405-line television system

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 introduced with the BBC Television Service in 1936, suspended for the duration of World War II, and remained in operation in the UK until 1985. It was also used between 1961 and 1982 in Ireland, as well as from 1957 to 1973 for the Rediffusion Television cable service in Hong Kong. 405-line was approved as System A in the CCIR assignment of broadcast systems.

Sometimes called the Marconi-EMI system, it was developed in 1934 by the EMI Research Team led by Isaac Shoenberg.^[1] The figure of 405 lines had been chosen following discussions over Sunday lunch at the home of Alan Blumlein.^[2] The system used interlacing; EMI had been experimenting with a 243-line all-electronic interlaced system since 1933. In the 405 system the scanning lines were broadcast in two complementary fields, 50 times per second, creating 25 frames per second. The actual image was 376 lines high and interlaced, with additional unused lines making the frame up to 405 lines to give the slow circuitry time to prepare for the next frame; in modern terms it would be described as "376i".

At the time of its introduction the 405-line system was referred to as "high definition" – which it was, compared to earlier systems, although of lower definition than 625-line and later standards.

In the United States, the FCC had briefly approved a 405-line color television standard in October 1950, which was developed by CBS.^[3] The CBS system was incompatible with existing black-and-white receivers. It used a rotating color wheel, reduced the number of scan lines from 525 to 405, and increased the field rate from 60 to 144, but had an effective frame rate of only 24 frames per second.