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{{Short description|405-line analog television transmission format}}
{{redirect|System A|other uses|Systema (disambiguation)}}
{{Redirect|System A|other uses|Systema (disambiguation)}}
[[Comité consultatif international pour la radio|CCIR]] System '''A''' was the [[405-line television system|405 line]] analog broadcast [[television]] system broadcast in the [[United Kingdom|UK]] and [[Ireland]]. CCIR service was discontinued in 1985.
{{Use dmy dates|date=January 2020}}
'''[[ITU-R#CCIR|CCIR]] System A''' was the [[405-line television system|405-line]] analog [[Broadcast television systems|broadcast television system]] adopted in the UK and Ireland. System A service started in 1936 and was discontinued in 1985.


== Specifications ==
== Specifications ==
{{Main|405-line television system}}
{{Main|405-line television system}}
[[File:Channel spacing for CCIR television System A (VHF Bands).svg|frame|Channel spacing for CCIR television System A (VHF Bands)<br />The separation between the audio and video carriers is −3.5&nbsp;MHz.]]

Some of the important specs are listed below.
Some of the important specs are listed below.
* [[Frame rate]]: 25 Hz
* [[Interlaced video|Interlace]]: 2/1
* [[Refresh rate|Field rate]]: 50 Hz<ref>Not an independent value: 25 Hz•2=50 Hz</ref>
* Lines per [[Film frame#Video frames|frame]]: 405
* [[Scan line|Line rate]]: 10.125 kHz<ref>Not an independent value: 25 Hz•405=10,125 Hz</ref>
* [[Bandwidth (signal processing)|Visual bandwidth]]: 3&nbsp;MHz
* [[Modulation|Vision modulation]]: AC3 positive
* Preemphasis: none
* Sound modulation: A3
* Sound offset: −3.5&nbsp;MHz
* [[Broadcasting|Channel bandwidth]]: 5&nbsp;MHz


A frame is the total picture. The '''frame rate''' is the number of pictures displayed in one second. But each frame is actually scanned twice '''interleaving''' odd and even lines. Each scan is known as a field (odd and even fields.) So '''field rate''' is twice the frame rate. In each frame there are 405 '''lines''' (or 202.5 lines in a field.) So the '''line rate''' (line frequency) is 405 times the frame frequency or 405•25=10,125&nbsp;Hz.
{| class="wikitable"
|+System A specifications
|-
! Frame rate
! Interlace
! Field rate
! Line/frame
! Line rate
! Visual b/w
! Vision mod.
! Preemphasis
! Sound mod.
! Sound offset
! Channel b/w
|- align="center"
| align="center" | 25 || 2/1 || 50<ref>Not an independent value: 25•2=50</ref> || 405 || 10125<ref>Not an independent value: 25•405=10125</ref> || 3&nbsp;MHz. || AC3 pos. || - || A3 ||-3.5&nbsp;MHz.||5&nbsp;MHz.
|}
[[File:Channel spacing for CCIR television System A (VHF Bands).jpg|frame|Channel spacing for CCIR television System A (VHF Bands)<br>The separation between the audio and video carriers is -3.5 MHz.]]
A frame is the total picture. The '''frame rate''' is the number of pictures displayed in one second. But each frame is actually scanned twice '''interleaving''' odd and even lines. Each scan is known as a field (odd and even fields.) So '''field rate''' is twice the frame rate. In each frame there are 405 '''lines''' (or 202.5 lines in a field.) So the '''line rate''' (line frequency) is 405 times the frame frequency or 405•25=10125&nbsp;Hz.


