How HDTV - High Definition TV Works

Look closely at your TV screen and you will see that the image is made up of many little colored rectangles, spaced slightly apart.

High definition TV, in effect, squeezes more of them onto the screen closer together and changes the shape of the screen.

Standard TVs rely on some clever physics. Shoot electrons, which are miniscule charged particles, from a 'gun' at the back of a picture tube to illuminate the little rectangles, called pixels. Old sets used phosphor, which would glow briefly after being hit. Now various other methods are used but the end result is similar.

Paper clips are not the only thing magnets can push around. They can push electrons, too. Take a pair of magnets, one on each side, and vary the magnetic field so that the electrons move in a curve to prevent them all hitting the screen in one spot.

Do that to illuminate 480 lines of pixels on a screen four units wide by 3 units high. Do that many times per second and change the intensity and color just right to create the illusion of real life motion. That is standard TV. with an aspect ration of 4:3. No matter what the actual dimensions of the TV, the ratio of width to height is 4 to 3.

Now, increase the number of lines to 1080 and change the aspect ratio to 16:9. That is High Definition TV or HDTV.

How HDTV - High Definition TV Works

Standard TV broadcasts send the radio waves they consist of in an analog fashion. They modulate the signal to provide changes that the television can process into images.

Digital signals use a changing pattern of 1's and 0's, just like computers. The result is an image with much higher resolution, less fuzzy, fewer sharp corners in parts of the picture and overall a better looking display. If you have ever seen a HDTV set with a good signal, and by now most people have, the difference really is striking.

The results are clearer pictures, sharper, more realistic colors and a look that's much closer to the 35mm film resolution on which most movies were originally shot.

There are a few commonly added features which are not strictly part of the HDTV standard but nevertheless improve the experience even more.

Most HDTV sets support 5.1 Dolby surround sound stereo, for sound of the type you would expect from a good stereo system. Home theater systems take advantage of that and add multiple speakers to provide movie theater style audio coverage.

Different manufacturers offer different frame rates and techniques for 'painting' the screen.

A 'frame rate' is the number of times per second the gun moves completely across the screen. Anywhere from 25 to 60 frames per second.

The 'painting' is done by moving the gun from top to bottom, in one or two passes. When it is done in two passes, the gun skips every other line, then comes back and does the skipped lines. That is called interlacing whereas in 'progressive scan' systems all the lines are illuminated in one pass.

For example, 1080i50 is equivalent to 1920 x 1080 pixels, interlaced, projecting 50 fields (25 frames) per second.

The visual results of the different frame rates and of using 'i' vs 'p' methods is often debated. In general a higher frame rate results in a smoother looking image. Interlacing helps to reduce flicker under certain conditions.

The bottom line for the consumer is to watch the different sets under good lighting conditions and use a good signal source, such as a quality DVD or digital TV broadcast. Those conditions are not always easy to find, but it is worth the effort, considering the cost and number of years you will use the High Definition TV.