AFT Circuit
First Claim
1. Automatic fine tuning circuit for controlling a heterodyne tuning stage of a television receiver in which a local oscillator generates a local oscillator signal at a tunable local oscillator frequency to be mixed with a broadcast-frequency video signal to which the receiver is to be tuned to produce a video IF signal constituted by a video information signal carried on a video IF carrier having a video IF frequency, comprising:
- frequency discriminator means having an input receiving said video IF signal and an output providing an AFT signal representing any departure of the video IF frequency from a predetermined frequency and which is present only when said video IF frequency is within an AFT capture range defined by predetermined frequency limits;
detecting means for detecting the existence of a synchronizing signal occurring in said video information signal and providing a detecting signal indicative of the presence of such synchronizing signal; and
control means for selectively generating control signals supplied to said local oscillator in response to said AFT signal and said detecting signal;
said control means generating a first control signal to control the local oscillator frequency in accordance with said AFT signal whenever said AFT signal is present, said control means generating a second control signal operative to increase the local oscillator frequency when said AFT signal is not present and said detecting signal is detected to be present and said control means generating a third control signal operative to decrease the local oscillator frequency when said AFT signal is not present and said detecting signal is not present.
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Accused Products
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
80 Citations
17 Claims
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1. Automatic fine tuning circuit for controlling a heterodyne tuning stage of a television receiver in which a local oscillator generates a local oscillator signal at a tunable local oscillator frequency to be mixed with a broadcast-frequency video signal to which the receiver is to be tuned to produce a video IF signal constituted by a video information signal carried on a video IF carrier having a video IF frequency, comprising:
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frequency discriminator means having an input receiving said video IF signal and an output providing an AFT signal representing any departure of the video IF frequency from a predetermined frequency and which is present only when said video IF frequency is within an AFT capture range defined by predetermined frequency limits; detecting means for detecting the existence of a synchronizing signal occurring in said video information signal and providing a detecting signal indicative of the presence of such synchronizing signal; and control means for selectively generating control signals supplied to said local oscillator in response to said AFT signal and said detecting signal;
said control means generating a first control signal to control the local oscillator frequency in accordance with said AFT signal whenever said AFT signal is present, said control means generating a second control signal operative to increase the local oscillator frequency when said AFT signal is not present and said detecting signal is detected to be present and said control means generating a third control signal operative to decrease the local oscillator frequency when said AFT signal is not present and said detecting signal is not present. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A method of automatically adjusting the tuning of a heterodyne tuning stage of a television receiver in which a local oscillator generates a local oscillator signal at a tunable local oscillator frequency to be mixed with a broadcast-frequency video signal to which the receiver is to be tuned to produce a video IF signal constituted by a video information signal carried on a video IF carrier having a video IF frequency;
- comprising;
providing an AFT signal representing any departure of the video IF frequency from a desired frequency and which is present only when said video IF signal is within an AFT capture range defined by predetermined frequency limits; detecting the existence of a synchronizing signal occurring in said video information signal to provide a detecting signal indicative of the presence of such synchronizing signal; and generating a control signal to be supplied to said local oscillator in response to said AFT signal and in response to said detecting signal;
said control signal being generated to control the local oscillator frequency in response to said AFT signal when said AFT signal has a value indicating that such video IF signal is within said capture range but, when said AFT signal is not present said control signal being provided in response to said detecting signal such that when said synchronizing signal is detected to be present in said video information signal carried on said video IF signal, said control signal is operative to increase the local oscillator frequency but, when said synchronizing signal is not detected to be present and said AFT signal is not present, said control signal is operative to decrease the local oscillator frequency. - View Dependent Claims (17)
- comprising;
Specification