Question: For how long did Rex Brough's employer alternate different broadcasting systems?

full_context: Document 1: Rob King: Rob King (born 1970 in Los Angeles) has (along with Paul Romero) composed the music for the Heroes of Might and Magic and the Might and Magic series. He also created music and character sound effects for Everquest: Gates of Discord for Sony Online. In the early 2000s (decade), he was a member of the alternative rock band Red Delicious.
Document 2: Brian Rust: Rust was born in London, and collected records from the age of five. He worked in the BBC's record library from 1945 to 1960, and supervised broadcasting selections. He wrote for "The Gramophone" from 1948 to 1970, and wrote freelance from 1960, including liner notes for record releases , such as  Bernard Miles' ' Over the Gate ' , ep., in 1956.
Document 3: BBC Television: After a series of test transmissions and special broadcasts that began in August, regular BBC television broadcasts officially resumed on 1 October 1936, from a converted wing of Alexandra Palace in London, which housed two studios, various scenery stores, make-up areas, dressing rooms, offices, and the transmitter itself, now broadcasting on the VHF band. BBC television initially used two systems, on alternate weeks: the 240-line Baird intermediate film system and the 405-line Marconi-EMI system, each making the BBC the world's first regular high-definition television service, broadcasting Monday to Saturday from 15:00 to 16:00 and 21:00 to 22:00.
Document 4: John Baker (Radiophonic musician): He was educated at the Royal Academy of Music where he studied piano and composition. In 1960 he joined the BBC as a sound mixer, before transferring, in 1963, to the BBC Radiophonic Workshop where he remained until 1974. He was the most prolific of the early Workshop composers, developing a trademark style, creating music by manipulating tapes of everyday sounds such as blowing across the top of an empty bottle. A rare snippet of Baker at work was included in the 1968 documentary film "Music", which also featured the Beatles working on "Hey Jude" in the studio. A jazz pianist, he brought a sense of rhythm to the Workshop which some of the other more mathematical composers lacked. His work included many signature tunes for BBC television and radio. He was also particularly interested in combining recorded electronic music with live musicians.
Document 5: BBC Television: The two systems were to run on a trial basis for six months; early television sets supported both resolutions. However, the Baird system, which used a mechanical camera for filmed programming and Farnsworth image dissector cameras for live programming, proved too cumbersome and visually inferior, and ended with closedown (at 22:00) on Saturday 13 February 1937.
Document 6: Rex Forrester: Rex Forrester (8 November 1928 – 31 August 2001) was a recognised New Zealand hunting and fishing specialist. He was known for his deer culling, helicopter hunting, live deer capture, pig hunting and trout fishing within New Zealand and Australia. He was also an outdoor sports author who published nine books.
Document 7: BBC Television: Mechanically scanned, 30-line television broadcasts by John Logie Baird began in 1929, using the BBC transmitter in London, and by 1930 a regular schedule of programmes was transmitted from the BBC antenna in Brookmans Park. Television production was switched from Baird's company to what is now known as BBC One on 2 August 1932, and continued until September 1935. Regularly scheduled electronically scanned television began from Alexandra Palace in London on 2 November 1936, to just a few hundred viewers in the immediate area. The first programme broadcast – and thus the first ever, on a dedicated TV channel – was "Opening of the BBC Television Service" at 15:00. The first major outside broadcast was the coronation of King George VI and Queen Elizabeth in May 1937. The service was reaching an estimated 25,000–40,000 homes before the outbreak of World War II which caused the service to be suspended in September 1939. The VHF broadcasts would have provided an ideal radio beacon for German bombers homing in on London, and the engineers and technicians of the service would be needed for the war effort, in particular the radar programme.
Document 8: High-definition television: Since the formal adoption of digital video broadcasting's (DVB) widescreen HDTV transmission modes in the early 2000s; the 525-line NTSC (and PAL-M) systems, as well as the European 625-line PAL and SECAM systems, are now regarded as standard definition television systems.
