Editing Colorburst

{{Short description|Synchronisation signal in colour video}}
{{for|the instant camera range|Kodak Colorburst}}
{{Refimprove|date=September 2014}}
<!-- Dab via redirect as per [[WP:DABREDIR]] -->

[[File:Commodore 64 PAL horizontal sync and colourburst.jpg|200px|right|thumb|Horizontal sync and color burst of the composite output of a [[Commodore 64]] computer]]

'''Colorburst''' is an [[analog video|analog]] and [[composite video]] signal generated by a [[video-signal generator]] used to keep the [[chrominance subcarrier]] synchronized in a [[color television]] signal. By synchronizing an [[oscillator]] with the colorburst at the [[Analog television#Structure of a video signal|back porch]] (beginning) of each [[scan line]], a television receiver is able to restore the suppressed carrier of the [[chrominance]] (color) signals, and in turn decode the color information. The most common use of colorburst is to [[genlock]]  equipment together as a common reference with a [[vision mixer]] in a [[television studio]] using a [[multi-camera setup]].

== Explanation ==

In [[NTSC]], its frequency is exactly 315/88&nbsp;= 3.579{{Overline|54}}{{Efn|name=repeating decimal|[[Repeating decimal#Notation|Repeating decimal notation]]}}&nbsp;[[MHz]] with a phase of 180°. [[PAL]] uses a frequency of exactly 4.43361875&nbsp;MHz, with its phase alternating between 135° and 225° from line to line. Since the colorburst signal has a known amplitude, it is sometimes used as a reference level when compensating for amplitude variations in the overall signal.

[[SECAM]] is unique in not having a colorburst signal, since the chrominance signals are encoded using [[Frequency modulation|FM]] rather than [[QAM]], thus the signal phase is immaterial and no reference point is needed.

== Rationale for NTSC Color burst frequency ==
{{Details|NTSC#Color encoding}}

The original black and white NTSC television standard specified a frame rate of 30&nbsp;Hz and 525 lines per frame, or 15750 lines per second. The audio was frequency modulated 4.5&nbsp;MHz above the video signal.  Because this was black and white, the video consisted only of luminance (brightness) information. Although all of the space in between was occupied, the line-based nature of the video information meant that the luminance data was not spread uniformly across the [[frequency domain]]; it was concentrated at multiples of the line rate. Plotting the video signal on a [[spectrogram]] gave a signature that looked like the teeth of a comb or a gear, rather than smooth and uniform.

[[RCA]] discovered<ref>{{Cite journal|last=Brown and Luck|date=June 1953|title=PRINCIPLES AND DEVELOPMENT OF COLOR TELEVISION SYSTEMS|url=http://www.americanradiohistory.com/ARCHIVE-RCA/RCA-Review/RCA-Review-1953-June.pdf|journal=RCA Review|volume=XIV|pages=155–156}}</ref> that if the [[chrominance]] (color) information, which had a similar spectrum, was modulated on a carrier that was a [[half-integer]] multiple of the line rate, its signal peaks would fit neatly between the peaks of the luminance data and interference was minimized. It was not eliminated, but what remained was not readily apparent to human eyes. (Modern televisions attempt to reduce this interference further using a [[comb filter]].)

To provide sufficient bandwidth for the chrominance signal, yet interfere only with the highest-frequency (and thus least perceptible) portions of the luminance signal, a chrominance subcarrier near 3.6&nbsp;MHz was desirable. 227.5&nbsp;= 455/2 times the line rate was close to the right number, and 455's small factors (5&nbsp;× 7&nbsp;× 13) make a divider easy to construct.

However, additional interference could come from the [[audio signal]]. To minimize interference there, it was similarly desirable to make the distance between the chrominance [[carrier frequency]] and the audio carrier frequency a half-integer multiple of the line rate. The sum of these two half-integers implies that the distance between the frequency of the luminance carrier and audio carrier must be an integer multiple of the line rate. However, the original NTSC standard, with a 4.5&nbsp;MHz carrier spacing and a 15750&nbsp;Hz line rate, did not meet this requirement: the audio was 285.714 times the line rate.

