the networks. As radio prospered throughout the 1920s and 1930s, experimental television stations, which broadcast both an audio and a video signal, began sporadic broadcasts. Licenses for these experimental stations were often granted to experienced radio broadcasters, and thus advances in television technology closely followed breakthroughs in radio technology. As interest in television grew, and as early television stations began regular broadcasts, the idea of networking television signals (sending one station's video and audio signal to outlying stations) was born. However, the signal from an electronic television system, containing much more information than a radio signal, required a broadband transmission medium. Transmission by a nationwide series of radio relay towers would be possible but extremely expensive. Researchers at AT&T subsidiary Bell Telephone Laboratories patented coaxial cable in 1929, primarily as a telephone improvement device. Its high capacity (transmitting 240 telephone calls simultaneously) also made it ideal for long-distance television transmission, where it could handle a frequency band of 1 MHz. German television first demonstrated such an application in 1936 by relaying televised telephone calls from Berlin to Leipzig, away, by cable. AT&T laid the first L-carrier coaxial cable between New York City and Philadelphia, with automatic signal booster stations every , and in 1937 it experimented with transmitting televised motion pictures over the line. Bell Labs gave demonstrations of the New York–Philadelphia television link in 1940 and 1941. AT&T used the coaxial link to transmit the Republican National Convention in June 1940 from Philadelphia to New York City, where it was televised to a few hundred receivers over the NBC station W2XBS (which evolved into WNBC) as well as seen in Schenectady, New York via W2XB (which evolved into WRGB) via off-air relay from the New York station. NBC had earlier demonstrated an inter-city television broadcast on 1 February 1940, from its station in New York City to another in Schenectady, New York by General Electric relay antennas, and began transmitting some programs on an irregular basis to Philadelphia and Schenectady in 1941. Wartime priorities suspended the manufacture of television and radio equipment for civilian use from 1 April 1942 to 1 October 1945, temporarily shutting down expansion of television networking. However, in 1944 a short film, "Patrolling the Ether", was broadcast simultaneously over three stations as an experiment. AT&T made its first postwar addition in February 1946, with the completion of a cable between New York City and Washington,