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 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. ;
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. ;
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 1962).
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.). 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 (European among other countries with 50 Hz mains supply), the content is
PAL speedup, while a technique known as "
3:2 pulldown" is used for 30 frame/s formats (North America among other countries with 60 Hz mains supply) to match the film frame rate to the video frame rate without speeding up the play back.
Viewing technology Analog television signal standards are designed to be displayed on a
cathode ray tube (CRT), and so the physics of these devices necessarily controls the format of the video signal. The image on a CRT is painted by a moving beam of electrons which hits a
phosphor coating on the front of the tube. This electron beam is steered by a magnetic field generated by powerful
electromagnets close to the source of the electron beam. In order to reorient this magnetic steering mechanism, a certain amount of time is required due to the
inductance of the magnets; the greater the change, the greater the time it takes for the electron beam to settle in the new spot. For this reason, it is necessary to shut off the electron beam (corresponding to a video signal of
zero luminance) during the time it takes to reorient the beam from the end of one line to the beginning of the next (
horizontal retrace) and from the bottom of the screen to the top (
vertical retrace or
vertical blanking interval). The horizontal retrace is accounted for in the time allotted to each scan line, but the vertical retrace is accounted for as
phantom lines which are never displayed but which are included in the number of lines per frame defined for each video system. Since the electron beam must be turned off in any case, the result is gaps in the television signal, which can be used to transmit other information, such as test signals or color identification signals. The temporal gaps translate into a comb-like
frequency spectrum for the signal, where the teeth are spaced at line frequency and concentrate most of the energy; the space between the teeth can be used to insert a color subcarrier.
Hidden signaling Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for
teletext and
closed captioning: •
PALplus uses a
hidden signaling scheme to indicate if it exists, and if so what operational mode it is in. •
NTSC was modified by the
Advanced Television Systems Committee to support an
anti-ghosting signal that is inserted on a non-visible scan line. •
Teletext uses hidden signaling to transmit information pages. •
NTSC Closed Captioning signaling uses signaling that is nearly identical to
teletext signaling. •
Widescreen signaling enables a flag to indicate that a 16:9 widescreen image is being broadcast, and allows the TV set to switch to the appropriate display mode.
Overscan Television images incorporate regions of the picture with reasonable-quality content outside the normal display.
Interlacing In a purely analog system, field order is merely a matter of convention. For digitally recorded material it becomes necessary to rearrange the field order when conversion takes place from one standard to another.
Image signal polarity Another parameter of analog television systems, minor by comparison, is the choice of whether vision modulation is positive or negative. Some of the earliest electronic television systems such as the British 405-line (System A) used positive modulation. It was also used in the two Belgian systems (System C, 625 lines, and System F, 819 lines) and the two French systems (System E, 819 lines, and System L, 625 lines). In positive modulation systems, as in the earlier
white facsimile transmission standard, the maximum luminance value is represented by the maximum carrier power; in negative
modulation, the maximum luminance value is represented by zero carrier power. All newer analog video systems use negative modulation with the exception of the French System L. Impulse noise, especially from older automotive ignition systems, caused white spots to appear on the screens of television receivers using positive modulation but they could use simple synchronization circuits. Impulse noise in negative-modulation systems appears as dark spots that are less visible, but picture synchronization was seriously degraded when using simple synchronization. The synchronization problem was overcome with the invention of
phase-locked synchronization circuits. When these first appeared in Britain in the early 1950s one name used to describe them was "flywheel synchronisation." Older televisions for positive-modulation systems were sometimes equipped with a peak video signal inverter that would turn the white interference spots dark. This was usually user-adjustable with a control on the rear of the television labeled "White Spot Limiter" in Britain or "Antiparasite" in France. If adjusted incorrectly it would turn bright white picture content dark. Most of the positive modulation television systems ceased operation by the mid-1980s. The French System L continued on up to the transition to digital broadcasting. Positive modulation was one of several unique technical features that originally protected the French electronics and broadcasting industry from foreign competition and rendered French TV sets incapable of receiving broadcasts from neighboring countries. Another advantage of negative modulation is that, since the synchronizing pulses represent maximum carrier power, it is relatively easy to arrange the receiver
automatic gain control to only operate during sync pulses and thus get a constant amplitude video signal to drive the rest of the TV set. This was not possible for many years with positive modulation as the peak carrier power varied depending on picture content. Modern digital processing circuits have achieved a similar effect but using the front porch of the video signal.
Modulation Given all of these parameters, the result is a mostly continuous
analog signal which can be modulated onto a radio-frequency carrier and transmitted through an antenna. All analog television systems use
vestigial sideband modulation, a form of
amplitude modulation in which one sideband is partially removed. This reduces the bandwidth of the transmitted signal, enabling narrower channels to be used.
Audio In analog television, the
analog audio portion of a broadcast is invariably modulated separately from the video. Most commonly, the audio and video are combined at the transmitter before being presented to the antenna, but separate aural and visual antennas can be used. In all cases where negative video is used,
FM is used for the standard
monaural audio; systems with positive video use AM sound and intercarrier receiver technology cannot be incorporated. Stereo, or more generally multi-channel, audio is encoded using a number of schemes which (except in the French systems) are independent of the video system. The principal systems are
NICAM, which uses a digital audio encoding; double-FM (known under a variety of names, notably
Zweikanalton, A2 Stereo, West German Stereo, German Stereo or IGR Stereo), in which case each audio channel is separately modulated in FM and added to the broadcast signal; and BTSC (also known as
MTS), which multiplexes additional audio channels into the FM audio carrier. All three systems are compatible with monaural FM audio, but only
NICAM may be used with the French AM audio systems. == Digital television systems ==