The operators of radio and television stations are required to broadcast brief station identification messages every hour, and full messages daily – when signing on and off. This aids in the diagnosis and repair of any technical issues pertaining to the broadcast equipment and tells the listener or viewer what channel they have tuned to. When a station broadcasts over multiple channels/stations, known as simulcasting, each station self-identifies according to their rules. The Federal Communications Commission is the national authority for radio and television broadcasting in the United States.
Here is one of the earlier network sign-offs from our archives, courtesy Youtube.
Reference:
This is the first roll opener broadcast by BADCOP.ONLINE operated, at the time, by Signal High Networks Inc. (Canada). This later became Signal High News Corporation in the USA.