CCIR System A was the
405-line
The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting. The number of television lines influences the image resolution, or quality of the picture.
It was ...
analog
broadcast television system
Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.
Analog television systems were standardized by ...
adopted in the UK and Ireland. System A service started in 1936 and was discontinued in 1985.
Specifications
Some of the important specs are listed below.
![Channel spacing for CCIR television System A (VHF Bands)](https://upload.wikimedia.org/wikipedia/commons/3/36/Channel_spacing_for_CCIR_television_System_A_%28VHF_Bands%29.svg)
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 Hz.
The video bandwidth was 3.0 MHz. The video signal modulates the carrier by
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
, but a portion of the upper sideband is suppressed. This technique is known as
vestigial sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
modulation (AC3). The polarity of modulation is positive, meaning that an increase in the instantaneous brightness of the
video signal
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) systems ...
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
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
was modulated by
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
.
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
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
,
PAL
Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
and
SECAM
SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog color television system that was used in France, some parts of Europe and Africa, and Russia. It was one of th ...
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
Alexandra Palace is a Grade II listed entertainment and sports venue in London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. It is built on the site of Tottenham Wood and the later Tottenham Wood Farm. Origi ...
from 7 October 1954. 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.
[http://downloads.bbc.co.uk/rd/pubs/archive/pdffiles/monographs/bbc_monograph_18.pdf ]
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
![VHF Usage](https://upload.wikimedia.org/wikipedia/commons/c/cc/VHF_Usage.svg)
System A was the first formal broadcasting standard in the world. A European 41–68 MHz
Band I
Band I is a range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum. The first time there was defined "for simplicity" in Annex 1 of "Final acts of the European Broadcasting Conference in the VHF and U ...
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 limits of the original transmitter at
Alexandra Palace
Alexandra Palace is a Grade II listed entertainment and sports venue in London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. It is built on the site of Tottenham Wood and the later Tottenham Wood Farm. Origi ...
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.
§ Allocated, but never used.
See also
*
Broadcast television systems
Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.
Analog television systems were standardized by ...
*
Television transmitter
A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized a ...
*
Transposer
In broadcasting, a transposer or translator is a device in or beyond the service area of a radio or television station transmitter that rebroadcasts signals to receivers which can’t properly receive the signals of the transmitter because of a p ...
Notes and references
External links
World Analogue Television Standards and Waveforms405-line TV in the Republic of IrelandBBC Report on the state of Television in Southern Ireland
{{Analogue TV transmitter topics
ITU-R recommendations
Television technology
A, System
Broadcast engineering
CCIR System
Standards of the United Kingdom