While existing black and white receivers could not decode a signal with a different audio carrier frequency, they could easily use the copious timing information in the video signal to decode a slightly slower line rate.  Thus, the new color television standard reduced the line rate by a factor of 1.001 to 1/286 of the 4.5&nbsp;MHz audio subcarrier frequency, or about 15734.2657&nbsp;Hz. This reduced the frame rate to 30/1.001&nbsp;≈ 29.9700&nbsp;Hz, and placed the color subcarrier at 227.5/286 = 455/572 = 35/44 of the 4.5&nbsp;MHz audio subcarrier.<ref>{{Cite web|url=https://www.antiqueradio.org/art/NTSC%20Signal%20Specifications.pdf|title=NTSC SIGNAL SPECIFICATIONS|date=23 May 2018|website=Antique Radio.org}}</ref>

== Crystals ==
<!-- "Colorburst crystal" redirects here. -->
An NTSC or PAL television's color decoder contains a colorburst [[crystal oscillator]].

Because so many analog color TVs [[Digital television transition|were produced]] from the 1960s to the early 2000s, [[economies of scale]] drove down the cost of colorburst crystals, so they were often used in various other applications, such as [[Electronic oscillator|oscillators]] for [[microprocessor]]s or for [[amateur radio]]: 3.5795&nbsp;MHz has since become a common [[QRP operation|QRP]] calling frequency in the [[80-meter band]], and its [[frequency multiplier|doubled]] frequency of 7.159&nbsp;MHz is a common calling frequency in the [[40-meter band]]. Tripling this frequency is also how [[FM broadcasting|FM radio]] circuits came to use a nominally 10.7 MHz [[intermediate frequency]] in [[Superheterodyne transmitter|superheterodyne]] conversion.

{{Table alignment}}
{| class="wikitable sortable col3center"
|+ Non-television uses of NTSC color burst frequency<br/>
(generally only in the NTSC version of each device):<br/>
''f''&nbsp;= 315&nbsp;MHz/88&nbsp;= 3.579{{Overline|54}}{{Efn|name=repeating decimal}}&nbsp;MHz
! Component || Frequency || Ratio
|-
| [[Intellivision]] CPU ||rowspan=2| 0.8949&nbsp;MHz ||rowspan=2| {{Sfrac|4}}''f''
|-
| [[TRS-80 Color Computer]] CPU (normal speed)
|-
| [[Apple II]] CPU (short cycles only, one in 65 cycles is longer)||rowspan=4| 1.0227&nbsp;MHz ||rowspan=4| {{Sfrac|2|7}}''f''
|-
| [[VIC-20]] CPU
|-
| [[Commodore 64]] CPU
|-
| [[Commodore 128]] CPU (SLOW & [[Commodore 64|C64]] compatible modes)
|-
| [[Atari 2600]] CPU<ref name="atari2600">
 Atari 2600 Specifications http://problemkaputt.de/2k6specs.htm</ref> ||rowspan=2| 1.1932&nbsp;MHz ||rowspan=2| {{Sfrac|3}}''f''
|-
| [[Intel 8253]] interval timer in IBM PC (remains in use {{As of|2008|alt=today}})
|-
| [[Fairchild Channel F|Fairchild Video Entertainment System]] CPU ||rowspan=6| 1.7898&nbsp;MHz ||rowspan=6| {{Sfrac|2}}''f''
|-
| [[Magnavox Odyssey 2|Odyssey 2]] CPU
|-
| [[Atari 8-bit computers]]<ref name="altirraHRM">{{cite book |title=Altirra Hardware Reference Manual |chapter=3.3 Cycle Timings |page=43 |first=Avery |last=Lee |date=2024-09-21 |access-date=2024-11-27 |url=http://www.virtualdub.org/downloads/Altirra%20Hardware%20Reference%20Manual.pdf#page=43}}</ref> and [[Atari 7800]] CPU
|-
| [[Commodore Plus/4]] CPU
|-
| [[Nintendo Entertainment System]] CPU
|-
| TRS-80 Color Computer 3 CPU (fast mode)
|-
| [[Commodore 128]] CPU (FAST & CP/M modes) || 2.0454&nbsp;MHz || {{Sfrac|4|7}}''f''
|-
|rowspan=2| [[Super Nintendo Entertainment System]] CPU || 2.6847&nbsp;MHz || {{Sfrac|3|4}}''f''
|-
<!--For some reason, adding "rowspan=7" to the following doesn't work.-->
| 3.5795&nbsp;MHz ||style=text-align:center| ''f''
|-
| [[Master System]] CPU ||rowspan=6| 3.5795&nbsp;MHz ||rowspan=6| ''f''
|-
| [[MSX]] CPU
|-
| [[Amateur radio]] Tx/Rx crystal for 80m band
|-
| [[ColecoVision]] CPU
|-
| [[Yamaha_OPL|Yamaha OPL and OPL2]] FM synthesis sound chips
|-
| [[Advanced Configuration and Power Interface|ACPI]] power management timer
|-
| CPU of [[IBM Personal Computer]] 5150 || 4.7727&nbsp;MHz || {{Sfrac|4|3}}''f''
|-
| Commodore [[Amiga]] CPU || rowspan="5" | 7.1591&nbsp;MHz || rowspan="5" | 2''f''
|-
| CPU of [[Tandy 1000#Tandy 1000 TX and Tandy 1000 SX|Tandy 1000 SX]]<ref>In normal (high speed) mode. The 1000 SX also has a slower 4.77 MHz mode ({{Frac|4|3}}''f''&nbsp;) for IBM PCjr compatibility.</ref> (and many other IBM PC-XT clones)
|-
| NEC [[TurboGrafx-16]] CPU
|-
| [[Yamaha TX81Z]] synthesizer CPU
|-
| [[Amateur radio]] Tx/Rx crystal for 40m band
|-
| [[Sega Genesis]] CPU || 7.6705&nbsp;MHz || {{Sfrac|15|7}}''f''  <!-- The Genesis has a 53.693 MHz (15 times f) crystal oscillator, as can be seen here: https://d3nevzfk7ii3be.cloudfront.net/igi/U5BMTgUf53DjnSxe -->
|-
|Intermediate frequency of FM radio superheterodyne circuits
|10.7386 MHz
|3''f''
|-
| [[High Precision Event Timer]] (typical) || 14.3181&nbsp;MHz || 4''f''
|}

