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Abstract

An AFT circuit for a television receiver automatically adjusts the frequency of a tuner local oscillator, not only on the basis of an error signal provided from an AFT discriminator, but also on the basis of horizontal synchronizing pulses contained in the video signal to which the receiver is tuned. A synch signal detector detects the presence or absence of the synchronizing pulses. A control circuit provides a control signal to the local oscillator in response to the AFT error signal when the latter is above or below a reference voltage such that the desired frequency is within an AFT capture range. However, when the AFT signal is at the reference voltage, the control circuit provides the control signal based on the presence or absence of the synchronizing pulses, such that when the pulses are present, the local oscillator frequency is raised, but when the pulses are absent, the local oscillator frequency is lowered. Then, when the AFT signal is present, the local oscillator is controlled thereby. This AFT circuit has an expanded effective capture range, and is prevented from locking onto the sound carrier of an adjacent channel. 00202

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First Claim