HDTV in Plain English
September 23, 2004,
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.