Editing Broadcast television systems (section)

=== Technical aspects ===

==== Frames ====
{{Main|Film frame}}

Ignoring color, all television systems work in essentially the same manner. The monochrome image seen by a camera (later, the [[Luma (video)|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 (signal processing)|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 video|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 ''[[Field (video)|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 [[Electric power distribution|electricity distribution]] system operates, to avoid flicker resulting from the [[Beat (acoustics)|beat]] between the television screen deflection system and nearby mains generated magnetic fields. All digital, or "fixed pixel," displays have [[progressive scan]]ning and must [[Deinterlacing|deinterlace]] an interlaced source. Use of inexpensive deinterlacing hardware is a typical difference between lower- vs. higher-priced [[Flat-panel display|flat panel displays]] ([[Plasma display]], [[Liquid-crystal display|LCD]], etc.).

All [[film]]s and other filmed material shot at 24&nbsp;frames per second must be transferred to video [[frame rate]]s using a [[telecine]] in order to prevent severe motion jitter effects. Typically, for 25&nbsp;frame/s formats (European among other countries with 50&nbsp;Hz mains supply), the content is [[576i#PAL speed-up|PAL speedup]], while a technique known as "[[Three-two pull down|3:2 pulldown]]" is used for 30&nbsp;frame/s formats (North America among other countries with 60&nbsp;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|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 [[electromagnet]]s 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 [[Spectral density|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 [[Widescreen signaling|hidden signaling]] scheme to indicate if it exists, and if so what operational mode it is in.
* [[NTSC]] was modified by the [[ATSC standards|Advanced Television Systems Committee]] to support an [[ghost-canceling reference|anti-ghosting signal]] that is inserted on a non-visible scan line.
* [[Teletext]] uses hidden signaling to transmit information pages.
* [[NTSC]] [[Closed captioning|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 ====
{{Main|Overscan}}

Television images are unique in that they must incorporate regions of the picture with reasonable-quality content, that will never be seen by some viewers.{{Vague|date=September 2017}}

==== Interlacing ====
{{Main|Interlaced video}}

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 loop|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 [[Single-sideband modulation#Vestigial sideband (VSB)|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 recording|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, [[frequency modulation|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 [[Multichannel Television Sound|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.