automatic tuning control (ATC), method or device to keep a radio or television receiver automatically tuned to a desired frequency or channel. Assuming that the receiver is at least approximately tuned to the desired frequency, a circuit in the receiver develops an error voltage proportional to the degree to which the receiver is mistuned. This error voltage is then fed back to the tuning circuit in such a way that the tuning error is reduced. In most frequency modulation (FM) detectors an error voltage of this type is easily available; in television receivers extra circuits may be used to develop it. In an FM receiver, automatic frequency control (AFC) may make it difficult to receive a weak signal located near in frequency to a strong one. In tuning circuits that use frequency synthesis, in which digital circuitry, in conjunction with a crystal oscillator, produces precise signals, automatic tuning control is not necessary.