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The Difference Between HDTV, EDTV, and SDTV

By Evan Powell


The consumer electronics industry has done a spectacular job spreading mass confusion about video. Time was when there was just TV. Now we've got SDTV, EDTV, HDTV, 480i, 480p, 525p, 720p, 1080i, progressive scan, component, composite, blah blah blah. Enough to make you feel like you need an engineering degree to buy a projector or TV.

And if you think you are confused now, just go talk to any sales rep on the floor of your local Buster's Big Screen Megastore and you will get some insight into what true confusion is all about. Many of those reps have little training and can't tell you anything meaningful about good video. Invariably however they are pushing the deal of the week and selling equipment that may be exactly wrong for your needs.

Let's talk basics. You want the best picture you can get for your money, right? OK. Getting there is actually easier than you think. Reading the rest of this article will give you most of what you need to sort out the good stuff from the junk.

Our Television/Video System. There are over 250 million televisions in the United States. Almost all of them work exactly the same way. A video signal pumps information into a TV at the rate of 30 frames per second. Each frame is a still picture. But they are displayed so rapidly that they give the appearance of continuous motion, just like an animated cartoon.

Each frame of video contains about 480 active lines of information (482.5 actually, but we will talk round numbers here to communicate the concept). Now a single frame of video is actually painted on the screen line-by-line in two passes. On the first pass, the beam paints all of the odd numbered lines from 1 to 479, top to bottom. That takes 1/60 second. On the second pass it paints all of the even numbered lines from 2 to 480. That also takes 1/60 second. So it takes a total of 1/30 second to display all 480 lines of the frame. This display technique is known as "interlacing."

When they broadcast video information, they need to give CRT-type TVs time to reset the electronic beam to the top of the screen so it can get ready to paint the next sequence of lines. So they build in an interframe gap that equals about 45 lines. There is no picture information in this 45 line gap—it is there just to allow the TV time to get ready to receive the next frame. So the total number of lines in each frame is 480 + 45 = 525. You've probably heard that a TV set has 525 lines. Not so. The signal has 525 lines, but only 480 of them contain active video information that ends up on your screen.

Sometimes you will see this standard analog TV format designated as 525i, which means 525-interlaced. In common usage, a lot of people also use the term "480i" to refer to analog interlaced 480-line active video. However, the industry has recently defined a digital interlaced 480-line format under the array of DTV formats which is known as Standard Definition Television, or SDTV, and 480i is the correct designation for this format.

The Problem with Interlacing: Screen Size

For most of the 50 years or so that plain ole "525i" television has been in existence, it has worked just fine. That's because TVs were small. On a 19" TV set the picture looks terrific because the scan lines and the errors introduced by interlacing are too small to see. But as TVs have gotten larger, the scan lines have become more visible.

Not only that, but the interlacing system creates weird "artifacts" when blown up to big-screen proportions. When there is motion in the picture, an object will have moved between the time the first half and the second half of the frame are painted. That makes straight lines look like they've got jagged edges. And on a 60" TV or a genuine really big-screen image from a front projector, the visible scan lines and jaggies are enough to drive you nuts.

If you want a good demo of really bad video, just go into any Buster's Big Screen in your area and look at the 60" TVs they have set up down there. Most of them look terrible—enough to make you want to give up TV altogether. Of course, if you stand back about 30 feet, they look great. But at the distance from which you intend to watch them, you see scan lines, jaggies, and overall picture disintegration that'll make you go blind in a hurry.

The fact is that the 525-line interlaced system we have today was never meant to be blown up to big screen proportions. What works beautifully at 19" is a disaster at 60". And TV designers and marketers know that they won't be able to sell really bad video forever just on the WOW factor of the screen size. So they've come up with ways to clean up the picture.

The New Solutions

The single largest step that can be taken toward better big-screen video is to eliminate the interlacing. Interlacing was originally invented to save transmission bandwidth, since with an interlaced signal you only need to send half the frame at a time. But now we have media such as DVD from which we can read and transmit picture information much faster than ever before. So there is no need to stay with an interlaced format.

