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MPEG-2 is a widely used format for digital video signals which have been compressed. It is used for digital television broadcasts, DVDs and computer video files. This format includes support for the interlaced picture system used on many TV sets.
The name MPEG stands for Moving Pictures Experts Group. This is a panel of experts formed by the International Standards Organization, which oversees standardization across a wide range of products and services. MPEG has developed several video standards which bear its name.
The MPEG series of video standards uses compression techniques to minimize file sizes. The techniques used are very technical and complicated, but an extremely simplified example of the principles involved would be to imagine a section of the picture showing a blue sky. The signal could say "Pixel 1 is light blue, Pixel 2 is light blue, Pixel 3 is light blue…" and so on. In compressed form, the signal could say "The first 10 pixels are light blue," which carries the same information in less space.
MPEG-2, developed in 1995, has several key changes from the original MPEG-1, developed in 1993. It allows the associated audio track to carry information for 5.1 surround sound systems rather than merely carry a stereo signal. It also allows for signals at a higher bitrate, meaning more detail but with a larger filesize.
Arguably the most important development with MPEG-2 is that it adds support for interlaced video. This is a system by which only half the lines of pixels on a screen update each time the screen is refreshed, though the human eye cannot usually detect this. The updating switches between odd-numbered lines and even-numbered lines on alternate updates. This cuts the amount of detail which is needed for each refresh, making the signal more efficient.
Interlaced video support was important as it allowed MPEG-2 to be used for digital television broadcasts. Until high-definition televisions became popular, the majority of viewers could only receive and view interlaced TV signals. The alternative system, progressive scan, refreshes the entire screen at the same time. Most computer screens use progressive scan.
MPEG-2 should not be confused with MP2, an abbreviation for MPEG-1 Audio Layer 2. It is one of a series of audio compression formats which were developed alongside as the MPEG-1 video format. The best known of these audio formats is MP3.
There have been two subsequent MPEG video formats. MPEG-3 was designed to cope with high-definition video signals, but was later considered unnecessary and was merged into MPEG-2. MPEG-4 involves a greater rate of compression, meaning it can fit more detail into the same size. It can be used in high-definition DVD formats such as Blu-Ray®.