{{Table alignment}}
{| class="wikitable sortable col3center"
|+ Non-television uses of PAL colour burst frequency<br/>
(generally only in the PAL version of each device):<br/>
''f''&nbsp;= 283.75 × 15625&nbsp;Hz + 25&nbsp;Hz = {{val|4.43361875|u=MHz}}
! Component || Frequency || Ratio
|-
| [[Commodore 64]] CPU ||rowspan=2| 0.9852&nbsp;MHz ||rowspan=2| {{Sfrac|2|9}}''f''
|-
| [[Commodore 128]] CPU (SLOW & [[Commodore 64|C64]] compatible modes)
|-
| [[Atari 2600]] CPU || 1.182298&nbsp;MHz<ref name="atari2600"/> || {{Sfrac|4|15}}''f''
|-
| [[VIC-20]] CPU || 1.1084&nbsp;MHz || {{Sfrac|1|4}}''f''
|-
| [[Nintendo Entertainment System]] CPU<ref name="nesdev">[http://wiki.nesdev.com/w/index.php/Clock_rate NES DEV wiki: Clock Rate]</ref> || 1.662607&nbsp;MHz || {{Sfrac|3|8}}''f''
|-
| [[Atari 8-bit computers]] CPU<ref name="altirraHRM"/> ||rowspan=2| 1.7734475&nbsp;MHz ||rowspan=2| {{Sfrac|2|5}}''f''
|-
| [[Dendy (console)]] NES clone CPU<ref name=nesdev/>
|-
| [[Commodore 128]] CPU (FAST & CP/M modes) || 1.9704&nbsp;MHz || {{Sfrac|4|9}}''f''
|-
|rowspan=2| [[Super Nintendo Entertainment System]] CPU ||  2.6601712&nbsp;MHz || {{Sfrac|3|5}}''f''
|-
| 3.546895&nbsp;MHz ||style=text-align:center| {{Sfrac|4|5}}''f''
|-
| Commodore [[Amiga]] CPU || 7.09379&nbsp;MHz || {{Sfrac|8|5}}''f''
|}

== See also ==

* [[Camera control unit]]
* [[Color framing]]
* [[Color killer]]
* [[Sync pulse]]
* [[Glossary of video terms]]

== References ==

{{Reflist}}

== Notes ==

{{Notelist}}

{{Analogue TV transmitter topics}}

[[Category:Video signal]]
[[Category:Television terminology]]