Mediumwave radio broadcasts are those between the frequencies of 500 kHz and 2000 kHz. In most of the world, mediumwave serves as the most common band for broadcasting. The standard AM broadcast band is 535 kHz to 1705 kHz.
Mediumwave signals have the properties of following the curvature of the earth (the groundwave) and reflecting or refracting off the ionosphere at night (skywave). This makes this frequency ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km in New York City, due to groundwave propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably.
In the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of +/- 5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of +/- 4.5 kHz. This provides adequate audio quality for voice, but is insufficient for high-fidelity broadcasting, which is reserved for the VHF FM bands.
In the United States, in September 2002, the Federal Communications Commission approved the iBiquity in-band on-channel (IBOC) system of digital broadcasting, which is meant to improve the audio quality of signals. The Digital Radio Mondiale (DRM) IBOC system has been approved by the ITU for use outside the Americas.