Digital TV for Dummies
There are so many aspects of digital TV to understand. But it’s not that hard to figure out when someone takes the time to explain it.
Analog TV is what we have had for over 60 years. Analog means that a raw RF transmission carries the video and audio signals. The system was developed by the NTSC, National Television Standards Committee. NTSC was a board of over 300 experts in broadcasting, sound, video, manufacturers, scientists, movie studios…etc. They decided what TV was going to be made of and how it would get from the studio to your home. They finally decided on a standard that all TV stations and manufacturers would have to follow by law.
An analog TV station would use a 6MHz wide channel. It would contain 2 transmissions. One is the 2.5MHz video signal which was AM. The second is the 200KHz audio signal which was FM. It was a monochrome (Black & White) signal that provided each pixel with a different brightness to form the many hues of gray from black and white. This was done with a screen resolution of 480 interlaced lines. (We will get into that a bit later.) The audio was sent by FM for clarity and noise reduction. This FM signal was transmitted the same way as today’s analog FM radio station.
The screen size would be a ratio of 4:3. This is not inches or metric. It is a ratio of the width vs the height. If you took the width measurement and divided it into 4 equal units, the height would equal 3 of those units. Back when Thomas Edison was inventing motion picture film, one of his co-workers asked him, “what size do we make the frames?” Edison thought about it for a while and said, “1 inch by 3/4 inch.” The 4:3 format was then the standard for film. 1 inch is 4 quarters. 3/4 is 3 quarters. =4 by 3 or 4:3. This was the film standard for many years until CinemaScope. So the NTSC knew that TV would show lots of theatrical movies, and filmed shows would be done with movie equipment. So they decided to make the TV standard as 4:3.
Color was decided as 3 separate colors sent as red, green, and blue. Each pixel on the screen would now contain the 3 separate bars of color. If the color on the screen was red, then the green and blue bars were turned way down in brightness and only the red bar would stay at 50% brightness. A mixture of brightnesses of each color bar would determine the main color you saw. If only the red and blue bars were on 50% brightness, the color you saw was purple. These 3 colors alone show us all of the colors we see. 100% of all 3 colors gave us pure white. 0% of all 3 gave us black 50% of all 3 gave us gray. Varying the brightness of the 3 colors is what gives us all the colors of the rainbow.