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About HDTV Types & Differences



ABOUT HDTV TYPES & DIFFERENCES

By Evan Powell


The consumer electronics industry has done a remarkable job spreading confusion about video in general, and HDTV in particular. Time was when you could go to the store and just buy a TV with confidence. You could choose a big one or a little one depending on your room size and budget. But those days are gone. Now we've got DTV, SDTV, EDTV, HDTV, RPTV, DVI, DLP, LCD, LCOS, plasma, progressive scan, component video, S-video, composite video, etc. With all the alphabet soup, it seems only video engineers can buy a projector or TV and know what they are doing.

If you are confused now, you won't get much help from the sales rep on the floor of your local Buster's Electronics Megastore. The good news is that some of the personnel in these stores are better trained than they used to be. However, most of them lack the knowledge required to help you sort it all out.

So let's talk basics. First let's look at where we have been with the current television system, then we can see where we are going with the new technologies.

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. (By the way, this discussion describes the way television works in the US and other countries using the "NTSC" television standards. Things work a little differently in countries using PAL or SECAM.)

At any rate, 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 on a regular TV 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 even numbered lines from 2 to 480, top to bottom. That takes 1/60 second. On the second pass it paints all of the odd numbered lines from 1 to 479. 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.

You've probably heard that a TV has 525 lines. So what's this 480-line business, you may wonder? Well, when they broadcast video information, they need to give standard 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 timing 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 of video is 480 + 45 = 525. Therefore 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 Digital Television (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 that the 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 great 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 recorded by the video camera. That makes straight lines break up and look like they've got jagged edges. And on a 60" TV or a genuine large-screen image from a front projector, the visible scan lines and jaggies can be enough to drive you nuts. Many will remember the very poor picture quality from the earlier generation big-screen TVs—if you sat too close to them you'd go blind in a hurry.

The fact is that the 525-line interlaced system we have today was invented in 1953 when televisions were small. The picture was never designed to be blown up to large screen proportions. What works beautifully at 19" is a disaster at 60". And TV designers and marketers know that they couldn't continue to sell really bad video forever just on the WOW factor of the screen size. So they came up with ways to clean up the picture.

The New Solutions

The single largest step that was 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 (either the even lines or the odd lines) 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.

The alternative to the interlaced presentation of the odd and even numbered lines is a process called progressive scanning. Progressive scanning simply means that all of the lines in the frame of video are painted onto the screen in sequential order from 1, 2, 3, 4, . . . up to 480 in one pass. This has the potential to give us a cleaner and more stable picture. However, the video signals are still being broadcast in interlaced format, half the frame information at a time. So the frames need to be assembled into full 480-line frames before they can be painted progressively on the screen. This process is commonly referred to as deinterlacing or line doubling (more on this in a minute). 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. But it's basically the same thing.

Of course the marketers needed to come up with a snazzy name for this simple 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 major step 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. Most DVD players on the market output both interlaced and progressive signals, and they are getting better and cheaper by the month. (At the moment, the Costco down the street is selling a little progressive scan DVD player for $79.99). That means that every DVD movie and video on the market can be played in EDTV right now.

So here is your first absolute rule for buying a new video display, whether it is a projector or a TV: make sure that it is 480p compatible.

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

Well, no problem. You simply feed your new projector or TV the interlaced signal just as you always did with your regular TV or VCR. All of the newest video systems will accept interlaced signals as well as progressive. They can do this because they have a device on board called a deinterlacer or line doubler.

Here is what a line doubler (deinterlacer) does: It takes the two halves of the 480-line frame that is coming in at 30 frames per second from your cable TV or VCR or laserdisc player or DVD player and creates two complete 480-line frames out of them. Furthermore, each of these sets of 480-line frames can be painted on the screen in 1/60 second rather than 1/30 second.

But the good line doublers do more than that. Remember that if an object is in motion the video camera will capture the odd lines and the even lines at slightly different moments in time. Recombining these two fields into a single frame could look a bit fuzzy when motion is present, and straight lines can appear jaggy. Therefore when good line doublers recombine the lines, they look for motion offsets (the jaggies), and make adjustments to smooth them out (technically this is called motion adaptive deinterlacing). So their job is to convert an interlaced signal into a much cleaner progressive scan signal. Some of them do this better than others, but most of them produce a much better picture than they started with.

So, contrary to what you might assume, a line doubler does NOT double the number of lines in a frame from 480 to 960. Rather it doubles the number of times 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.

It used to be that the internal line doublers in projectors and TVs were not very good (in the cheap ones they still aren't). They could recombine the lines easily enough, but they weren't capable of smoothing out the motion artifacts. So many folks invested in separate external line doublers that were much more sophisticated. These days the internal line doublers that are onboard projectors and high end TVs are MUCH more comprehensive than they used to be. So the need for an external doubler has diminished. Today, whether you could benefit from investing in an external line doubler or not depends on the quality of the doubler already built into the projector.

