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A tuner is a subsystem that receives radio frequency (RF) transmissions like radio broadcasts 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 convert 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. uses a wider frequency bandwidth, often with several subcarriers. These are transmitted inside the receiver as an intermediate frequency (IF). The next step is usually either to process subcarriers like real radio transmissions or to sample the whole bandwidth with A/D at a rate faster than the nyquist rate that is at least the IF frequency. The term ''tuner'' can also refer to a radio receiver or standalone audio component that are part of an audio system, to be connected to a separate amplifier. The verb "tuning" in radio contexts means adjusting the radio receiver to receive the desired radio signal carrier frequency that a particular radio station uses. == Design == The simplest tuner consists of an inductor and capacitor connected in parallel, where the capacitor or inductor is made to be variable. This creates a resonant circuit which responds to an alternating current at one frequency. Combined with a detector, also known as a demodulator (diode D1 in the circuit), it becomes the simplest radio receiver, often called a crystal set. Practical radio tuners use a superheterodyne receiver. Older models would realize manual tuning by means of mechanically operated ganged variable capacitors. Often several sections would be provided on a tuning capacitor, to tune several stages of the receiver in tandem, or to allow switching between different frequency bands. A later method used a potentiometer supplying a variable voltage to varactor diodes in the local oscillator and tank circuits of front end tuner, for electronic tuning. Still later, phase locked loop methods were used, with microprocessor control. In a self-contained radio receiver for audio, the signal from the detector after the tuner is run through a volume control and to an amplifier stage. The amplifier feeds either an internal speaker or headphones. In a tuner component of an audio system (for example, a home high-fidelity system or a public address system in a building), the output of the detector is connected to a separate external system of amplifiers and speakers. The broadcast audio FM band ( in most countries) is around higher in frequency than the AM band and provides enough space for a bandwidth of This bandwidth is sufficient to transmit both stereo channels with almost the full bandwidth of the human ear. Sometimes, additional subcarriers are used for unrelated audio or data transmissions. The left and right audio signals must be combined into a single signal which is applied to the modulation input of the transmitter; this is done by the addition of an inaudible subcarrier signal to the FM broadcast signal. FM stereo allows left and right channels to be transmitted. The availability of FM stereo, a quieter VHF broadcast band, and better fidelity led to the specialization of FM broadcasting in music, tending to leave AM broadcasting with spoken-word material. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tuner (radio)」の詳細全文を読む スポンサード リンク
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