Video-recording systems are dependent, in their mechanical and electrical details, on the format of the television signals to be recorded. These signals vary in different parts of the world. For example, electrical-power standards vary from region to region, with two of the most common power frequencies being 50 Hz and 60 Hz. In order to match the frequency of the power supply, the television signals have been adapted to these standards. In countries with 60-Hz power, like the United States, Canada, and Japan, 30 video frames per second are transmitted, while in countries with 50-Hz power, like Australia, India, and some European countries, 25 frames per second are transmitted.
A T.V. picture consists of a series of dots. For a black and white picture the dots are black, gray, and white; in a color picture, they are usually red, green, and blue. These dots are usually very small and, if viewed from more than a meter or so away, invisible to the human eye. A series of these dots are synchronized in the form of horizontal lines and vertical lines, forming the image. The structure of the image lines also determine the bandwidth of the television signal, and, as with electrical power, the number of horizontal lines varies from country to country. Countries with 60-Hz power use 525 horizontal lines, while others use 625 lines. There is no world standard yet for these horizontal lines and the number of frames for transmission. This has led to the development of various incompatible video recording systems. There are three major types of video recording: NTSC (National Television System Committee); PAL (phase alternating line); and SECAM (from the French for "sequential color with memory"). The major differences in the three systems are:
Horizontal lines: NTSC = 525, PAL = 625, SECAM = 625. Fields per second: NTSC = 60, PAL = 50, SECAM = 50. Frames per second NTSC = 30, PAL = 25, SECAM = 25
Besides these major differences, there are variations in their subcarrier frequency, luminance, and chrominance bandwidths. (PAL and SECAM differ in these variables, not in their arrangemtns of lines, fields, and frames per second.) There are also variants of these video systems, the differences between these minor variants being mainly in FM bandwidth.
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