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 1982 in Ireland and 1985 in Britain.

Specifications

Some of the important specs are listed below. *

Frame rate Frame rate, most commonly expressed in frame/s, or FPS, is typically the frequency (rate) at which consecutive images (Film frame, frames) are captured or displayed. This definition applies to film and video cameras, computer animation, and moti ...

: 25 Hz * Interlace: 2/1 *

Field rate Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a gras ...

: 50 Hz * Lines per

frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

: 405 *

Line rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...

: 10.125 kHz * Visual bandwidth: 3 MHz * Vision modulation: AC3 positive * Preemphasis: none * Sound modulation: A3 * Sound offset: -3.5 MHz * Channel bandwidth: 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=10,125 Hz. The video bandwidth was 3.0 MHz. The video signal modulates the carrier by

amplitude modulation Amplitude modulation (AM) is a signal modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the instantaneous amplitude of the wave is varied in proportion t ...

, 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 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) ...

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 signal (signal processing), digital signals. Audio signals have frequencies i ...

was modulated by

. 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 NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...

PAL Phase Alternating Line (PAL) is a color encoding system for analog 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 ''sequential colour memory''), is an analog color television system that was used in France, Russia and some other countries or territories of Europe and Africa. ...

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 an entertainment and sports venue in North London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. A listed building, Grade II listed building, it is built on the site of Tottenham Wood and th ...

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. 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.

Audio

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

Transmission channels

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 ...

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

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.

Notes and references

External links

World Analogue Television Standards and Waveforms

405-line TV in the Republic of Ireland

BBC Report on the state of Television in Southern Ireland
{{Analogue TV transmitter topics ITU-R recommendations Television technology Video formats Broadcast engineering CCIR System Standards of the United Kingdom