HDTV - What's All The (Signal to) Noise About? - The Sets
Look closely at your TV screen and you'll see that the image is made up of many little colored rectangles, spaced slightly apart. HDTV, in effect, squeezes more of them onto the screen closer together and changes the shape of the screen.
A (very) little Physics
Standard TVs themselves rely on clever physics. Shoot electrons (small, charged particles) from a 'gun' at the back of a screen to illuminate the little rectangles, called pixels. Old sets used phosphor, which would glow briefly after being hit. Now various methods are used.
So what good does that do?
Paper clips aren't the only thing magnets can push around. They can push electrons, too. Take a pair of magnets, one on each side, and vary the strength so that the electrons move in a curve. Otherwise they all would hit 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's standard TV. 4:3 is the 'aspect ratio'. No matter the TV's dimensions, 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's High Definition TV, HDTV.
So, what's 'digital' about that?
Standard TV broadcasts send the radio waves they consist of in analog fashion. They wiggle (modulate) the signal to provide changes that the set's electronics can process into images.
Digital signals instead 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 ('jaggies') and overall a better looking display. If you've ever seen an 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 - not strictly part of the HDTV standard - that improve the experience even more.
Most HDTV sets support 5.1 Dolby stereo, for sound of the type you'd 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's done in two passes, the gun skips every other line, then comes back and does the skipped lines. That's 'interlacing'. In 'progressive' 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: watch the different sets under good lighting conditions and use a good signal source, such as a quality DVD or digital broadcast. Those conditions aren't always easy to find, but it's worth the effort, considering the cost and number of years you'll use the TV.
