System A
CCIR System A was the 405 line analog broadcast television system broadcast in the UK and Ireland. It was discontinued in 1985.
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 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. 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 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 specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.
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. 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.
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