HDTV
High-definition television (or HDTV) is a digital television broadcasting system with higher resolution than traditional television systems (standard-definition TV, or SDTV). HDTV is digitally broadcast; the earliest implementations used analog broadcasting, but today digital television (DTV) signals are used, requiring less bandwidth due to digital video compression.
The term high definition once described a series of television systems originating from the late 1930s, 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
- 1949: French (monochrome) 819 line @ 50 Hz system launched, discontinued 1983
- 1952-1956: European adoption of 625 lines @ 50 Hz with PAL and SECAM color coming in 1967
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 1983, before the final British 405-line broadcast. Experimental 405 line color 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.
Color
In Mexico, Guillermo González Camarena (1917–1965), invented an early color television transmission system. He received patents for color television systems in 1942 (U.S. Patent 2,296,019), 1960 and 1962. The 1942 patent (filed in Mexico on August 19, 1940) was for a synchronized color filter wheel adapter for monochrome television, similar to the field sequential color receiver demonstrated by Baird in England in July 1939 and by CBS in the United States in August 1940.
On August 31, 1946 González Camarena sent his first color transmission from his lab in the offices of The Mexican League of Radio Experiments at Lucerna St. #1, in Mexico City. The video signal was transmitted at a frequency of 115 MHz and the audio in the 40 meter band. He made the first publicly announced color broadcast in Mexico, on February 8, 1963, of the program Paraíso Infantil on Mexico City's XHGC-TV.
In 1958, the Soviet Union created Тransformator (Russian: Трансформатор, "Transformer"), the first high-resolution (definition) television system capable of producing an image composed of 1,125 lines of resolution for the purpose of television conferences among military commands; as it was a military product, it was not commercialized.
Modern systems
In 1969, the Japanese state broadcaster NHK first developed consumer high-definition television with a 5:3 aspect ratio, a slightly wider screen format than the usual 4:3 standard. However, the system was not launched publicly until late in the 1990s.
In 1981, the first HDTV demonstration in the United States was held. It had the same 5:3 aspect ratio as the Japanese system. Upon visiting a demonstration of the Japanese Multiple sub-nyquist sampling Encoding system (MUSE) HDTV system in Washington, US President Ronald Reagan was most impressed and officially declared it "a matter of national interest" to introduce HDTV to the USA. Several systems were proposed as the new standard for the USA, including the Japanese MUSE system, but all were rejected by the FCC because of their higher bandwidth requirement.
A new standard had to be radically efficient, needing less bandwidth for HDTV than the existing NTSC standard for SDTV. It was commonly understood only a digital system could possibly bring desired results; however, nothing such had yet been developed.
Rise of digital compression
As soon as the MPEG-1 standard provided the foundation for digital TV, development of modern TV standards started worldwide. After finalization of MPEG-2 in mid 1993, the DVB organization within the International Telecommunication Union's radio telecommunications sector (ITU-R) developed the ETSI standard 300-327 by the end of December 1993.
It became known as DVB-T for digital terrestrial TV. DVB-S and DVB-C standards soon followed for terrestrial, satellite and cable transmission of SDTV and HDTV. In the USA the Grand Alliance proposed ATSC as the new standard for SDTV and HDTV. Both ATSC and DVB were based on the MPEG-2 standard. The DVB-S2 standard is based on the newer and more efficient H.264/MPEG-4 AVC compression standards. Common for all DVB standards is the use of highly efficient modulation techniques for further reducing bandwidth, and foremost for reducing receiver-hardware and antenna requirement.
In 1983, the International Telecommunication Union's radio telecommunications sector (ITU-R) set up a working party (IWP11/6) with the aim of setting a single international HDTV standard. One of the thornier issues concerned a suitable frame/field refresh rate, with the world already strongly demarcated into two camps, 25/50Hz and 30/60Hz, related by reasons of picture stability to the frequency of their mains electrical supplies.
The WP considered many views and through the 1980s served to encourage development in a number of video digital processing areas, not least conversion between the two main frame/field rates using motion vectors, which led to further developments in other areas. While a comprehensive HDTV standard was not in the end established, agreement on the aspect ratio was achieved.
Initially the existing 5:3 aspect ratio had been the main candidate, but due to the influence of widescreen cinema, the aspect ratio 16:9 (1.78) eventually emerged as being a reasonable compromise between 5:3 (1.67) and the common 1.85 widescreen cinema format. (It has been suggested that the 16:9 ratio was chosen as being the geometric mean of 4:3, Academy Ratio, and 2.35:1, the widest cinema format in common use, in order to minimize wasted screen space when displaying content with a variety of aspect ratios.)
An aspect ratio of 16:9 was duly agreed at the first meeting of the WP at the BBC's R & D establishment in Kingswood Warren. The resulting ITU-R Recommendation ITU-R BT.709-2 ("Rec. 709") includes the 16:9 aspect ratio, a specified colorimetry, and the scan modes 1080i (1,080 actively-interlaced lines of resolution) and 1080p (1,080 progressively-scanned lines). The current BBC freeview trials of HD use MBAFF, which contains both progressive and interlaced content in the same encoding.
It also includes the alternative 1440 x 1152 HDMAC scan format. (According to some reports, a mooted 720p format (720 progressively-scanned lines) was viewed by some at the ITU as an "enhanced" television format rather than a true HDTV format, and so was not included, although 1920x1080 and 1280x720p systems for a range of frame and field rates were defined by several US SMPTE standards.)
Demise of analog HD systems
However, even that limited standardization of HDTV did not lead to its adoption, principally for technical and economic reasons. Early HDTV commercial experiments such as NHK's MUSE required over four times the bandwidth of a standard-definition (SDTV) broadcast, and despite efforts made to shrink the required bandwidth down to about two times that of SDTV, it was still only distributable by satellite. In addition, recording and reproducing a HDTV signal was a significant technical challenge in the early years of HDTV. Japan remained the only country with successful public broadcast analog HDTV, known as "Hi-vision", featuring a 5:3 aspect ratio screen with 1,125 interlaced lines (1,035 active lines) at the rate of 60 fields per second. The single satellite transponder MUSE service was turned off on January 1, 2007.
In Europe, analogue 1,250-line HD-MAC test broadcasts were performed in the early 1990s, but did not lead to any established public broadcast service.