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 interleavingodd and even lines. Each scan is known as a field So field rate is twice the frame rate. In each frame there are 405 lines So the line rate 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 sideband is suppressed. This technique is known as vestigial sideband modulation. 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 result in minimum 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. When testing with NTSC between November 1956 and April 1958, the colour subcarrier was 2.6578125 MHz with a 'Q' bandwidth of 340 kHz. 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. On the low-frequency side, a full 1.0 MHz sideband was radiated. 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
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 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 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.