The '''video bandwidth''' was 3.0&nbsp;MHz. The video signal modulates the carrier by [[Amplitude Modulation]], but a portion of the upper side band is suppressed. This technique is known as vestigial side band modulation ('''AC3'''). The polarity of modulation is positive, meaning that an increase in the instantaneous brightness of the [[video signal]] results in an increase in RF power and vice versa. Specifically, the sync pulses (being "blacker than black") result in minimum power (possibly zero power) from the vision transmitter.
The '''video bandwidth''' was 3.0&nbsp;MHz. The video signal modulates the carrier by [[amplitude modulation]], but a portion of the upper sideband is suppressed. This technique is known as [[Single-sideband modulation#Vestigial sideband (VSB)|vestigial sideband]] modulation ('''AC3'''). The polarity of modulation is positive, meaning that an increase in the instantaneous brightness of the [[Video|video signal]] results in an increase in RF power and vice versa. Specifically, the sync pulses (being "blacker than black") result in minimum power (possibly zero power) from the vision transmitter.


The [[audio signal]] was modulated by [[Amplitude modulation]].
The [[audio signal]] was modulated by [[amplitude modulation]].


The separation between the audio AM carrier and the video carrier is -3.5&nbsp;MHz.
The separation between the audio AM carrier and the video carrier is −3.5&nbsp;MHz.


The total '''RF bandwidth''' of System A was 4.26&nbsp;MHz, allowing System A to be transmitted in the 5.0&nbsp;MHz wide channels specified for television in the British VHF bands with an ample 740&nbsp;kHz guard zone between channels.
The total '''RF bandwidth''' of System A was 4.26&nbsp;MHz, allowing System A to be transmitted in the 5.0&nbsp;MHz wide channels specified for television in the British VHF bands with an ample 740&nbsp;kHz guard zone between channels.


In specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.
In specifications, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.


=== Colour TV ===
=== Colour TV ===
System '''A''' has variously been tested with the [[NTSC]], [[PAL]] and [[SECAM]] colour systems. However, apart from out-of-hours technical tests in the 1950s and 1960s, colour was never transmitted officially on System A.
System '''A''' has variously been tested with the [[NTSC]], [[PAL]] and [[SECAM]] colour systems. However, apart from out-of-hours technical tests in the 1950s and 1960s, colour was never transmitted officially on System A.


Colour tests were first radiated from [[Alexandra Palace transmitting station|Alexandra Palace]] from 7 October 1954.<ref>[http://www.earlytelevision.org/british_ntsc_color.html British Experiments with NTSC Color]</ref> When testing with NTSC between November 1956 and April 1958, the colour subcarrier was 2.6578125&nbsp;MHz with a 'Q' bandwidth of 340&nbsp;kHz (matching the rolloff of the luminance signal at +3.0&nbsp;MHz). On the low-frequency side, a full 1.0&nbsp;MHz single-sideband of the 'I' signal was radiated.<ref>http://downloads.bbc.co.uk/rd/pubs/archive/pdffiles/monographs/bbc_monograph_18.pdf</ref>
Colour tests were first radiated from [[Alexandra Palace television station|Alexandra Palace]] from 7 October 1954.<ref>[http://www.earlytelevision.org/british_ntsc_color.html British Experiments with NTSC Color]</ref> When testing with NTSC between November 1956 and April 1958, the colour subcarrier was 2.6578125&nbsp;MHz with a 'Q' bandwidth of 340&nbsp;kHz (matching the rolloff of the luminance signal at +3.0&nbsp;MHz). On the low-frequency side, a full 1.0&nbsp;MHz single-sideband of the 'I' signal was radiated.<ref>http://downloads.bbc.co.uk/rd/pubs/archive/pdffiles/monographs/bbc_monograph_18.pdf {{Bare URL PDF|date=March 2022}}</ref>


When testing with PAL, the colour subcarrier was 2.66034375&nbsp;MHz and the sidebands of the PAL signal had to be truncated on the high-frequency side at +330&nbsp;kHz (matching the rolloff of the luminance signal at +3.0&nbsp;MHz). On the low-frequency side, a full 1.0&nbsp;MHz sideband was radiated. ( This behaviour would cause massive U/V crosstalk in the NTSC system, but delay-line PAL hides such artefacts. )
When testing with PAL, the colour subcarrier was 2.66034375&nbsp;MHz and the sidebands of the PAL signal had to be truncated on the high-frequency side at +330&nbsp;kHz (matching the rolloff of the luminance signal at +3.0&nbsp;MHz). On the low-frequency side, a full 1.0&nbsp;MHz sideband was radiated. (This behaviour would cause massive U/V crosstalk in the NTSC system, but delay-line PAL hides such artefacts.)


