Broadcast television systems

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, 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.

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 chrominance 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.

A number of experimental and broadcast pre-WW2 systems were tested. The first ones were mechanically based and of very low resolution, sometimes with no sound. Later TV systems were electronic, and usually mentioned by their line number: 375-line (used in Germany, Italy, US), 405-line (used in the UK), 441-line (used in Germany, France, Italy, US) or 567-line (used in the Netherlands). These systems were mostly experimental and national, with no defined international standards, and didn't resume broadcasting after the war. An exception was the UK 405-line system, that resumed broadcasts and was the first to be standardized by ITU as System A, remaining in operation until 1985.

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:

On UHF bands Bands IV and V only 625-line systems were adopted, with the difference being transmission parameters like channel bandwidth.