A tuner is an ajustable device which passes one radio frequency, or band of frequencies, and excludes others, by using electrical resonance. A tuner performs the process of selecting the desired signal but it's output is not directly usable and must be sent to another device. Typically tuners are sold with built-in audio amplifiers, loudspeakers, and/or a video display to form a radio receiver or television set. Tuners can be either stereo or mono, and are available for TV, FM, AM, and other types of radio signals.