Have you heard this one yet from a projector salesman.... "You don't want to buy THAT projector...it doesn't do TRUE HDTV." Well, certainly nobody would want to buy a projector that didn't do real HDTV, right? But they all claim to do HDTV. So what's the scoop?

It is easy to understand why the confusion exists. But it is also easy to sort it all out. First, let's start by defining HDTV. There are two common HDTV formats in use today, usually referred to as 1080i and 720p. The numbers refer to the number of horizontal lines in each frame of video (also known as "vertical resolution" since it is the number of horizontal lines as counted vertically from top to bottom of the screen). So in a 1080i signal, there are 1,080 lines per frame of video, and in a 720p signal there are 720 lines per frame.

The "i" and "p" indicate whether the signal is interlaced or progressive. In an interlaced signal, all of the even numbered lines are transmitted in one batch, followed by all of the odd numbered lines. (This is done to reduce transmission bandwidth.) In a progressive signal, all lines of the frame are transmitted at once in sequence. So with the interlaced 1080i signal, only 540 lines are recorded by the camera and transmitted at a time; they are then reassembled at the time of display. Meanwhile, with 720p, all 720 lines are recorded and transmitted in sequence.

Both of these signal formats maintain a 16:9 aspect ratio. That means the picture is 16 units in width for every 9 units in height. This is what has become known as the standard widescreen television format-all widescreen format TVs, plasmas, and projectors have a native 16:9 aspect ratio these days.

In order for an HDTV signal to maintain a 16:9 aspect ratio that matches the widescreen format, it needs to have 16 pixels on each line for every 9 lines of video in the frame. So a 1080i signal has 1920 pixels horizontally. That is why you will sometimes see the actual resolution of the 1080i format designated as 1920x1080. (If you divide 1920 by 16, then multiply the result by 9, you get 1080.)

Similarly, a 720p format signal has 1280 pixels on each line. So the physical resolution of the 720p format is often noted as 1280x720. (Once again, if you divide 1280 by 16, then multiply the result by 9, you get 720.)

So far, so good. Now....what is TRUE HDTV? This is where it gets confusing, because people use the term to mean different things. Some people think that the only real, legitimate HDTV format is 1080i because it has the highest physical resolution. So they refer to 1920x1080 as true HDTV. Others have been calling 1080i "full HDTV," presumably to distinguish it from the less full 1280x720.

Fans of the 720p format object to this. They point out that progressive scanning produces a cleaner, higher resolution signal when the subject is in fast motion. It has no deinterlacing fuzziness. And since the 1080i camera captures only 540 lines at a time, the actual resolution of 1080i when the subject is in motion is only 540 lines, not 1080. So many folks think 720p is better for rapid motion sports like football and soccer, while 1080i is better for, say, golf, where people are just basically standing around.

The fact is that both 1080i and 720p are great HDTV formats that look a lot better than standard television. Both formats are being broadcast by the major networks today, so your projector needs to be able to display both of them, and all projectors that are HDTV compatible do in fact display both of them.

So what does it mean to ask "does your projector display true HDTV?" Often what is really meant is, "does it need to re-scale the image?" In other words, does the video information coming in on the HDTV signal need to be either compressed or expanded to fit the physical resolution of the projector? In most cases, it does.

Any given projector has just one physical resolution, usually called the native resolution. Native resolution is the number of pixels actually available on the display. So an SVGA projector, for example, has display panels or chips with a native 800x600 pixel matrix. In order to display a 16:9 signal, it uses an active area of 800x450 on the display. So any HDTV signal that it gets, whether it is 1280x720 or 1920x1080, it must reformat (compress) that incoming signal into 800x450 before feeding it to its internal display. So no matter what, it cannot display any HDTV signal without compressing it, and losing a bit of image detail in the process.

This is true of standard XGA resolution projectors as well. They have a native resolution of 1024x768. In order to display a 16:9 image, they use an active portion of their display that is 1024x576, which is a 16:9 matrix. Therefore the HDTV signals, whether 1920x1080 or 1280x720, must be compressed to fit into a 1024x576 matrix before they are displayed.

Many new home theater projectors have native 1280x720 LCD panels or DLP chips. These are built expressly for the purpose of displaying HDTV 720p without needing to compress it or expand it. Some would say that projectors with the 1280x720 matrix are true HDTV projectors. However, some wouldn't, because when they get a 1080i signal these projectors still need to compress the 1920x1080 information into their native 1280x720 displays.

For the purist with unlimited funds, the only real, genuine HDTV projector is one with 1920x1080 internal resolution. These will display 1080i without any compression. There are a small handful of projectors on the market with this resolution, and at the moment they cost $20,000 and up. But these units need to reformat 720p signals, scaling them up to fit their native 1920x1080 displays. Technically, then, you could say that even these units are not true HDTV when it comes to 720p format.

The bottom line is that all projectors are built to scale a wide variety of incoming signal formats into their one native display. They will all do standard television, they will all do DVD, and almost all of them will do HDTV 1080i and 720p as well. In addition, most of them will display a variety of computer resolutions, including SVGA, XGA, and so forth. Really, when it comes to HDTV, there are only two circumstances where scaling is not required: 720p for a projector with 1280x720 native resolution, and 1080i for a projector with 1920x1080 resolution. Other than for those two unique matches, scaling is always required no matter what.

So this whole issue about "true HDTV" misses the point. Even the cheapest low resolution projectors will display HDTV pictures that look better than any television you ever saw. The fact that you are seeing a compressed signal is quite beside the point. Scalers have gotten so good these days that even low resolution projectors deliver amazing HDTV quality for the money, even after the compression. So who cares if it isn't "true HDTV?"

The real question is how much are you willing to spend on a projector? Generally, the projectors with higher native resolutions tend to cost more than those of lower resolution. With higher resolution you get reduced pixelation, and usually a smoother, cleaner, more filmlike image. And you usually get these improvements, to varying degrees, on all video sources whether they be television, DVD or HDTV. Getting better image quality across the board is usually a more important key to your overall viewing satisfaction than the question of whether the HDTV image is scaled or not.

Yes, it is true that today's 1280x720 format projectors are indeed particularly impressive for 720p display. But the amount of 720p material you will view compared to everything else will probably be rather small unless your weekly video entertainment consists mainly of HD sports broadcasts from ABC, ESPN, and Fox. And meanwhile, 1080i can look spectacular on a 1280x720 projector, even though the 1080i signal is compressed and not "true" 1080i.

Therefore, next time a salesman says, "Don't buy that projector, it doesn't do true HDTV," think twice and don't take his word for it. That relatively inexpensive projector you are considering just might deliver the best possible HDTV picture for the money on the market.