Question: How long did the network that broadcast Pigeon street alternate different broadcasting systems?

full_context: Document 1: Pigeon Street: Pigeon Street is an animated children's television series, written by Michael Cole, originally shown on the BBC in 1981 as part of its 'See-Saw' strand for preschoolers. There were two series with eight and five episodes respectively, each programme lasting 15 minutes. The series has been repeated a number of times.
Document 2: High-definition television: Colour broadcasts started at similarly higher resolutions, first with the US NTSC color system in 1953, which was compatible with the earlier monochrome systems and therefore had the same 525 lines of resolution. European standards did not follow until the 1960s, when the PAL and SECAM color systems were added to the monochrome 625 line broadcasts.
Document 3: 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 4: Only Fools and Horses: Only Fools and Horses (stylised as Only Fools and Horses...) is a British television sitcom created and written by John Sullivan. Seven series were originally broadcast on BBC One in the United Kingdom from 1981 to 1991, with sixteen sporadic Christmas specials aired until the end of the show in 2003. Episodes are regularly repeated on UKTV comedy channel Gold, Yesterday and occasionally repeated on BBC One.
Document 5: Bagpuss: Bagpuss is a British children's television series, made by Peter Firmin and Oliver Postgate through their company Smallfilms. The series of 13 episodes was first broadcast from 12 February 1974 to 7 May 1974. The title character was "a saggy, old cloth cat, baggy, and a bit loose at the seams". Although only 13 episodes were made, it remains fondly remembered, and was frequently repeated in the UK for 13 years. In 1999 "Bagpuss" topped a BBC poll for the UK's favourite children's TV programme.
Document 6: 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 7: 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 8: 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 9: Street Cents: Street Cents is a teen-themed newsmagazine TV series that originally aired on CBC Television between 1989 and 2006. It was one of a few shows focused on consumer and media awareness for young people. The series was created by producer John Nowlan. He cites Britain's "Pocket Money" as one of the inspirations for "Street Cents". The show has won several Gemini Awards and even an International Emmy for Best Youth Programming or Series. The series was lauded by critics for its efforts to be inclusive and representative of Canada's youth.
Document 10: List of Peppa Pig episodes: Peppa Pig is a British preschool animated television series produced by Astley Baker Davies. The show features the eponymous pig along with her family and friends. Each episode is approximately five minutes long and there have been five seasons to date. Peppa Pig has been broadcast in over 180 countries.
Document 11: Pigeon Boy: Pigeon Boy is a children's cartoon created by Millimages of France, and is shown on a number of television networks, including the Australian Broadcasting Corporation in Australia. It is set in the fictional city of Ultrapolis and centres on the lives of three teenagers:Pigeon Boy, and his two companions, one who is extremely fat and the other who is the complete opposite.
Document 12: The Kids of Degrassi Street: The Kids of Degrassi Street is a Canadian children's TV series that aired from 1979 to 1986, and is the first series in the "Degrassi" franchise. It follows the lives of a group of children living on Degrassi Street in Toronto, Ontario, Canada. The series grew out of four short films: "Ida Makes a Movie", "Cookie Goes to the Hospital", "Irene Moves In" and "Noel Buys a Suit", which originally aired as after-school specials on CBC Television in 1979, 1980, 1981 and 1982, respectively. The show was acclaimed for its realistic depiction of everyday children's lives and tribulations, and it remains memorable to many Canadians because of this.
Document 13: 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 14: 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.
Document 15: 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.
ground_truth: six months
sub_queries: What network broadcast Pigeon Street? Answer: BBC
For how long did the BBC alternate different broadcasting systems? Answer: six months