If we can paint all of the scan lines sequentially from 1, 2, 3…up to 480 on one pass, we can eliminate the jaggies that come from interlacing. This is called "progressive scanning." Note that we don't have any more lines of information—it's still the same 480 lines. But we paint them in sequence from top to bottom. This 480 line progressive scan technique is commonly referred to as 480p. However, there is still the interframe gap, and there are still 525 total lines. So some people call it 525p instead of 480p.

Of course the marketers needed to come up with a snazzy name for this marvelous new concept. So they did—Enhanced Definition Television, or EDTV.

EDTV is a major advance

EDTV, or 480p, doesn't sound like much compared to HDTV. But it is in fact a great leap forward in picture quality. On a big screen it looks closer in quality to HDTV than it does regular television. And it is here today in its full glory. DVD players are now on the market that can output both interlaced and progressive signals, and they are getting better and cheaper by the month. So every DVD on the market can be played in EDTV right now! And digital broadcasting is being done now in 480p format as well.

Now in order to take advantage of 480p, you need two things: (1) a video source such as a DVD player that outputs that signal, and (2) a television or projector that can take that signal as an input. Warning: Most of them can't.

So here is your first absolute rule for buying a new video display, whether it is a projector or a TV: if you want maximum video quality, make sure to buy a TV or projector that is 480p compatible.

But, you ask, what about regular interlaced video sources like cable television, VCR, laserdisc, and so on—how do I play those signals on a progressive scan video system?

Well, you've got two options. First is simply to feed your new projector or TV the interlaced signal. All progressive scan display systems will take interlaced signals as well. They have a device on board called a deinterlacer or line doubler. If it is cheap, you will usually end up with the same jaggies that you are trying to avoid. But if the internal line doubler is good quality, you will get a much better picture from your interlaced source than you would on a cheaper projector or TV.

Second, if you like how a projector or TV looks with a progressive signal, but don't like its internal line doubler, you can get a good external doubler and use it to bypass the internal doubler. Silicon Image makes one called the DVDO iScan that works beautifully for the money (about $700).

Here is what a line doubler (deinterlacer) does, whether it's external or built-in: It takes a 480-line interlaced signal from your cable TV or VCR or laserdisc player or DVD player and recombines the odd and even lines into a 480-line progressive signal. When it recombines the lines, it looks for motion offsets (the jaggies), and makes adjustments to smooth them out. So its function is to convert a 525i signal into a much cleaner 480p signal.

Furthermore, since the projector/TV scans from top to bottom in 1/60 second, all 480 lines are displayed in that time. So what does the line doubler do with the second 1/60 second, since new frames are arriving at the speed of only 30 frames per second? Simple. It feeds the same 480-line frame to the display a second time.

So, please note that a line doubler does NOT double the number of lines. Rather it doubles the number of times the 480 lines are painted on the screen during the 1/30 second frame display time. This increases brightness and stability of the image. But most importantly, the elimination or reduction of the jaggies gives you a much cleaner picture. Whether you need an external doubler or not depends on the quality of the doubler already built into the projector.

So let's forget about HDTV for a moment. The big leap forward that is accessible to everyone right now is EDTV, which is simply progressive scanning. I had friends over recently and I put on the Eagles "Hell Freezes Over" DVD, using an NEC MT1055 projector on a Da-lite Cinema Vision 105" screen. They were stunned at the picture quality—"I've never seen ANY big screen ever look that good," said one. "So that's HDTV huh?" Nope. That's just regular DVD my friends, played on a progressive scan DVD player.

Well then, what about HDTV?

The broadcast industry is struggling toward conversion of our system to High Definition Television (HDTV), a conversion which presumably will be complete in another five years. HDTV does two things. First, it increases the number of scan lines on the screen. Second, it widens the aspect ratio of the screen from the standard 4:3 which is what most televisions are today, to 16:9. The wider screen format has a more theatrical look.

The most popular HDTV format is 1080i, or 1080 lines interlaced. As with 525i, the system paints the odd lines first, then the even lines in a second pass. But since there are so many more scan lines, both the lines themselves and the motion jaggies are much less visible.