Furthermore, some of the newer projectors have such comprehensive line doublers on board that they need no help at all from your DVD player in producing a great progressive scan signal. In this situation you may actually be better off using the interlaced output from your DVD player instead of its progressive scan output. This is contrary to what you would expect. But the fact is that if the deinterlacing system in the projector is better than the one in the DVD player, you will get a better picture by feeding your projector the interlaced signal. How can you tell which you should use? Just run it both ways and see if you can tell a difference, and go with the one you think is better. It is really not any more technical than that. But the point is that just because your DVD player is capable of outputting progressive scan does not mean you should always use it with every display system. Your projector or TV may already have a much better internal line doubler on board than does that cheap little progressive scan DVD player at Costco.

So let's ignore HDTV for a moment. The big leap forward that is accessible to everyone right now is EDTV, which is simply progressive scanning. You get progressive scanning with most new digital televisions, as well as all digital projectors, such as LCD projectors, or DLP projectors, or LCOS projectors…it does not matter, they all do progressive scanning internally, they all have deinterlacers onboard, and the vast majority of the new ones take both interlaced as well as progressive scan signals from your DVD player. The bottom line is that DVD movies never looked cleaner and sharper than they do on the new digital display products. If you never cared a lick about HDTV but you like to see DVD movies and music videos at their best, this is plenty reason by itself to get rid of that old TV you have sitting around and upgrade to a new digital system.

Well then, what about HDTV?

The broadcast industry is moving toward conversion of our system to High Definition Television (HDTV), a conversion which presumably will be complete sometime in this decade. This is good news. HDTV is better than EDTV, and much better than regular TV for two reasons. First, it increases the number of scan lines on the screen, so you get much more picture clarity and detail. Second, it gives you a wider picture that looks more like a movie theater screen rather than the old, almost square TV.

So how many more scanlines do you get? Well, there are two popular HDTV formats in use. One is called 1080i, and the other is 720p. But don't worry, there are no format wars to worry about. All digital projectors and digital HDTVs take both formats. The fact is that ABC, ESPN, and Fox broadcast in 720p, while CBS, NBC, HDNet and others use 1080i. Both formats produce great pictures that are a leap forward over regular television. And you will never know the difference. If you are watching Everybody Loves Raymond in high definition on Monday night, and you flip to ABC to see what is happening on Monday Night Football, your projector or HDTV will automatically switch from 1080i to 720p without you even knowing it. All you will be aware of is that both channels look a lot better than they used to with an amazingly clear and detailed widescreen picture.

For those who want a bit more technical explanation, we can say that both formats feature the widescreen 16:9 aspect ratio that defines HDTV. However, the 1080i format has 1080 lines for each frame of video, whereas the 720p format has 720 lines. A lot of people think 1080i is preferred because it has more scanlines, and thus is capable of more picture detail. However, since there is so much data in the 1080-line format, each frame must be separated into even and odd lines and broadcast a half frame at a time in interlaced format, just like the current TV broadcast system. This can introduce the same type of artifacts as we see today, but they are smaller and less noticeable since there are so many scanlines.

The alternative HDTV format is 720-lines progressive scan, or 720p. Though it has fewer lines, the native progressive scan format eliminates motion artifacts that originate in interlacing. So for subject matter that contains a lot of rapid motion—NFL Football for example--you can get a clearer, more stable picture from 720p than you can from 1080i. Alternatively, for subject matter that has very little motion, 1080i is capable of rendering more picture detail.

The truth is that 1080i and 720p are both excellent HDTV formats. One is not better than the other; they are just each better with particular types of subject matter. When done right, both are clearly superior to the 480-line format we have today. And best of all, you will never have to worry about it because your projector or HDTV will sort it all out for you anyway.

Up until recently there has not been much HDTV programming on the airwaves. HDNet has traditionally delivered lots of beautiful images to look at, but no serious programming content worth paying attention to (although they've gotten better lately--seeing the new and vastly improved Hogan's Heroes in HD is truly a nostalgic trip back in time for those of us who remember the original broadcasts as young kids). HDNet is to be commended for leading the way in demonstrating the full potential of HDTV. Discovery HD Theater produces magnificent content, but people generally want a wider selection of programming. HBO, Showtime, and others have done a good job as well. But the major networks had not begun to convert primetime programming to HDTV--until this season. All of the networks are offering their top prime time shows in HD this fall. Just a partial list of shows currently being broadcast in HDTV widescreen format includes:

The West Wing
According to Jim
Everybody Loves Raymond
Hope & Faith
Jag
8 Simple Rules
Wonderful World of Disney
CSI: Miami
CSI: New York
Law & Order
NYPD Blue
ER
Judging Amy
King of Queens

And most importantly for the large contingent of football fans, many NFL football games including ABC Monday Night Football and ESPN Sports HD Sunday Night Football. So, if HDTV content is what you've been waiting for, it is here in spades.

Therefore, you now have two reasons to get rid of the old television and step into the new world of high definition video. The first is broadcast HDTV, and the second is the excellent quality of DVD movies and other materials that can be viewed in EDTV. You get it all with the new digital televisions and digital projectors on the market today.

The big decision you now need to make is whether you want to get a larger box television, or some of the new, very thin flat screen TVs, or whether you want to step up to the full large screen experience with a digital projector. But that is a fun decision to make, and any way you go, your home movie and TV viewing experience will be vastly improved.

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