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PAL

PAL is the acronym for a television signal encoding system (see below) and also an acronym for a Permissive Action Link, a security device for nuclear weapons. In the UK there was also a brand of dog food called PAL, allegedly an acronym for "Prolongs Active Life". In computer science, PAL is also the acronym for "processor abstraction layer", a set of functions for the programmer to use that works on all processors.

Also an acronym for "programmable array logic".


PAL is the analogue video format used in television transmission in most of Europe (except France, Bulgaria, Russia, Yugoslavia, and some other countries in Eastern Europe, where SECAM is used), Australia and some Asian, African, and South American countries.

The name PAL comes from Phase Alternating Line, which describes the way that part of the colour information on the video signal is reversed in phase with each line, which automatically corrects phase errors in the transmission of the signal (NTSC receivers have a tint control to perform the correction manually). This is why some engineers jokingly expand NTSC to "Never Twice the Same Colour" while referring to PAL as "Peace At Last"! However, the alternation of colour information can lead to picture grain on pictures with extreme phase errors. PAL was developed in Germany by Walter Bruch[?]. It was first introduced in 1967.

The PAL colour system is usually used with a video format that has 625 lines per frame and a refresh rate of 25 frames per second. Like NTSC this is an interlaced format. Each frame consists of two fields (half-a-frame), each field has half of the lines of a frame (one has all the even lines, one has all the odd lines). Fields are transmitted and displayed successively. There are 50 fields per second. At the time of its design the interlacing of fields was a compromise between flicker and bandwidth.

There are many variants of PAL. PAL-M is a hybrid of NTSC and PAL used in Brazil and PAL-N is a variant of PAL with narrow bandwidth which is used in Argentina, Paraguay, and Uruguay. PAL-I is the version used in the United Kingdom. Other European countries use PAL-B/G. A variant named PAL60 uses, like NTSC, 59.94 fields per second. It is used mainly for displaying NTSC video or DVD on a PAL TV set.

References

  • The standard that defines the PAL system was published by the International Telecommunications Union in 1998 and has the title "Recommendation ITU-R BT.470-6, Conventional Television Systems".
  • More information about TV standards can be found at http://www.ee.surrey.ac.uk/Contrib/WorldTV/broadcast




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