When used with SECAM, the FM carrier was nominally 2.66&nbsp;MHz with a deviation of ±250&nbsp;kHz.
When used with SECAM, the FM carrier was nominally 2.66&nbsp;MHz with a deviation of ±250&nbsp;kHz.
Line 48: Line 43:
None of the above colour encoding systems had any effect on the bandwidth of system '''A''' as a whole.
None of the above colour encoding systems had any effect on the bandwidth of system '''A''' as a whole.


=== Improved Audio===
=== Improved audio ===
No changes were made to the audio specification over the 49 years that the 405-line system was in use.
No changes were made to the audio specification over the 49 years that the 405-line system was in use.


== Transmission Channels ==
== Transmission channels ==
[[File:VHF Usage.svg|thumb|400px|right|Plan showing VHF frequency ranges for ITU Systems]]
System '''A''' was the first formal broadcasting standard in the world. A European 41-68&nbsp;MHz [[Band I]] television allocation was agreed at the 1947 ITU (International Telecommunications Union) conference in 1947, effectively "grandfathering in" the VHF allocation that has been used in Britain since 1936.
System '''A''' was the first formal broadcasting standard in the world. A European 41–68&nbsp;MHz [[Band I]] television allocation was agreed at the 1947 ITU (International Telecommunication Union) conference in 1947, effectively "grandfathering in" the VHF allocation that has been used in Britain since 1936.


=== United Kingdom: 1936 - 1985, Ireland 1961 - 1982 ===
=== United Kingdom: 1936 – 1985, Ireland 1961 – 1982 ===
{| class="wikitable"
{| class="wikitable"
! {{No2}}Channel
! {{No2}}Channel limits (MHz)
! {{No2}}Vision carrier frequency (MHz)
! {{No2}}Audio carrier frequency (MHz)
|-
|-
| B1 †
!{{No2}}Channel
| 41.25 – 46.25
!{{No2}}Channel Limits (MHz)
| 45.00
!{{No2}}Vision Carrier Frequency (MHz)
| 41.50
!{{No2}}Audio Carrier Frequency (MHz)
|-
|-
| B2
|B1 †||41.25 - 46.25||45.00||41.50
| 48.00 – 53.00
| 51.75
| 48.25
|-
|-
| B3
|B2||48.00 - 53.00||51.75||48.25
| 53.00 – 58.00
| 56.75
| 53.25
|-
|-
| B4
|B3||53.00 - 58.00||56.75||53.25
| 58.00 – 63.00
| 61.75
| 58.25
|-
|-
| B5
|B4||58.00 - 63.00||61.75||58.25
| 63.00 – 68.00
|-
|B5||63.00 - 68.00||66.75||63.25
| 66.75
| 63.25
|}
|}


† Channel limits of the original transmitter at [[Alexandra Palace transmitting station|Alexandra Palace]] in [[London]] were 41.25 - 48.00 DSB from 1936 to 1956. Every other transmitter on channel B1 used VSB to save bandwidth and transmission power.
† Channel limits of the original transmitter at [[Alexandra Palace transmitting station|Alexandra Palace]] in London were 41.25 – 48.00 DSB from 1936 to 1956. Every other transmitter on channel B1 used VSB to save bandwidth and transmission power.


