FCC regulations:
Stations must make identification announcements when they sign on and off for the day.. They must also make the announcements hourly, as close to the hour as possible, at a natural programming break. TV stations may make these announcements on-screen or by voice only. Official station identification includes the station's call letters followed by the community or communities specified in its license as the station's location. Between the call letters and its community, the station may insert the name of the licensee, the station's channel number, and/or its frequency.