The switch from analog to digital broadcast television is called the digital TV (DTV) transition.
Back in 1996, the U.S. Congress allowed each broadcast TV station to distribute programming using a digital broadcast channel in addition to their existing analog channels.
Currently broadcast stations in all U.S. markets use both analog and digital signals to broadcast their programs.
U.S. & World
Congress has mandated that all stations must stop broadcasting in analog and switch entirely to a digital broadcast. The final date of dual broadcasting will be June 12, 2009. After that full-power television stations will broadcast using only their digital signal.