Question: For how long did the network that was discussed alternate different broadcasting systems?

full_context: Document 1: Abu Hamza Rabia: According to American intelligence officials, Rabia was al-Qaeda's third in command. Few details have been made available about his background, although CNN reports confirmed his Egyptian origins and indicated that he was in his thirties. He and four other men, two of them also Arabs, were killed in the village of Asoray, near Miranshah, the capital of North Waziristan. His death stirred controversy because it was Pakistani policy that US forces were not allowed in the country.
Document 2: Assassination of Robert F. Kennedy: In November 2006, the BBC's Newsnight program presented research by filmmaker Shane O'Sullivan alleging that several CIA officers were present on the night of the assassination. Three men who appear in films and photographs from the night of the assassination were positively identified by former colleagues and associates as former senior CIA officers who had worked together in 1963 at JMWAVE, the CIA's main anti-Castro station based in Miami. They were JMWAVE Chief of Operations David Morales, Chief of Maritime Operations Gordon Campbell and Chief of Psychological Warfare Operations George Joannides. However, several people who had known Morales, including family members, were adamant that he was not the man who O'Sullivan said was Morales. After O'Sullivan published his book, assassination researchers Jefferson Morley and David Talbot also discovered that Campbell had died of a heart attack in 1962, six years prior to the assassination of Kennedy. In response, O'Sullivan stated that the man on the video may have used Campbell's name as an alias. He then took his identifications to the Los Angeles Police Department whose files showed the men he identified as Campbell and Joannides to be Michael Roman and Frank Owens, two Bulova sales managers attending the company's convention in the Ambassador. O'Sullivan stood by his allegations stating that the Bulova watch company was a "well-known CIA cover".
Document 3: 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 4: 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 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: 2004 United States presidential election: During August and September 2004, there was an intense focus on events that occurred in the late 1960s and early 1970s. Bush was accused of failing to fulfill his required service in the Texas Air National Guard. However, the focus quickly shifted to the conduct of CBS News after they aired a segment on 60 Minutes Wednesday introducing what became known as the Killian documents. Serious doubts about the documents' authenticity quickly emerged, leading CBS to appoint a review panel that eventually resulted in the firing of the news producer and other significant staffing changes.
Document 7: 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 8: 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 9: Assassination of Martin Luther King Jr.: President Lyndon B. Johnson was in the Oval Office that evening, planning a meeting in Hawaii with Vietnam War military commanders. After press secretary George Christian informed him at 8:20 p.m. of the assassination, he canceled the trip to focus on the nation. He assigned Attorney General Ramsey Clark to investigate the assassination in Memphis. He made a personal call to King's wife, Coretta Scott King, and declared April 7 a national day of mourning, on which the U.S. flag would be flown at half-staff.
Document 10: John Kerry: Meanwhile, Kerry's staff began their own investigations and, on October 14, issued a report that exposed illegal activities on the part of Lieutenant Colonel Oliver North, who had set up a private network involving the National Security Council and the CIA to deliver military equipment to right-wing Nicaraguan rebels (Contras). In effect, North and certain members of the President's administration were accused by Kerry's report of illegally funding and supplying armed militants without the authorization of Congress. Kerry's staff investigation, based on a year-long inquiry and interviews with fifty unnamed sources, is said to raise "serious questions about whether the United States has abided by the law in its handling of the contras over the past three years."
Document 11: 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 12: 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 13: Federal Bureau of Investigation: In 2005, fugitive Puerto Rican Nationalist leader Filiberto Ojeda Ríos died in a gun battle with FBI agents in 2005 in what some charged was an assassination.[citation needed] Puerto Rico Governor Aníbal Acevedo Vilá criticized the FBI assault as "improper" and "highly irregular" and demanded to know why his government was not informed of it. The FBI refused to release information beyond the official press release, citing security and agent privacy issues. The Puerto Rico Justice Department filed suit in federal court against the FBI and the US Attorney General, demanding information crucial to the Commonwealth's own investigation of the incident. The case was dismissed by the U.S Supreme Court. Ojeda Rios' funeral was attended by a long list of dignitaries, including the highest authority of the Roman Catholic Church in Puerto Rico, Archbishop Roberto Octavio González Nieves, ex-Governor Rafael Hernández Colón, and numerous other personalities.
Document 14: Federal Bureau of Investigation: The FBI has maintained files on numerous people, including celebrities such as Elvis Presley, Frank Sinatra, John Denver, John Lennon, Jane Fonda, Groucho Marx, Charlie Chaplin, the band MC5, Lou Costello, Sonny Bono, Bob Dylan, Michael Jackson, and Mickey Mantle. The files were collected for various reasons. Some of the subjects were investigated for alleged ties to the Communist party (Charlie Chaplin and Groucho Marx), or in connection with antiwar activities during the Vietnam War (John Denver, John Lennon, and Jane Fonda). Numerous celebrity files concern threats or extortion attempts against them (Sonny Bono, John Denver, John Lennon, Elvis Presley, Michael Jackson, Mickey Mantle, Groucho Marx, and Frank Sinatra).
Document 15: John Kerry: During their investigation of Noriega, Kerry's staff found reason to believe that the Pakistan-based Bank of Credit and Commerce International (BCCI) had facilitated Noriega's drug trafficking and money laundering. This led to a separate inquiry into BCCI, and as a result, banking regulators shut down BCCI in 1991. In December 1992, Kerry and Senator Hank Brown, a Republican from Colorado, released The BCCI Affair, a report on the BCCI scandal. The report showed that the bank was crooked and was working with terrorists, including Abu Nidal. It blasted the Department of Justice, the Department of the Treasury, the Customs Service, the Federal Reserve Bank, as well as influential lobbyists and the CIA.
ground_truth: six months
sub_queries: What network was discussed? Answer: BBC
For how long did the BBC alternate different broadcasting systems? Answer: six months