{| class="wikitable"
{| class="wikitable"
! {{No2}}Channel
! {{No2}}Channel limits (MHz)
! {{No2}}Vision carrier frequency (MHz)
! {{No2}}Audio carrier frequency (MHz)
|-
|-
| B6
!{{No2}}Channel
| 176.00 – 181.00
!{{No2}}Channel Limits (MHz)
| 179.75
!{{No2}}Vision Carrier Frequency (MHz)
| 176.25
!{{No2}}Audio Carrier Frequency (MHz)
|-
|-
| B7
|B6||176.00 - 181.00||179.75||176.25
| 181.00 – 186.00
| 184.75
| 181.25
|-
|-
| B8
|B7||181.00 - 186.00||184.75||181.25
| 186.00 – 191.00
| 189.75
| 186.25
|-
|-
| B9
|B8||186.00 - 191.00||189.75||186.25
| 191.00 – 196.00
| 194.75
| 191.25
|-
|-
| B10
|B9||191.00 - 196.00||194.75||191.25
| 196.00 – 201.00
| 199.75
| 196.25
|-
|-
| B11
|B10||196.00 - 201.00||199.75||196.25
| 201.00 – 206.00
| 204.75
| 201.25
|-
|-
| B12
|B11||201.00 - 206.00||204.75||201.25
| 206.00 – 211.00
| 209.75
| 206.25
|-
|-
| B13
|B12||206.00 - 211.00||209.75||206.25
| 211.00 – 216.00
| 214.75
| 211.25
|-
|-
| B14 §
|B13||211.00 - 216.00||214.75||211.25
| 216.00 – 221.00
|-
| 219.75
|B14 §||216.00 - 221.00||219.75||216.25
| 216.25
|}
|}


Line 104: Line 140:


== See also ==
== See also ==
* [[Broadcast television systems]]

* [[Television transmitter]]
*[[Broadcast television systems]]
* [[Transposer]]
*[[Television transmitter]]
*[[Transposer]]


== Notes and references ==
== Notes and references ==
{{reflist}}
<references/>


== External links ==
== External links ==
* [https://web.archive.org/web/20120830232230/http://www.pembers.freeserve.co.uk/World-TV-Standards/Transmission-Systems.html World Analogue Television Standards and Waveforms]
* [https://web.archive.org/web/20120830232230/http://www.pembers.freeserve.co.uk/World-TV-Standards/Transmission-Systems.html World Analogue Television Standards and Waveforms]
* [http://www.paradiso-design.net/weltweite_fernsehsysteme.html Fernsehnormen aller Staaten und Gebiete der Welt]
* [http://www.paradiso-design.net/weltweite_fernsehsysteme.html Fernsehnormen aller Staaten und Gebiete der Welt]{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}
* [http://www.stjarnhimlen.se/tv/tv.html Analog TV Broadcast Systems]
* [http://www.stjarnhimlen.se/tv/tv.html Analog TV Broadcast Systems]
* [https://web.archive.org/web/20070518102958/http://www.irish-tv.com/405.asp 405-line TV in the Republic of Ireland]
* [https://web.archive.org/web/20070518102958/http://www.irish-tv.com/405.asp 405-line TV in the Republic of Ireland]
Line 124: Line 159:
[[Category:ITU-R recommendations]]
[[Category:ITU-R recommendations]]
[[Category:Television technology]]
[[Category:Television technology]]
[[Category:Video formats|A, System]]
[[Category:Video formats]]
[[Category:Broadcast engineering]]
[[Category:Broadcast engineering]]
[[Category:CCIR System]]
[[Category:CCIR System]]
[[Category:Standards of the United Kingdom]]

Revision as of 16:32, 13 August 2024

"System A" redirects here. For other uses, see Systema (disambiguation).

CCIR System A was the 405-line analog broadcast television system adopted in the UK and Ireland. System A service started in 1936 and was discontinued in 1985.

Specifications

Main article: 405-line television system
Channel spacing for CCIR television System A (VHF Bands)
The separation between the audio and video carriers is −3.5 MHz.

Some of the important specs are listed below.

A frame is the total picture. The frame rate is the number of pictures displayed in one second. But each frame is actually scanned twice interleaving odd and even lines. Each scan is known as a field (odd and even fields.) So field rate is twice the frame rate. In each frame there are 405 lines (or 202.5 lines in a field.) So the line rate (line frequency) is 405 times the frame frequency or 405•25=10,125 Hz.

