Friday, December 14, 2007

High-Definition Video

High-definition (HD) video generally refers to any video system of higher resolution than standard-definition (SD) video, most commonly at display resolutions of 1280x720 (720p) or 1920x1080 (1080i or 1080p). This article discusses the general concepts of high-definition video, as opposed to its specific applications in television broadcast (HDTV), video recording formats (HDCAM, HDCAM-SR, DVCPRO-HD, D5-HD, XDCAM-HD, HDV and AVCHD), and optical disc delivery systems (Blu-ray and HD-DVD).


Original HD specifications date back to the early 1980s, when Japan developed an 1125-line TV standard operating at 30 frames per second (fps). Japan presented their standard at an international meeting of television engineers in Algiers in 1981 and Japan's NHK presented its analog HDTV system at Swiss conference in 1983. The NHK system was standardized in the United States as SMPTE (Society of Motion Picture and Television Engineers) standard #240M in the early 1990s.

Historically, the term high-definition television was used to refer to television standards developed in the late 1930s to replace the early experimental mechanically-scanned systems that ranged from 15 lines to about 220 lines of resolution. John Logie Baird of the UK was a major proponent of these early mechanically scanned systems, but they were quickly replaced by all-electronic systems developed by engineers such as Philo T. Farnsworth, Vladimir Zworykin and the EMI team including Alan Blumlein under Isaac Shoenberg.

The United Kingdom was the first to start regular broadcast television – the BBC Television Service – in 1936 from Alexandra Palace, initially with a 240-line, 25 frames-per-second (fps) mechanically-scanned system by Baird Television Limited alternating with a 405-line Marconi-EMI interlaced system at 50 fields per second (each frame consisting of two fields). The Baird system was dropped after the end of 1936. This was referred to as the world's first scheduled 'high definition' television service, and thus the term must be regarded as originally identifying systems offering 240-line resolution or better. The Marconi-EMI specification went on to be adopted across Europe as CCIR System A.

In the United States, the National Television System Committee (for which the NTSC standard is named) standardized on 525 lines at 30 fps in 1940, with regular broadcasts starting on July 1, 1941. The NTSC standard was updated to include first a non-compatible 441-line color standard in 1950, which was then replaced by a compatible 525-line, 29.97fps color standard approved in 1953 and used to this day. PAL (Phase Alternating Line) was developed in the late 1950s with 625 lines at 25 fps and went on the air in 1964. SECAM (SÉquentiel Couleur À Mémoire, French for "sequential colour with memory") was developed by France as the first European color television standard independent to the American NTSC standard, and soon competed by the West German PAL, also using 625 lines and 25 fps. SECAM was adopted during the Cold War by France and its colonial territories, as well as the Belgian colonies, and later adopted by countries rejecting the American standard, namely the Soviet Union, the Peoples' Republic of China, and their satellite communist governments.

The current high definition video standards were developed during the course of the advanced television process initiated by the Federal Communications Commission in 1987 at the request of American broadcasters. The FCC process, led by the Advanced Television Systems Committee (ATSC) adopted a range of standards from interlaced 1080 line video (a technical descendant of the original analog NHK 1125/30fps system) with a maximum frame rate of 30 fps, and 720 line video, progressively scanned, with a maximum frame rate of 60 fps. The FCC officially adopted the ATSC transmission standard (which included both HD and SD video standards) in 1996, with the first broadcasts on October 28, 1998.

The world has transmitted analog PAL, NTSC, SECAM for over 60 years. However, with the advent of digital broadcasting including HD formats, analog transmissions will cease in the coming years and NTSC, PAL and SECAM will pass into history, or so goes the most optimistic point-of-view. It remains to be seen if and when this can be achieved, due to the vast amounts of analog video equipment (TV stations and home TVs) which are currently installed.

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