High-definition television (or HDTV, or just HD) refers to video having resolution substantially higher than traditional television systems (standard-definition TV, or SDTV, or SD). HD has one or two million pixels per frame, roughly five times that of SD. Early HDTV broadcasting used analog techniques, but today HDTV is digitally broadcast using video compression.
History of High-definition Television
The term high definition once described a series of television systems originating from the late 1930s; however, these systems were only high definition when compared to earlier systems that were based on mechanical systems with as few as 30 lines of resolution.
The British high definition TV service started trials in August 1936 and a regular service in November 1936 using both the (mechanical) Baird 240 line and (electronic) Marconi-EMI 405 line (377i) systems. The Baird system was discontinued in February 1937. In 1938 France followed with their own 441 line system, variants of which were also used by a number of other countries. The US NTSC system joined in 1941. In 1949 France introduced an even higher resolution standard at 819 lines (768i), a system that would be high definition even by today's standards, but it was monochrome only.
All of these systems used interlacing and a 4:3 aspect ratio except the 240 line system which was progressive (actually described at the time by the technically correct term 'sequential') and the 405 line system which started as 5:4 and later changed to 4:3. The 405 line system adopted the revolutionary idea of interlaced scanning to overcome the flicker problem of the 240 line with its 25 Hz frame rate.
HDTV broadcast systems are identified with three major parameters:
Frame size in pixels is defined as number of horizontal pixels × number of vertical pixels, for example 1280 × 720 or 1920 × 1080. Often the number of horizontal pixels is implied from context and is omitted, as in the case of 720p and 1080p.
Scanning system is identified with the letter p for progressive scanning or i for interlaced scanning.
Frame rate is identified as number of video frames per second.
For interlaced systems an alternative form of specifying number of fields per second is often used.
If all three parameters are used, they are specified in the following form: [frame size][scanning system][frame or field rate] or [frame size]/[frame or field rate][scanning system]. Often, frame size or frame rate can be dropped if its value is implied from context. In this case the remaining numeric parameter is specified first, followed by the scanning system.
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