital Video Essentials. The best I'll be able to do is tell you what we at Joe Kane Productions did in the production process then let you evaluate what each DVD player does with the video on playback.

If we pick the numbers commonly quoted for DVD, our National Television Standards Committee (NTSC) SD system has a pixel count of about 178,800 per 60th of a second. The 720p system should be delivering about 921,600 pixels and the 1080i system has a potential of about 1,036,800 for that same 60th of a second.

In early discussion about the coming of HDTV, it was written that the format had to be ten times better than the current system or consumers wouldn't notice. Certainly the potential of the 1080p format, at a pixel count of 2,073,600, fits the criteria of being better than SD by a factor of ten. The 1080p pixel count is exactly ten times the PAL field rate, something that was probably not lost on those trying to include the PAL community in HDTV. It is our position that the factor of ten is based on implementation of HDTV not reaching the goals of the system. We see clear justification for HDTV in our 1.78:1 direct-view TV set that can only display about 576,000 pixels, about three times the SD rate.

The set I am talking about is the Princeton AF3.0HD, which is no longer available. Finding a better picture than this $4,100 set could deliver will cost the consumer at least $15,000. The Princeton wasn't perfect by any means, but it set a standard that has yet to be duplicated by any other manufacture. The irony is that the Princeton, being the first good set of its kind, was an easy target for anyone wanting to do better. There are a number of inexpensive options that could have seriously improved the set.

It is my fear that sets promising the capability of HDTV are not enough above the capability of SDTV that consumers will be justified in staying away from it. The factor of ten in improved capability of HDTV isn't showing up at much more than one in many sets. In other words, the coming of HDTV has forced them to try to catch up to the capability of SDTV.

I base this on my view of displays at the Consumer Electronics Show (CES) in Las Vegas. It's interesting to observe how a lower price seems to be the driving force behind many of the equipment introductions. I was hard pressed to find many who were offering improved picture quality as the primary attraction to their product. High quality pictures were hard to find on the show floor.

With that in mind it's time to revisit display device specifications. What should consumers be looking for in their purchase of display devices in the world of DTV? If a set like the Princeton AF3.0HD can be attractive at two times the capability of SDTV there should be room for every TV set manufacturer to participate in the improvements offered by DTV.