Digital TV for Dummies
The screen size has also been enhanced. The standard screen size for digital TV is a ratio of 16:9. As before, if you took the width of an HDTV set and divided it into 16 equal units, the height of the HDTV screen would be equal to 9 of those units. It is almost twice as wide as it is high. Many widescreen movies use several different widescreen ratios. The widest was the movie “Ben Hur”. The ratio was 2.45:1. Meaning the width was 2 and a half times the height. But the lowest ratio widescreen movie was 1.85:1, or 16:9. So the ATSC decided the HDTV screen would be 16:9. Wider movies would have black bars on the top and bottom to allow the full width of the original movie to be viewed.
Pixels and lines.
Digital TV uses the same red, green and blue color system that analog used. so the pixels are the same design. (Kind of). If you get real close to your TV screen, you will see a bunch of tiny dots that fill the screen. They are called pixels. Pixel is short for Picture Elements. The size and amount of those pixels determine the resolution of the TV. Resolution means the amount of detail the screen can display. Here is how pixels make the difference between the old and new TV systems.
Analog TV used 480 lines of resolution. That means there was 480 lines of pixels from top to bottom of the screen. The picture is made in the TV circuitry by painting one line at a time until all lines are painted to make a single picture. The analog TV does this 30 times every second, but does it as an interlaced framed picture. Before we go any further, let’s explain how the picture is made.
Just like with movies on film, there are several frames scanned each second to form moving pictures. When looking at TV resolutions, you may have noticed the letters (i) or (p) after the line numbers such as, 480i, 480p, 720p, 1080i and so on. The (i) stands for Interlaced. With interlaced scanning, the TV scans 480 lines from top to bottom, but it does this in 2 fields. The TV paints line number 1, then line number 3, then 5…etc until it gets to the bottom.
Only the odd numbered lines were painted on the screen to complete field 1. The TV then goes back to the top and scans lines 2, 4, 6…etc to the bottom. Only the even numbered lines were painted to make field number 2. Then the TV interlaces both fields to make 1 complete single frame (Picture). The TV does this 30 times every second, which is fast enough for the human eye to see a seamless clear picture. This system has worked for more than 60 years.
Continue to next page =>