High-definition television (HDTV) is a digital television broadcasting system with higher resolution than traditional television systems (standard-definition TV, or SDTV). HDTV is digitally broadcast because digital television (DTV) requires less bandwidth if sufficient video compression is used.
History of high-definition television:
The term high definition once described a series of television systems from the 1930s and 1940s, starting with the British 240 line and 405 line black-and-white systems introduced in 1936, and including the American 525-line NTSC system established in 1941. However, these systems were only "high definition" when compared to earlier systems.
The British high definition TV service started trials in August 1936 and a regular service in November 1936 using both the Baird 240 line and Marconi-EMI 405 line systems. The Baird system was discontinued in February 1937.
A brief itemized history of early analog HD systems follows; these would be considered standard definition television systems today.
1936: System-A, UK: 405 lines @ 50 Hz, discontinued 1986
1938: Several countries used a 441 line system, France in 1956 being the last to discontinue it
1939: System-M, USA: 525 lines @ 60 Hz
1952-1956: European adoption of 625 lines @ 50 Hz with PAL and SECAM color coming in 1956
1956: French (monochrome) 819 line @ 50 Hz system launched, discontinued 1986
All used interlacing and a 4:3 aspect ratio except the 405 line system which started as 5:4 and later changed to 4:3.
The post–WWII French 819-line black-and-white system was high definition in the contemporary sense, but was discontinued in 1986, a year after the final British 405-line broadcast. Experimental 405 line colour transmissions were made in the 1950s using a modified NTSC system.
Since the formal adoption of DVB's widescreen HDTV transmission modes in the early 2000s the 525-line NTSC (and PAL-M) systems as well as the European 625-line PAL and SECAM systems are now regarded as standard definition television systems. In Australia, the 625-line digital progressive system (with 576 active lines) is officially recognized as high definition.
more info/ visit: en.wikipedia.org/wiki/HD_TV