History of the National Football League on television
NBC was the first major television network to cover an NFL game, when on October 22, 1939, it broadcast a game between the Philadelphia Eagles and the Brooklyn Dodgers; the network was still only in its infancy, with only two affiliates, the modern day WRGB (now a CBS affiliate) in Schenectady and W2XBS in New York City. Portions of that game still survive via films, but the film is not footage from the telecast (recordings of television broadcasts did not begin until 1948). The use of a sideline camera, the sole camera used in the 1939 broadcast, would become the standard for all future NFL broadcasts until 2017; the angle is particularly suited for estimating yardage, compared to more mobile camera angles that began to appear in the 21st century.
NFL on CBS
From 1956 to 1959, the Baltimore Colts, Pittsburgh Steelers and Philadelphia Eagles only had their away games telecast on CBS. When these three played at home, there was no need for the usage of split audio. Instead, the away team's telecasts were produced in a simple singular audio-video feed. In 1959, 1960 and 1961, NBC had the rights to televise Colts and Steelers home games. While the game broadcasts were blacked out (as per NFL policy) in those cities, they were available to other NBC-affiliated stations.