The Difference Between HDTV, EDTV, and SDTV

Evan Powell, October 2, 2003
Contents
The Current System

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 video, composite video, 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.

Contents: The Current System Interlacing Issues EDTV HDTV
 
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