Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.

Analog television systems were standardized by the International Telecommunication Union (ITU) in 1961,Final acts of the European Broadcasting Conference in the VHF and UHF bands. Stockholm, 1961. with each system designated by a letter (A-N) in combination with the color standard used (NTSC, PAL or SECAM) - for example PAL-B, NTSC-M, etc.). These analog systems for TV broadcasting dominated until the 2000s.

With the introduction of digital terrestrial television (DTT), they were replaced by four main systems in use around the world: ATSC, DVB, ISDB and DTMB.

Analog television systems

Every analog television system bar one began as a black-and-white system. Each country, faced with local political, technical, and economic issues, later adopted a color television standard which was grafted onto an existing monochrome system such as CCIR System M, using gaps in the video spectrum (explained below) to allow color transmission information to fit in the existing channels allotted. The grafting of the color transmission standards onto existing monochrome systems permitted existing monochrome television receivers predating the changeover to color television to continue to be operated as monochrome television. Because of this compatibility requirement, color standards added a second signal to the basic monochrome signal, which carries the color information. The color information is called chroma with the symbol C, while the black and white information is called the luminance with the symbol Y. Monochrome television receivers only display the luminance, while color receivers process both signals. Though in theory any monochrome system could be adopted to a color system, in practice some of the original monochrome systems proved impractical to adapt to color and were abandoned when the switch to color broadcasting was made. All countries used one of three color standards: NTSC, PAL, or SECAM. For example, CCIR System M was often used in conjunction with NTSC standard, to provide color analog television and the two together were known as NTSC-M.

Pre–World War II systems

A number of experimental and broadcast systems had been developed before the WW2. The first ones were mechanical, had low resolution (180 lines in Germany, 240 lines in the UK), sometimes with no sound.

Later TV systems were electronic, and usually mentioned by their line number: 240-line (used in the US), 343-line (used in the US, USSR), 375-line (used in Germany, Italy, US), 405-line (used in the UK), 441-line (used in Germany, France, Italy, US, adopted but not widely used in the USSR) or 567-line (used in the Netherlands). These systems were mostly experimental and national, with no defined international standards, and did not resume broadcasting after the war except for the UK 405-line system, which resumed broadcasts and was the first to be standardized by ITU as System A, remaining in operation until 1985.

ITU standards On an international conference in Stockholm in 1961, the International Telecommunication Union designated standards for broadcast television systems (ITU System Letter Designation). Each standard is designated by a letter (A-M).

On VHF bands I, II and III the 405, 625 and 819-line systems could be used: A – 405-line system, 5 MHz video bandwidth B – 625-line system, 7 MHz video bandwidth C – Belgian 625-line system, 7 MHz video bandwidth D – I.B.T.O. 625-line system, 8 MHz video bandwidth E – French 819-line system, 14 MHz video bandwidth F – Belgian 819-line system, 7 MHz video bandwidth On UHF bands Bands IV and V only 625-line systems were adopted, with the difference being transmission parameters like channel bandwidth. G – 625-line system, 5 MHz video bandwidth H – 625-line system, 5 MHz video bandwidth I – 625-line system, 5.5 MHz video bandwidth K – 625-line system, 6 MHz video bandwidth L – 625-line system, 6 MHz video bandwidth

Following further conferences and the introduction of color television, by 1966 each standard was designated by a letter (A-M) in combination with a color standard (NTSC, PAL, SECAM). This completely specifies all of the monaural analog television systems in the world (for example, PAL-B, NTSC-M, etc.).

The following table gives the principal characteristics of each standard. Except for lines and frame rates, other units are megahertz (MHz). Also see: television channel frequencies

+ World analog television systems System Introduced Lines Frame rate (fps) Usual color standard Channel bandwidth (MHz) Video bandwidth (MHz) Vision/sound carrier separation (MHz) Vestigial sideband (MHz) Vision modulation (+, -) Sound modulation (AM, FM) Chrominance subcarrier frequency (MHz) Vision/sound power ratio Assumed display device gamma A 1936 405 25 5 3 −3.5 0.75 + AM none 4:1 2.5 - 2.0 B 1950 625 25 PAL/SECAM 7 5 +5.5 0.75 - FM 4.43 2.8 C 1953 625 25 7 5 +5.5 0.75 + AM none 2.0 D 1948 625 25 SECAM/PAL 8 6 +6.5 0.75 - FM 4.43 2.8 E 1949 819 25 14 10 ±11.15 2.00 + AM none 1.7 F 1953 819 25 7 5 +5.5 0.75 + AM none 2.0 G 1961 625 25 PAL/SECAM 8 5 +5.5 0.75 - FM 4.43 5:1 2.8 H 1961 625 25 PAL 8 5 +5.5 1.25 - FM 4.43 5:1 2.8 I 1962 625 25 PAL 8 5.5 +5.9996 1.25 - FM 4.43 5:1 2.8 J 1953 525 30 NTSC 6 4.2 +4.5 0.75 - FM 3.58 2.2 K 1961 625 25 SECAM/PAL 8 6 +6.5 0.75 - FM 4.43 5:1 2.8 K1 1964 625 25 SECAM 8 6 +6.5 1.25 - FM 4.43 2.8 L 1961 625 25 SECAM 8 6 -6.5 1.25 + AM 4.43 8:1 2.8 M 1941 525 30 NTSC/PAL-M 6 4.2 +4.5 0.75 - FM 3.58/3.56 2.2 N 1951 625 25 PAL 6 4.2 +4.5 0.75 - FM 3.58 2.8

