Control circuit for an afc system

Claim 2

In a receiver having tuner means for selecting a carrier signal having a recurring sync pulse and an information portion located between adjacent sync pulses, said carrier signal being located within a predetermined frequency passband, and an AFC circuit for varying the tuning of said tuner means to cause the receiver to lock onto the carrier signal, a control circuit comprising: sync separator means for detecting said recurring sync pulse, detector means coupled to said sync separator means for detecting the absence of a sync pulse within said passband, and AFC defeat means coupled to said detector means for effectively disabling said AFC circuit in response to the detected absence of the sync pulse.

Claim 3

The control circuit of claim 1 wherein the detector means includes time constant means responsive to the absence of a plurality of the sync pulses for producing a control signal, said AFC defeat means being responsive to said control signal to effectively disable said AFC circuit.

Claim 4

The control circuit of claim 2 wherein said detector means includes noise gate means for rendering said detector means inoperative except during the time period of a gating pulse, said receiver includes means for generating a periodic signal which occurs in synchronism with said sync pulse, and means coupling said periodic signal to said noise gate means whereby said periodic signal corresponds to said gating pulse.

Claim 5

The control circuit of claim 1 wherein said receiver includes tuning voltage means for establishing different tuning voltages representative of different frequency carrier signals to be received, said AFC circuit generates an AFC voltage which alters the frequency passband of said tuner means, combining means coupled to said tuning voltage means and said AFC circuit for combining said tuning voltage and AFC voltage to control the frequency passband of said tuner means, and said AFC defeat means disables said AFC circuit.

Claim 6

The control circuit of claim 4 wherein said AFC defeat means includes switch means controllablE between a first state which allows said AFC voltage to be coupled to said combining means and a second state which prevents said AFC voltage from being coupled to said combining means, means biasing said switch means into said first state, said AFC defeat means causing said switch means to change to said second state in order to defeat the operation of said AFC circuit.

Claim 7

In a television receiver having tuner means for tuning a composite television signal, AFC means for varying the tuning of said tuner means to cause the receiver to lock onto said composite television signal, image reproducing means coupled to said tuner means for demodulating the composite television signal and reproducing an image therefrom, a control circuit comprising: switchable means having a first state and a second state and coupled to said AFC means for altering the operation thereof in response to said second state, sampling means coupled to said tuner means for sampling a recurring selected portion of the composite television signal, detector means for generating a control signal in response to a detected absence of a plurality of sampled recurring selected portions of the composite television signal, and gate means responsive to said control signal for switching said switchable means to said second state.

Claim 8

In a television receiver having tuner means for tuning a composite television signal, AFC means for varying the tuning of said tuner means to cause the receiver to lock onto said composite television signal, image reproducing means coupled to said tuner means for demodulating the composite television signal and reproducing an image therefrom, a control circuit comprising: switchable means having a first state and a second state and coupled to said AFC means for altering the operation thereof in response to said second state, sampling means coupled to said tuner means for sampling a recurring selected portion of the composite television signal, detector means for generating a control signal in response to a detected absence of a plurality of sampled recurring selected portions of the composite television signal, and gate means responsive to said control signal for switching said switchable means to said second state.

Claim 9

The control circuit of claim 6 wherein said sampling means comprises sync detector means for detecting sync pulses which correspond to said recurring selected portion of the composite television signal, said television receiver includes scanning means for generating a retrace pulse during the time period of a sync pulse, and noise immunity means for rendering said sync detector means effective only during the occurrence of said retrace pulses.

Claim 10

The control circuit of claim 6 wherein said detector means comprises capacitor means, charge means for charging said capacitor means with the sampled recurring selected portions of the composite television signal, level means responsive to a predetermined voltage level across said capacitor means for generating said control signal, discharge means coupled to said capacitor means for discharging said capacitor means, said discharge means having a time constant greater than the sampling period.

Claim 11

The control circuit of claim 6 wherein said switchable means comprises AFC defeat means having a first state which allows continued operation of the AFC means and a second state which disables said AFC means.

Claim 12

The control circuit of claim 9 wherein said sampling means comprises sync separator means coupled to said tuner means for separating sync pulses from the composite television signal, scanning means for producing a raster synchronized with the occurrence of sync pulses from said sync separator means, said detector means being coupled to said sync separator means for generating said control signal in response to the absence of a plurality of sync pulses from said sync separator means.

Claim 13

The control circuit of claim 10 wherein said scanning means includes horizontal output means which generate retrace pulses, noise immunity means for normally disabling said detector means, and means responsive to the retrace pulses from said horizontal output means for enabling said detector means.