It is all an issue of Image Resolution

There are currently a number of different HDTV formats as adopted by the Advanced Television Systems Committee (ATSC), based on 720p, 1080i, and 1080p - using refresh rates that vary between 24 Hz and 60 Hz.

The main difference between these HDTV formats is one of image resolution.

The 720p is on the lower-end of the scale with an image resolution of 1280 pixels by 720 lines. The other two formats both support 1920 pixels by 1080 lines. In other words, both support the same image resolution but there is a significant difference in the way the 1080i (interlaced) and the 1080p (progressive) formats build up the image.

Surely, image resolution in fixed-pixel displays is a very HOT topic with many HDTV buyers. For many, the obvious choice is to opt for the latest 1080p HDTV - also referred to as ultra-HD or full-HD by some manufacturers ...but there is a price to pay to get the latest in HDTV technology.

On the other hand, the way the 1080i and the 720p formats build up the image may render the lower resolution 720p format more suitable to display certain image content.

In other words, do not simply jump to conclusions as to which HD format is best. As we will see in this article, each of these different HDTV formats has got its strengths and weaknesses.

1080i: Up to a few years ago, this was considered as the reference standard in HDTV. Nearly all first-generation HDTVs were rear-projection sets that supported this standard.

This format boasts a picture resolution of 1920 pixels by 1080 horizontal lines that are painted on the screen in two interlaced halves (hence the 'i' in the '1080i' format) - by first painting all 540 even-numbered lines on the screen (also referred to as the even-field), and then proceed with the painting of the odd-numbered lines (odd-field). These two fields together form a single frame of 1080 lines.

In the process, the screen is painted 60 times per second (50 times in a PAL signal) - each time painting only half of the lines per frame, with the entire screen being painted in two passes 30 times every second.

Because of the way the interlaced process paints the screen, all picture information contained in adjacent odd and even lines in an interlaced image is 1/60th of a second out-of-synch with the next or previous line. This difference between the two halves of the picture would show up in what are referred to as 'interlaced artifact'. This in itself will impact on the type of image content that is best displayed on an interlaced display.

720p and 1080p HDTV: In contrast, in all progressive-scan formats - 480p (EDTV), and 720p and the 1080p HDTV displays - all scanning lines in a single frame are displayed sequentially in a single pass. This means that a progressive scan image is complete in itself. This also explains why progressive images look better when objects in the pictures are in motion.

The Bottom Line

The 1080i format is more widely supported by manufacturers and broadcasters; broadcasters however would normally broadcast only in either 720p or 1080i but not both. This should not be much of an issue in that any HDTV set you buy should be able to display pictures in any HDTV format by up-converting or down-converting to its native format, i.e. the one in which it's designed to produce a picture.

Theoretically, from a pixel-count perspective, 1080i supports better spatial resolution than a 720p HDTV. In theory, 1080i supports a pixel count of over 2 million pixels as against the 0.92 million pixels supported by 720p HDTV, but due to limitations associated with interlacing, the actual difference in effective resolution between 720p and 1080i is almost negligible. What's more, a 720p display is capable of a better flicker-free picture when it comes to fast moving action movie scenes and sports.

The situation with 1080p HDTV is totally different than that of 1080i. 1080p supports the full 1920 x 1080 pixels (2.07 million pixels) while at the same time, enjoys the added benefit that all 1080 lines are displayed in a single pass.

In other words, 1080p HDTV combines the superior resolution of the 1080 format with the smooth stable image of a progressive scan format.

N.B: It is worth mentioning here that interlaced formats aren't really an option in fixed-pixel displays (DLP and LCD rear-projection as well as plasma and LCD flat-panel). This also explains why display manufacturers are shifting away from the 1080i and instead moving directly from 720p to 1080p HDTV.

And What about 1080p HDTV Sets?

Surely, 1080p HDTV represents the latest developments in HD Television technology. A few of these sets have already started to hit the market.

Among the latest HDTV models, one can find the much awaited Samsung's 2005 line-up of DLP 1080p HDTVs.

Not surprisingly, these sets do not come cheap. There is a premium one has to pay to get the latest and best in the HDTV world. As a minimum, expect to pay at least $1,000 to $2,000 more to jump from a 720p to a 1080p HDTV display. But when you are spending some $3,000 plus for a HDTV set, it is only human that you will be ready to fork out another one or two thousand dollars to get the best of breed.

Should you opt for a 1080p HDTV Set, and are these sets worth the extra expense?

Well, there is no straight answer here. It all depends on what are your requirements, yet there are a few facts you should know before making any decision:

• Surely, these sets are capable of producing spectacular results with ultra sharp images. But whether you will be able to see the difference in image quality between a 720p display and a 1080i signal displayed on a 1080p HDTV display depends on your TV viewing distance and screen size, more than on the screen native resolution. (It is not the scope of this article to discuss the TV viewing distance but more info in this respect is available at our site at http://www.practical-home-theater-guide.com/Tv-viewing-distance.html)

• In reality, it would be very difficult to detect any difference in image detail between 720p and 1080i/p HDTV material on the smaller sets from 10-feet away. Sit closer and feed your 1080p HDTV set with a good quality HD source, and you will start to see the difference.

• Further more, with most of today's HD broadcasts, you'll be hard pressed to see a difference in picture quality when you compare the image on current 720p sets versus the latest 1080p HDTV models. Surely, if you are thinking of going really big, then the extra image resolution would make the difference. But keep in mind that at present, true 1080p HDTV material is almost non-existent. What's more, none of the major networks has announced 1080p broadcasts. If and when such high-def formats as 'Blu-ray' or 'HD-DVD' take off, the equation may change. But it will probably be at least a few years from now before this stuff become truly within reach of many household budgets.

Finally, most 1080p HDTV sets presently available on the market do not offer a 1080p connection. Though the HDMI standard does support 1080p HDTV, yet the majority of today's 1080p HDTV sets do not offer a 1080p connection on their HDMI port. To a certain extent, this is explained by the lack of true 1080p HDTV gear with which these sets can interconnect.

(c) 2004/2005 www.practical-home-theater-guide.com. All rights reserved.

Andrew Ghigo A Telecoms/Electronics engineer by profession, with specialization in digital switching and telecoms fraud management systems.

Editor and publisher of http://www.practical-home-theater-guide.com - a site dedicated to all home theater enthusiasts with the scope of serving as a comprehensive home theater guide to home theater systems, product reviews and home theater design.

This article is an excerpt from a series of guides appearing under the Rear Projection TV Reviews section of the site.