The video bandwidth was 3.0 MHz. The video signal modulates the carrier by amplitude modulation, but a portion of the upper sideband is suppressed. This technique is known as vestigial sideband modulation (AC3). The polarity of modulation is positive, meaning that an increase in the instantaneous brightness of the video signal results in an increase in RF power and vice versa. Specifically, the sync pulses (being "blacker than black") result in minimum power (possibly zero power) from the vision transmitter.

The audio signal was modulated by amplitude modulation.

The separation between the audio AM carrier and the video carrier is −3.5 MHz.

The total RF bandwidth of System A was 4.26 MHz, allowing System A to be transmitted in the 5.0 MHz wide channels specified for television in the British VHF bands with an ample 740 kHz guard zone between channels.

In specifications, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.

Colour TV

System A has variously been tested with the NTSC, PAL and SECAM colour systems. However, apart from out-of-hours technical tests in the 1950s and 1960s, colour was never transmitted officially on System A.

Colour tests were first radiated from Alexandra Palace from 7 October 1954.[3] When testing with NTSC between November 1956 and April 1958, the colour subcarrier was 2.6578125 MHz with a 'Q' bandwidth of 340 kHz (matching the rolloff of the luminance signal at +3.0 MHz). On the low-frequency side, a full 1.0 MHz single-sideband of the 'I' signal was radiated.[4]

When testing with PAL, the colour subcarrier was 2.66034375 MHz and the sidebands of the PAL signal had to be truncated on the high-frequency side at +330 kHz (matching the rolloff of the luminance signal at +3.0 MHz). On the low-frequency side, a full 1.0 MHz sideband was radiated. (This behaviour would cause massive U/V crosstalk in the NTSC system, but delay-line PAL hides such artefacts.)

When used with SECAM, the FM carrier was nominally 2.66 MHz with a deviation of ±250 kHz.

None of the above colour encoding systems had any effect on the bandwidth of system A as a whole.

Improved audio

No changes were made to the audio specification over the 49 years that the 405-line system was in use.

Transmission channels

Plan showing VHF frequency ranges for ITU Systems

System A was the first formal broadcasting standard in the world. A European 41–68 MHz Band I television allocation was agreed at the 1947 ITU (International Telecommunication Union) conference in 1947, effectively "grandfathering in" the VHF allocation that has been used in Britain since 1936.

United Kingdom: 1936 – 1985, Ireland 1961 – 1982

Channel Channel limits (MHz) Vision carrier frequency (MHz) Audio carrier frequency (MHz)
B1 † 41.25 – 46.25 45.00 41.50
B2 48.00 – 53.00 51.75 48.25
B3 53.00 – 58.00 56.75 53.25
B4 58.00 – 63.00 61.75 58.25
B5 63.00 – 68.00 66.75 63.25

† Channel limits of the original transmitter at Alexandra Palace in London were 41.25 – 48.00 DSB from 1936 to 1956. Every other transmitter on channel B1 used VSB to save bandwidth and transmission power.

Channel Channel limits (MHz) Vision carrier frequency (MHz) Audio carrier frequency (MHz)
B6 176.00 – 181.00 179.75 176.25
B7 181.00 – 186.00 184.75 181.25
B8 186.00 – 191.00 189.75 186.25
B9 191.00 – 196.00 194.75 191.25
B10 196.00 – 201.00 199.75 196.25
B11 201.00 – 206.00 204.75 201.25
B12 206.00 – 211.00 209.75 206.25
B13 211.00 – 216.00 214.75 211.25
B14 § 216.00 – 221.00 219.75 216.25

§ Allocated, but never used.

See also

Notes and references

  1. ^ Not an independent value: 25 Hz•2=50 Hz
  2. ^ Not an independent value: 25 Hz•405=10,125 Hz
  3. ^ British Experiments with NTSC Color
  4. ^ http://downloads.bbc.co.uk/rd/pubs/archive/pdffiles/monographs/bbc_monograph_18.pdf [bare URL PDF]
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