Nevertheless, the world of videophiles who seek video perfection are looking forward to the day when even this 1080i signal will be presented progressively--1080p. Faroudja is marketing a video processor that will convert 1080i to 1080p, and there are a few very expensive projectors that will handle the scan rates required for this signal. But for those who don't want to spend $60,000 on a big-screen TV just yet, 1080i is what is coming.

An alternative HDTV format is 720-lines progressive scan, or 720p. There is a debate among videophiles as to whether 1080i or 720p is the better format. I can say without reservation that the most impressive HDTV pictures I've personally seen have been 720p. But 1080i requires less bandwidth and will probably be the more prevalent broadcast format as time goes forward. However, ABC is broadcasting its entire primetime lineup for the fall of 2001 in 720p, ensuring that this excellent format will be in use as well.

The funny thing is this. For the videophile who wants to spend a great deal of money to get the best possible picture, HDTV is the solution in theory. But the quality of HDTV signals will be dependent upon a host of variables. And economics will be the primary determinant. One can expect broadcasters to minimize production costs and save bandwidth whenever possible. There is one thing that is certain, and that is that broadcasters have absolutely no interest in boosting the cost of production in order to satisfy the desires of a small contingent of videophiles. They want to spend just enough money make picture quality acceptable to the mass market and no more.

That being the case, don't imagine that HDTV is going to be uniformly as glorious as the pristine demos you might see at trade shows or well-calibrated retail showrooms. Just as there are variations in picture quality today between channels, and between programs on given channels, so will the quality of production HDTV vary as well.

Meanwhile, the quality of the 480-line system continues to improve. Video decoders, deinterlacers, and scalers have all gotten much better at a rapid pace. New projectors today look amazingly good compared to similar products released just 18 months ago. And the fact is that for the mass consumer market that will be spending less than, say, $7,000 for a video system, there is not a great deal of difference between HDTV and today's new 480p. The quality difference is certainly nothing that the majority of the population would want to pay much extra for.

So the bottom line is this--don't worry about HDTV for now. It will continue to evolve over the next few years and it will be whatever it will be. Who cares? The unfortunate thing is that people are sitting on the sidelines waiting for HDTV to get serious, and they are missing out on EDTV, which is the biggest advance in video quality since color TV.

The real reason to step up to the latest video systems is not HDTV, but EDTV, or more simply 480p. If you are buying for home theater, get a projector or big-screen that handles 480p as well as 1080i and 720p to be compatible with all future needs. If a projector or TV takes 480p it will probably take 1080i and 720p as well. But that won't be its major benefit since there is not a huge amount of HDTV source material at hand. But today every DVD on the market can be played in EDTV.

So adjust your buying criteria just a bit. If you are at Buster's looking at big-screens, ask to see one demo'd not with their pre-canned HDTV loop, but with a 480p DVD player. Be aware that the sales rep might hook up a progressive scan DVD player to a progressive scan TV with a composite or S-video cable. If he does, he doesn't know what he is doing because composite and S-video feeds are interlaced signals. However, you can take advantage of this blunder by looking at the set with an interlaced feed to see how good the internal line doubler or deinterlacer is. If you see lots of jaggies in motion sequences, you will probably want an external doubler to go with that set.

In point of fact, when evaluating a big screen TV or projector you should look at the picture with both 480p and 525i feeds to evaluate the performance of the unit with both signals since most of the material available to you in the next few years will be in one of those two formats. Keep in mind that whatever the retail store's HDTV demo looks like is relevant only if the source they are using is (a) a source you can get in your home, and (b) a source that has material you intend to watch with some reasonable frequency.

Once you get a good 480p projector or TV, then get yourself a good progressive scan DVD player. And if you want, you can also get Silicon Image's DVDO iScan line doubler to clean up your 525i sources (cable TV, laserdisc, etc.) and turn them into good 480p.

Once you've got the fundamentals of 480p installed, then just sit back and enjoy your new vastly improved video system. Once you experience the quality of today's EDTV, you may find yourself thinking that you don't care quite as much about HDTV as you once did.