Vladimir K. Zworykin. In 1935, the all-electronic CRT system was authorized as a "field test" project and NBC converted a radio studio in the RCA Building in New York City's Rockefeller Center for television use. In mid-1936, small-scale, irregularly scheduled programming began to air to an audience of some 75 receivers in the homes of high-level RCA staff, and a dozen or so sets in a closed circuit viewing room in 52nd-floor offices of the RCA Building. The viewing room often hosted visiting organizations or corporate guests, who saw a live program produced in the studios many floors below. 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 World's Fair on April 30, 1939, the day that regularly scheduled television programming was to begin in New York on NBC with much fanfare. In response, NBC ceased all TV broadcasting for several weeks until RCA sets went on sale; regular NBC telecasts commenced the day the fair opened. Firsts for W2XBS As W2XBS broadcasting on "Channel 1" (44–50 MHz), the station scored numerous "firsts". These included: the first televised Broadway drama (June 1938); the first live news event covered by a mobile unit (a fire in an abandoned building in November 1938); the first live telecast of a presidential speech (Franklin D. Roosevelt opening the 1939 New York World's Fair); the first live telecasts of college and Major League Baseball (both in 1939), the first telecast of a National Football League game (also in 1939); the first telecast of a National Hockey League game (early 1940); the first broadcast of religious services (Easter Sunday 1940); the first network (multi-city) telecast of