Notes by system A Early United Kingdom and Ireland VHF system (B&W only). First electronic TV system, introduced in 1936. Vestigial sideband filtering introduced in 1949. Discontinued on 23 November 1982 in Ireland and on 2 January 1985 in the UK. B VHF-only in most Western European countries (combined with system G and H on UHF); VHF and UHF in Australia. Originally known as the Gerber standard. C Early VHF system; used only in Belgium, Italy, the Netherlands, and Luxembourg, as a compromise between Systems B and L. Discontinued in 1977. D The first 625-line system. Used on VHF only in most countries (combined with system K on UHF); VHF and UHF in China. E Early French VHF system (B&W only); very good (near HDTV) picture quality but uneconomical use of bandwidth. Sound carrier separation +11.15 MHz on odd numbered channels, -11.15 MHz on even numbered channels. Discontinued in 1984 (France) and 1985 (Monaco). F Early VHF system used only in Belgium and Luxembourg; allowed French 819-line television programming to be broadcast on the 7 MHz VHF channels used in those countries, at a substantial cost in horizontal resolution. Discontinued in 1968 (Belgium) and 1971 (Luxembourg). G UHF only; used in countries with System B on VHF, except Australia. H UHF only; used only in Belgium, Luxembourg, and the Netherlands. Similar to System G with a 1.25 MHz vestigal sideband. I Used in the UK, Ireland, Southern Africa, Macau, Hong Kong, and Falkland Islands. J Used in Japan (see system M below). Identical to system M except that a different black level of 0 IRE is used instead of 7.5 IRE. Although the ITU specified a frame rate of 30 fields, 29.97 was adopted with the introduction of NTSC color to minimize visual artifacts. Discontinued in 2011, when Japan transitioned to digital. K UHF only; used in countries with system D on VHF, except China, and identical to it in most respects. K1 Used only in French overseas departments and territories. L Used only in France. On VHF Band 1 only, the audio is at −6.5 MHz. Discontinued in 2005, when France transitioned to digital. It was the last system to use positive video modulation and AM sound. M Used in most of the Americas and Caribbean (except Argentina, Paraguay, Uruguay, and French Guiana), Myanmar, South Korea, Taiwan, Philippines (all NTSC-M), Brazil (PAL-M), Vietnam, Cambodia, and Laos (all SECAM-M). Although the ITU specified a frame rate of 30 fields, 29.97 was adopted with the introduction of NTSC color to minimize visual artifacts. N Adopted by Argentina, Paraguay, and Uruguay (all PAL-N since 1978).

Evolution For historical reasons, some countries use a different video system on UHF than they do on the VHF bands. In a few countries, most notably the United Kingdom, television broadcasting on VHF has been entirely shut down. The British 405-line system A, unlike all the other systems, suppressed the upper sideband rather than the lower—befitting its status as the oldest operating television system to survive into the color era (although was never officially broadcast with color encoding). System A was tested with all three color standards, and production equipment was designed and ready to be built; System A might have survived, as NTSC-A, had the British government not decided to harmonize with the rest of Europe on a 625-line video system, implemented in Britain as PAL-I on UHF only.

The French 819 line system E was a post-war effort to advance France's standing in television technology. Its 819 lines were almost high definition even by today's standards. Like the British system A, it was VHF only and remained black & white until its shutdown in 1984 in France and 1985 in Monaco. It was tested with SECAM standard in the early stages, but later the decision was made to adopt color in 625-lines L system only. Thus, France adopted system L both on UHF and VHF networks and abandoned system E.

Japan had the earliest working HDTV system (MUSE), with design efforts going back to 1979. The country began broadcasting wideband analog high-definition video signals in the late 1980s using an interlaced resolution of 1,125 lines, supported by the Sony HDVS line of equipment.

In many parts of the world, analog television broadcasting has been shut down completely, or in process of shutdown; see Digital television transition for a timeline of the analog shutdown.

Technical aspects

Frames

Ignoring color, all television systems work in essentially the same manner. The monochrome image seen by a camera (later, the luminance component of a color image) is divided into horizontal scan lines, some number of which make up a single image or frame. A monochrome image is theoretically continuous, and thus unlimited in horizontal resolution, but to make television practical, a limit had to be placed on the bandwidth of the television signal, which puts an ultimate limit on the horizontal resolution possible. When color was introduced, this limit necessarily became fixed. All analog television systems are interlaced: alternate rows of the frame are transmitted in sequence, followed by the remaining rows in their sequence. Each half of the frame is called a video field, and the rate at which fields are transmitted is one of the fundamental parameters of a video system. It is related to the utility frequency at which the electricity distribution system operates, to avoid flicker resulting from the beat between the television screen deflection system and nearby mains generated magnetic fields. All digital, or "fixed pixel," displays have progressive scanning and must deinterlace an interlaced source. Use of inexpensive deinterlacing hardware is a typical difference between lower- vs. higher-priced flat panel displays (Plasma display, LCD, etc.).