A scanner is a raio receiver that can automatically tune, or scan, two or more discrete frequencies, stopping when it finds a signal on one of them and then continuing to scan other frequencies when the initial transmission ceases. Many scanners cover the non-broadcast radio bands between 30 and 951 MHz using FM, although there are models that cover more of the radio spectrum and use other modulation types. Early scanners were slow, bulky, and expensive. Today, modern microprocessors have enabled scanners to store thousands of channels and monitor hundreds of channels per second. Recent models can follow trunked radio systems and decode APCO-P25 digital transmissions. Both hand held and desktop models are available. Scanners are often used to monitor police, fire and emergency medical services.