Before HD television could exist, video had to be squeezed about a hundredfold
Uncompressed 1080p video needs more than a gigabit every second, far too much to store or broadcast. High definition became practical only after a compression method first proposed by Nasir Ahmed in 1972 was adapted for moving pictures, cutting the data by around 100 to 1.
There is no single official definition of high-definition video. As a rule of thumb, anything with well over 480 scan lines in North America, or 576 in Europe, counts. Oddly, the label has a long history: the 405-line electronic system of the 1930s was called high definition because the mechanical sets it replaced had far fewer lines. France pushed hardest early on. René Barthélemy experimented with up to 1,042 lines during the war, and French broadcasts began in late 1949 at 819 lines, a standard dropped in 1984 for 625-line colour on TF1.
The modern story starts in Japan in the early 1980s with HighVision, also called MUSE, an interlaced 1,125-line system running at 60 frames a second. Sony showed its HDVS system to engineers in Algiers in April 1981, and NHK demonstrated analogue HDTV in Switzerland in 1983. Europe answered with HD-MAC at 1,250 lines. Neither analogue approach succeeded as a terrestrial broadcast format, and attempts to transmit HighVision over the air had been abandoned by the mid-1990s.
Digital compression changed everything. The discrete cosine transform, a lossy technique, was turned into motion-compensated coding for the H.26x formats from 1988 and MPEG from 1993. By the early 1990s it had become the standard way of carrying HDTV. In the United States, the FCC began an advanced television process in 1987 at broadcasters' request and formally adopted the ATSC transmission standard in 1996. Brazil and China later chose their own systems, dashing hopes of worldwide compatibility.
Three numbers describe any HD format. The line count is 720 or 1,080. A letter marks the scan method: p for progressive, where every line is redrawn each refresh, or i for interlaced, which paints odd lines and then even ones. Interlacing gives sharper stills for the same bandwidth but can lose up to half its detail on moving subjects and show combing. Finally comes the rate, so 720p60 means 1,280 by 720 pixels at 60 frames a second.
Source: High-definition video
