The goal of “tele-vision” at the beginning, as its name suggests, was able to transmit moving pictures remotely, making “live” that the cinema does not, because you have to develop the film.
But how, in the early 20th Century?
The idea, if the schematic is quite simple: an image is formed on a surface, as we know to do for a long time, using a simple darkroom. Then, at the back of this dark room, instead of placing, as photography and cinema, packed with sensitive products (which will react film and darken more or less depending on the amount of light received, to record the trace of the image was projected on the surface), we will place a tiny “sensor” (a photocell), which will sweep from left to right and top to bottom, the whole image. The light received by the “sensor” is variable depending on the areas of the images, of course. Depending on the amount of received light, the photocell produces an electrical current more or less strong. This electric current is carried by an electric wire. At the other end of the line, imagine a small light bulb, which is the same movement as the original cell, but to a fluorescent screen. Depending on the amount of electrical current that the bulb is, turns on more or less. So, it reconstructs the image on the fluorescent screen, line by line (this explanation is very simplified, to understand the principle of operation).
In television (or video), so there is no material images like in the movies, but a sweeping motion and recording the light variation during the scan. This variation, reproduced with the same scan can recompose the image.
For television work, it is of course the sweeping motion of the shooting and that of reproduction are exactly synchronized, ie the lines are covered at the same time on both sides, otherwise it would give anything visually.
At a time when television was invented, it did not exist in quartz clocks and electronic timing systems. How, then, at the time, to synchronize everyone, the transmitter and receiver in people?
It is found that the electrical current is alternating (in France, it changes direction 50 times per second, is said to have a frequency of 50 hertz) and managed nationally, because it is produced by rotating dynamos ( as bicycle dynamos). The solution was therefore to “stall” the entire television system on AC power. That’s why television in Europe is 25 frames per second, not 24 frames per second as the film, because 25 is half of 50, which was easy to set up, leaning on the current frequency.
The United States and Japan, for various historical reasons and industrial, electric power has always had a frequency of 60 hertz. That’s why television in both countries is at 30 frames per second (NTSC) and not 25 frames per second as in Europe (PAL and SECAM).
For a number of years, due to the evolution of electronics, the timing of the frequency of the alternating current is no longer necessary or even used in the devices. There are more technical reason that there is so such different standards around the world, with all the compatibility problems that entails. The reason for the survival of these standards is historical: whenever it is changing the broadcast devices (TVs), recording (VCR) and capture (cameras), keep compatibility with equipment old. Consumers should not be forced to change their equipment at each change in technology. This happens regularly, but the market dictates that happens as infrequently as possible. This is called the “backward compatibility”.