Listened to an AM radio station lately? Especially one that offers news? Especially one that offers CBS news? Well, you know that 'ping' sound, followed by music, that you hear when it goes from local programming to (national) CBS news? That 'ping' is called a "bee doop".
Strictly technically speaking, the bee doop is not needed—it was originally provided to signal to a human operator when to make the switch to national programming, which the station operator would then do manually. However, electronic systems were soon devised, so many stations did not need the bee doop. But enough refused or could not afford the change that it continued to be used—and heard—by listeners, since the bee doop came after the new signal. Finally, in the 70s the last stations made the change to an automatic system, and the bee doop was (briefly) eliminated.
Big mistake. Listeners started complaining about the missing bee doop; It had evolved from an operator signal to one understood by the general public. The bee doop was reestablished, and remains in use today.