Broadcast television systems

Broadcast 'television systems' 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 in 1961, with each system designated by a letter in combination with the color standard used. These analog systems for TV broadcasting dominated until the 2000s.
With the introduction of digital terrestrial television, 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 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.

Pre–World War II systems

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, 405-line, 441-line or 567-line. These systems were mostly experimental and national, with no defined international standards, and did not 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.

ITU standards

On an international conference in Stockholm in 1961, the International Telecommunication Union designated standards for broadcast television systems. Each standard is designated by a letter.
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 in combination with a color standard. This completely specifies all of the monaural analog television systems in the world.
The following table gives the principal characteristics of each standard. Except for lines and frame rates, other units are megahertz.

Also see: television channel frequencies

Usual color standard	System	Introduced	Lines	Frame rate	Channel bandwidth	Video bandwidth	Vision/sound carrier separation	Vestigial sideband	Vision modulation	Sound modulation	Chrominance subcarrier frequency	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
PAL/SECAM	B	1950	625	25	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
SECAM/PAL	D	1948	625	25	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
PAL/SECAM	G	1961	625	25	8	5	+5.5	0.75	-	FM	4.43	5:1	2.8
PAL	H	1961	625	25	8	5	+5.5	1.25	-	FM	4.43	5:1	2.8
PAL	I	1962	625	25	8	5.5	+5.9996	1.25	-	FM	4.43	5:1	2.8
NTSC	J	1953	525	30	6	4.2	+4.5	0.75	-	FM	3.58		2.2
SECAM/PAL	K	1961	625	25	8	6	+6.5	0.75	-	FM	4.43	5:1	2.8
SECAM	K1	1964	625	25	8	6	+6.5	1.25	-	FM	4.43		2.8
SECAM	L	1961	625	25	8	6	-6.5	1.25	+	AM	4.43	8:1	2.8
NTSC/PAL-M	M	1941	525	30	6	4.2	+4.5	0.75	-	FM	3.58/3.56		2.2
PAL	N	1951	625	25	6	4.2	+4.5	0.75	-	FM	3.58		2.8

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. 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, 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 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.
All films and other filmed material shot at 24 frames per second must be transferred to video frame rates using a telecine in order to prevent severe motion jitter effects. Typically, for 25 frame/s formats, the content is PAL speedup, while a technique known as "3:2 pulldown" is used for 30 frame/s formats to match the film frame rate to the video frame rate without speeding up the play back.