A police scanner app allows users to listen to police, fire and EMS frequencies with a computer or mobile device. Digital applications allow listeners who lack a radio receiver to monitor emergency bands and communications.
Police scanners first became popular in the 1970s. The earliest radio scanners were equipped to monitor a limited number of channels, requiring a separate radio crystal for each frequency received. Digital applications that run on a computer or smartphone permit anyone with an Internet connection to listen to digital broadcasts and emergency radio frequencies streamed by individuals who possess the appropriate radio receiver and equipment.