 Complete the following phrase:  Viewership of early NBC broadcasts was tightly restricted to those authorized by the company, whose installed set base eventually reached about 200. Technical standards for television broadcasting were in flux as well. Between the time experimental transmissions began in 1935 and the beginning of commercial television service in 1941, picture definition increased from 343 to 441 lines, and finally in 1941 to the 525-line standard used for analog television from the start of full commercial service until the end of analog broadcasts in mid-2009. The sound signal was also changed from AM to FM, and the spacing of sound and vision carriers was also changed several times. Shortly after NBC began a semi-regular television transmission schedule in 1938, DuMont Laboratories announced TV sets for sale to the public, a move that RCA was saving for the opening of the 