The NBC Chimes Museum A Celebration Of Old-Time Radio's Most Famous Signature
The National Broadcasting Company
In its early months of operation, NBC announcers would read a list of stations carrying each program at the end of that particular program. This was actually a continuation of the original method of identifying affiliate stations on the old AT&T network; however, as the network grew this practice became unwieldy and tedious. Some signal had to be given, however, because with two networks NBC had to be able to coordinate switching arrangments between the network feeds and the affiliate stations.
Most major metropolitan areas would eventually have large, well-known stations devoted solely to Red programming, with smaller stations (that often shared the same broadcasting frequency with other small stations) devoted solely to Blue programming. Smaller cities, towns, and rural areas were generally only served by one NBC affiliate, who, at the sponsor's discretion, could be broadcasting either a Red program or a Blue one at any given time.
Because of this arrangement, it was necessary to be able to inform engineers from NBC (who maintained the links between the originating studios and the network feeds) and AT&T (who maintained the telephone lines feeding affiliates, and who carried out the actual switching) in a single stroke when the program period had ended, so they could immediately switch over the next period's feed to the correct affiliates.