Analog Painting
Imagine you had to paint a billboard on the side of the road. To do this, a spray paint device moves left to right across the board spraying several different colors in a line one inch wide. You control valves that determine how much paint of each color is being sprayed on the current location. You can open each valve all the way, or shut off that one color entirely, or position the valve anywhere in the middle. When the spray head gets to the right end of the billboard, the paint shuts off, the spray head moves back to the left margin just under the line that was just painted, and the process starts again left to right painting the next line.
This is the way the TV screen generates a picture, except that the TV tube sprays electrons instead of paint. It is an analog system because the valve controlling the amount of paint moves smoothly from all the way off to all the way on and can generate any amount in between.
All that would be needed to produce a true TV-like system would be to connect the paint valves to electric regulators controlled remotely by a radio signal. Fifty years ago, an electronically controlled "valve" for electrons was available as a vacuum tube, and it was used to build the first TV sets. Today, computer chips make it possible to use other designs.
Digital Painting
Computers operate on numbers. To digitize the previous example, replace the analog valve with a system that delivers paint in an amount represented by a number. Each spot on the billboard has a set of numbers that represent the exact color for that location.
The entire board requires a massive set of numbers. Fortunately, there are large sections with the same color or pattern, so the data can be aggressively compressed. Using what is now relatively old MPEG 2 technology, the picture from 4 to 10 TV channels can be squeezed into the frequency now used by a single analog channel.
In the 1960s vacuum tubes were replaced by transistors, and in the '70s Sony replaced the round dots on the TV screen with rectangular spots. Otherwise, analog systems have not changed much. Digital technology, however, changes rapidly with increased speed of microprocessors. With today's faster CPUs, it is possible to get better quality or smaller files with MPEG 4.
Consumers may be willing to buy a new computer every three years, but they expect a TV or stereo to last for decades. TV stations are not willing to invest millions in new broadcast equipment every time someone comes up with a better compression algorithm. Therefore, public standards for consumer equipment change much less frequently than the format of computer media files.