A tuner is a subsystem that receives radio frequency (RF) transmissions, such as FM broadcasting, and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually because a lower frequency is used on the output. Broadcast FM/AM transmissions usually feed this intermediate frequency (IF) directly into a demodulator that converts the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker.
More complex transmissions like PAL/NTSC (TV), DAB (digital radio), DVB-T/DVB-S/DVB-C (digital TV) etc. use a wider frequency bandwidth, often with several subcarriers. These are transmitted inside the receiver as an intermediate frequency (IF). Subcarriers are then processed like real radio transmissions, but the whole bandwidth is sampled with an analog-to-digital converter (A/D) at a rate faster than the Nyquist rate (that is, at least twice the IF frequency).
A tuner can also refer to a radio receiver or standalone audio component that is part of an audio system such as a hi-fi or stereo, to be connected to a separate amplifier. The verb tuning in radio contexts means adjusting the receiver to detect the desired radio signal carrier frequency that a particular radio station uses.

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