Document 9: King Robbo: King Robbo (born John Robertson, 23 October 1969 – 31 July 2014) was an English underground graffiti artist. His feud with the artist Banksy was the subject of a Channel 4 television documentary called "Graffiti Wars", first shown in August 2011.
Document 10: John, King of England: Contemporary chroniclers were mostly critical of John's performance as king, and his reign has since been the subject of significant debate and periodic revision by historians from the 16th century onwards. Historian Jim Bradbury has summarised the contemporary historical opinion of John's positive qualities, observing that John is today usually considered a "hard-working administrator, an able man, an able general". Nonetheless, modern historians agree that he also had many faults as king, including what historian Ralph Turner describes as "distasteful, even dangerous personality traits", such as pettiness, spitefulness and cruelty. These negative qualities provided extensive material for fiction writers in the Victorian era, and John remains a recurring character within Western popular culture, primarily as a villain in films and stories depicting the Robin Hood legends.
Document 11: BBC Television: Baird Television Ltd. made Britain's first television broadcast, on 30 September 1929 from its studio in Long Acre, London, via the BBC's London transmitter, using the electromechanical system pioneered by John Logie Baird. This system used a vertically-scanned image of 30 lines – just enough resolution for a close-up of one person, and with a bandwidth low enough to use existing radio transmitters. Simultaneous transmission of sound and picture was achieved on 30 March 1930, by using the BBC's new twin transmitter at Brookmans Park. By late 1930, 30 minutes of morning programmes were broadcast Monday to Friday, and 30 minutes at midnight on Tuesdays and Fridays, after BBC radio went off the air. Baird broadcasts via the BBC continued until June 1932.
Document 12: Rex Brough: Rex Brough "aka King John and the Red King" (born 1960 in Meriden, Warwickshire) is an English record producer and editor of radio programmes, mainly for the BBC.
Document 13: Color television: After a series of hearings beginning in September 1949, the FCC found the RCA and CTI systems fraught with technical problems, inaccurate color reproduction, and expensive equipment, and so formally approved the CBS system as the U.S. color broadcasting standard on October 11, 1950. An unsuccessful lawsuit by RCA delayed the first commercial network broadcast in color until June 25, 1951, when a musical variety special titled simply Premiere was shown over a network of five East Coast CBS affiliates. Viewing was again restricted: the program could not be seen on black - and - white sets, and Variety estimated that only thirty prototype color receivers were available in the New York area. Regular color broadcasts began that same week with the daytime series The World Is Yours and Modern Homemakers.
Document 14: Bror Rexed: Bror Rexed (June 19, 1914 – August 21, 2002) was a Swedish neuroscientist and professor at Uppsala University. Internationally, he is best known today for his development of the system now known as Rexed laminae, but in Sweden, he is also known for his involvement in the "du-reformen" of the Swedish language.
Document 15: BBC Television: After a series of test transmissions and special broadcasts that began in August 1936, the BBC Television Service officially launched on 2 November 1936 from a converted wing of Alexandra Palace in London. ``Ally Pally ''housed two studios, various scenery stores, make - up areas, dressing rooms, offices, and the transmitter itself, which then broadcast on the VHF band. BBC television initially used two systems on alternate weeks: the 240 - line Baird intermediate film system and the 405 - line Marconi - EMI system. The use of both formats made the BBC's service the world's first regular high - definition television service; it broadcast from Monday to Saturday between 15: 00 and 16: 00, and 21: 00 and 22: 00. The first programme broadcast -- and thus the first ever, on a dedicated TV channel -- was`` Opening of the BBC Television Service'' at 15: 00. The first major outside broadcast was the coronation of King George VI and Queen Elizabeth in May 1937.
ground_truth: six months
sub_queries: What is Rex Brough's place of employment? Answer: BBC
For how long did the BBC alternate different broadcasting systems? Answer: six months