JPS5916465B2 - Horizontal oscillation frequency automatic control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic horizontal oscillation frequency control circuit used in television receivers and the like.

Automatic horizontal oscillation frequency control of television receivers: A circuit (hereinafter referred to as horizontal AFC circuit) functions to provide a stable screen that is not disturbed by external noise or the like. By the way, the noise resistance characteristics of such a horizontal AFC circuit are as follows.
Related to the noise bandwidth of a circuit, it is already known that the narrower the noise bandwidth, the better the noise resistance characteristics and the less the screen will be disturbed by noise. However, it is extremely difficult to set the noise bandwidth separately from other characteristics of the horizontal AFC circuit.

The noise bandwidth has a proportional relationship with the DC loop gain, which is an important factor when determining the pull-in range 5, for example. Therefore, if the noise bandwidth is narrowed by focusing only on noise immunity, the DC loop gain will become smaller. The ripple in range becomes narrower. Note that the DC loop gain is calculated by the product μ·β, where μ is the detection sensitivity of the horizontal 0AFC circuit, and β is the control sensitivity of the horizontal oscillator. As is clear from the above explanation, horizontal AFC
The circuit should have as narrow a noise bandwidth as possible, while
It can be said that it is ideal to have a configuration that allows the DC loop gain to be set to a desired value.

Figure 1 shows the conventionally used sawtooth horizontal AF.
It is a block diagram showing the configuration of the C circuit, and the phase comparator circuit 1
, a smoothing circuit 2, a horizontal oscillator 3, a horizontal output circuit (including a driver circuit) 4, and an integrating circuit 5. Note that 6 is a signal input terminal to which a horizontal synchronizing signal is applied, and T is a horizontal output terminal. In the phase comparator circuit 1 of the sawtooth horizontal AFC circuit 5 having the above configuration, the horizontal synchronizing signal applied to the signal input terminal 6 and the horizontal output pulse output from the horizontal output circuit 4 are integrated by the integrating circuit 5. A sawtooth wave signal obtained from the two signals is applied, and the phases of both signals are compared.

The smoothing circuit 2 outputs a DC voltage according to the phase difference between both signals, and the horizontal oscillation frequency of the horizontal oscillator 3 is controlled by this DC voltage. By the way, the DC output voltage of the smoothing circuit 2 changes in a direction that lowers the horizontal oscillation frequency according to the phase difference when the horizontal oscillation frequency fluctuates in the higher direction, and on the other hand, when the horizontal oscillation frequency fluctuates in the lower direction. The horizontal oscillation frequency changes in accordance with the phase difference between the two. Therefore, by repeating this operation, the horizontal oscillation frequency becomes
The frequency and phase match the horizontal synchronizing signal applied to the signal input terminal 6. In the sawtooth wave horizontal AFC circuit described above, the detection sensitivity μ is determined by the phase comparator circuit 1, the frequency characteristic of the horizontal AFC circuit, which is also related to noise immunity characteristics, is determined by the smoothing circuit 2, and the control sensitivity β of the horizontal oscillator is determined by the smoothing circuit 2. Each is uniquely set by the horizontal oscillator 3. As already explained, the noise resistance characteristics are determined by the product μ and β of the detection sensitivity μ and the control sensitivity β, as well as the frequency characteristics, so if μ and β are set small using the smoothing circuit and horizontal oscillator described above, In this case, the DC loop gain becomes smaller and the pull-in range becomes narrower, and there is also the problem that the circuit is maintained in this state even under no-noise conditions. The present invention has been made in view of the problems in the conventional sawtooth wave horizontal AFC circuit, and has a detection sensitivity switch that can change the detection sensitivity μ of the horizontal AFC circuit depending on the presence or absence of noise in the composite video signal. A circuit section is added to the horizontal AFC circuit, and when noise is mixed, emphasis is placed on improving noise resistance characteristics and the detection sensitivity μ is reduced.On the other hand, when no noise is mixed, emphasis is placed on DC loop gain and detection sensitivity is reduced. The present invention provides an automatic horizontal oscillation frequency control circuit designed to increase μ. The horizontal oscillation frequency automatic control circuit of the present invention will be explained below with reference to the block diagram of FIG. 2 showing its configuration.

