Editing Analog television (section)

==Synchronization==
Synchronizing pulses added to the video signal at the end of every [[scan line]] and video frame ensure that the sweep oscillators in the receiver remain locked in step with the transmitted signal so that the image can be reconstructed on the receiver screen.<ref name="tveng">
 {{cite book
 | last = Gupta
 | first = R. G.
 | title = Television Engineering and Video Systems
 | publisher = Tata McGraw-Hill
 | year = 2006
 | page = 62
 | isbn = 0-07-058596-2
 | url = https://books.google.com/books?id=P6BlcWaizHUC
 }}
</ref><ref name="analogtv">
 {{cite web
 | url = http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html
 | title = World Analogue Television Standards and Waveforms
 | first = Alan
 | last = Pemberton
 | date = 30 November 2008
 | work = Pembers' Ponderings
 | location = [[Sheffield]], England
 | archive-url = https://web.archive.org/web/20080220113153/http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html
 | archive-date = 20 February 2008
 | access-date =25 September 2010
 }}
</ref><ref name="tvreception">
 {{cite book
 | last = Wharton
 | first = W.
 | author2 = Douglas Howorth
 | title = Principles of Television Reception
 | url = https://books.google.com/books?id=dxOCAAAACAAJ
 | edition = illustrated
 | year = 1971
 | publisher = Pitman Publishing
 | isbn = 0-273-36103-1
 | oclc = 16244216
 }}
</ref>

A ''sync separator'' circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync.

===Horizontal synchronization===
The horizontal sync pulse separates the [[scan line]]s. The horizontal sync signal is a single short pulse that indicates the start of every line. The rest of the scan line follows, with the signal ranging from 0.3&nbsp;V (black) to 1&nbsp;V (white), until the next horizontal or [[#Vertical synchronization|vertical synchronization pulse]].

The format of the horizontal sync pulse varies. In the 525-line [[NTSC]] system it is a 4.85&nbsp;[[μs]] pulse at 0&nbsp;[[volt|V]]. In the 625-line [[PAL]] system the pulse is 4.7&nbsp;μs at 0&nbsp;[[volt|V]].  This is lower than the amplitude of any video signal (''blacker than black'') so it can be detected by the level-sensitive ''sync separator'' circuit of the receiver.

Two timing intervals are defined&nbsp;– the ''front porch'' between the end of the displayed video and the start of the sync pulse, and the ''back porch'' after the sync pulse and before the displayed video. These and the sync pulse itself are called the ''horizontal blanking'' (or ''retrace'') ''interval'' and represent the time that the electron beam in the CRT is returning to the start of the next display line.

===Vertical synchronization===
{{For|the graphic option provided by video games|Screen tearing#Vertical synchronization}}
Vertical synchronization separates the video fields. In PAL and NTSC, the vertical sync pulse occurs within the [[vertical blanking interval]]. The vertical sync pulses are made by prolonging the length of horizontal sync pulses through almost the entire length of the scan line.

The ''vertical sync'' signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or midway through).

The format of such a signal in 525-line [[NTSC]] is:
* pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines)
* long-sync pulses (5 pulses)
* post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines)

Each pre- or post-equalizing pulse consists of half a [[scan line]] of black signal: 2&nbsp;μs at 0&nbsp;V, followed by 30&nbsp;μs at 0.3&nbsp;V. Each long sync pulse consists of an equalizing pulse with timings inverted: 30&nbsp;μs at 0&nbsp;V, followed by 2&nbsp;μs at 0.3&nbsp;V.

In video production and computer graphics, changes to the image are often performed during the [[vertical blanking interval]] to avoid visible discontinuity of the image. If this image in the [[framebuffer]] is updated with a new image while the display is being refreshed, the display shows a mishmash of both frames, producing [[page tearing]] partway down the image.

=== Horizontal and vertical hold ===
The sweep (or deflection) oscillators were designed to run without a signal from the television station (or VCR, computer, or other composite video source). This allows the television receiver to display a raster and to allow an image to be presented during antenna placement. With sufficient signal strength, the receiver's sync separator circuit would split timebase pulses from the incoming video and use them to reset the horizontal and vertical oscillators at the appropriate time to synchronize with the signal from the station.

The free-running oscillation of the horizontal circuit is especially critical, as the horizontal deflection circuits typically power the [[flyback transformer]] (which provides acceleration potential for the CRT) as well as the filaments for the high voltage rectifier tube and sometimes the filament(s) of the CRT itself. Without the operation of the horizontal oscillator and output stages in these television receivers, there would be no illumination of the CRT's face.

The lack of precision timing components in early equipment meant that the timebase circuits occasionally needed manual adjustment. If their free-run frequencies were too far from the actual line and field rates, the circuits would not be able to follow the incoming sync signals. Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.

Older analog television receivers often provide manual controls to adjust horizontal and vertical timing. The adjustment takes the form of ''horizontal hold'' and ''vertical hold'' controls, usually on the front panel along with other common controls. These adjust the free-run frequencies of the corresponding timebase oscillators.

A slowly rolling vertical picture demonstrates that the vertical oscillator is nearly synchronized with the television station but is not locking to it, often due to a weak signal or a failure in the sync separator stage not resetting the oscillator.

Horizontal sync errors cause the image to be torn diagonally and repeated across the screen as if it were wrapped around a screw or a barber's pole; the greater the error, the more copies of the image will be seen at once wrapped around the barber pole.

By the early 1980s the efficacy of the synchronization circuits, plus the inherent stability of the sets' oscillators, had been improved to the point where these controls were no longer necessary.  Integrated Circuits which eliminated the horizontal hold control were starting to appear as early as 1969.<ref>{{cite web |last1=Mills |first1=Thomas |title=A five function IC for television receivers |url=https://www.researchgate.net/publication/3478408 |website=ResearchGate |publisher=IEEE |access-date=11 May 2019}}</ref>

The final generations of analog television receivers used IC-based designs where the receiver's timebases were derived from accurate crystal oscillators. With these sets, adjustment of the free-running frequency of either sweep oscillator was unnecessary and unavailable.

Horizontal and vertical hold controls were rarely used in CRT-based computer monitors, as the quality and consistency of components were quite high by the advent of the computer age, but might be found on some composite monitors used with the 1970s–80s home or personal computers.