In FIG. 2, the horizontal AFC circuit itself has the same configuration as the horizontal AFC circuit shown in FIG.
A detection sensitivity switching circuit section consisting of a noise amplifier 10, a switch drive circuit 11, a switch 12 whose switching operation is controlled by the circuit 11, and load resistors 13 and 14 for setting detection sensitivity generates a complex signal applied to a terminal 15. The configuration is such that the magnitude of the detection sensitivity μ of this horizontal AFC circuit can be switched depending on the presence or absence of noise mixed into the video signal.

When no noise is mixed in the composite video signal applied to the terminal 15, the noise detection circuit 8 does not generate an output, and the noise amplifier 10 and the switch drive circuit 11 do not operate.

At this time, assuming that the switch 12 selects the a side contact, the load resistor 13 is connected to the horizontal AF via the switch 12.
Connect to C circuit. Therefore, if the value of the load resistance 13 is selected to a value that does not reduce the detection sensitivity μ, then
Although a significant improvement in noise immunity cannot be expected, a sufficiently large DC loop gain can be obtained. Note that even if the noise resistance characteristics cannot be improved, since no noise is mixed in, this does not pose a problem to the circuit operation. On the other hand, when noise is mixed into the composite video signal applied to the terminal 15, an output is generated in the noise detection circuit 8, which is amplified by the noise amplification circuit 10.

The switch and drive circuit 11 switches and drives the switch 12 using the output from the noise amplification circuit 10.
selects the b-side contact, and the load resistor 14 is connected to the horizontal AFC circuit via the switch 12. Therefore, the load resistance 14
If the value of is selected to make the detection sensitivity μ sufficiently small, the detection sensitivity μ of the horizontal AFC circuit when noise is mixed will be a sufficiently small value, and the product μ of the detection sensitivity μ and the control sensitivity β・The DC loop gain caused by β becomes smaller,
Furthermore, the noise bandwidth, which is proportional to the DC loop gain, is narrowed, and the noise resistance is improved. By the way, when the DC loop gain decreases as described above, the pull-in range becomes narrower and synchronization becomes more likely to be disrupted. Therefore,
Avoiding unilaterally lowering the detection sensitivity μ, it is possible to guarantee a predetermined detection sensitivity μ by considering the use of the AFC, the size of the pull-in range, the characteristics of the synchronization separation circuit and the noise cancellation circuit, etc. It is necessary to select the value of the load resistor 14 to the value. Note that the load resistors 13 and 14 can also be constant current sources that can flow a predetermined current.

As is clear from the above explanation, the horizontal oscillation frequency automatic control circuit of the present invention changes the noise bandwidth by changing the detection sensitivity by detecting noise mixed into the composite video signal. Therefore, compared to conventional circuits with fixed detection sensitivity, the DC loop gain can be increased at least when no noise is mixed in, and the noise bandwidth can be narrowed when noise is mixed in. This is effective in improving noise resistance characteristics, and can be said to be a step closer to ideal horizontal oscillation frequency automatic control.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a conventional sawtooth wave horizontal oscillation frequency automatic control circuit, and FIG. 2 is a block diagram showing the configuration of the horizontal oscillation frequency automatic control circuit of the present invention. 1...Phase comparison circuit, 2...Smoothing circuit, 3...Horizontal oscillation circuit, 4...Horizontal output circuit, 5...... Integrating circuit, 6...Horizontal synchronization signal application terminal, 7...Horizontal output terminal, 8...
... Noise detection circuit, 9 ... DC power supply for detection level setting, 10 ... Noise amplification circuit, 11.
...Switch drive circuit, 12...Switch, 13, 14...Load resistance for setting detection sensitivity,
15...Composite video signal application terminal.

Claims (1)

[Claims] 1. A phase comparison circuit that compares the phases of a horizontal synchronization signal and a sawtooth signal, a smoothing circuit that outputs a DC voltage according to the phase difference between the two signals in the phase comparison circuit, and oscillation with the output voltage of the smoothing circuit. A horizontal oscillator whose frequency is controlled, a horizontal output circuit connected to the horizontal oscillator, an integrating circuit that converts the horizontal output pulse output from the horizontal output circuit into the sawtooth signal, and different detection sensitivities are set. a noise detection circuit that detects noise mixed into the composite video signal; and a switch drive circuit that drives a switch that switches the detection sensitivity setting means depending on the presence or absence of an output from the noise detection circuit. , setting the detection sensitivity of the phase comparison circuit by switching the detection sensitivity setting means with the output of the noise detection circuit, and making the noise bandwidth during the noise signal arrival period narrower than the noise bandwidth during the non-noise arrival period. Horizontal oscillation frequency automatic control circuit featuring: