JPH06233212A - Circuit for processing video intermediate frequency signal - Google Patents

Abstract

PURPOSE:To obtain the processing circuit of a video intermediate frequency signal capable of suppressing the fluctuation of the detection sensitivity of the automatic frequency adjusting output based on the change of the average value level APL of a video signal. CONSTITUTION:An inputted video intermediate frequency signal is amplified and controlled by an automatic gain control circuit 3 and a video intermediate frequency amplifier 1 and is inputted in an automatic gain control amplifier 9. At this time, a gain control is performed for the automatic gain control amplifier 9 according to the output of an integrator 8 which is made so as to detect an average value level APL by integrating the output of the video detection from a video detector 2, gain is raised based on the output of the integrator 8 since the amplitude of the components of a video signal is decreased when the value of the average value level APL is high, gain is lowered when the value of the average value is low and an output having more than constant level amplitude always is transmitted to an automatic frequency adjusting circuit 4. Therefore, video signals of more than a constant level is inputted in the automatic frequency adjusting circuit 4 regardless of the fluctuation of the average value level APL of the video signal and the fluctuation of detection sensitivity is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video intermediate frequency signal processing circuit for demodulating a video intermediate frequency signal after frequency-converting a television broadcast signal into an intermediate frequency signal, and more particularly to an automatic frequency function in its function. The present invention relates to improvement of the adjustment circuit.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing a conventional video intermediate frequency signal processing circuit. In the figure, 1 is a video intermediate frequency (Video Inter Frequency, hereinafter referred to as VIF) after frequency conversion of a television broadcast signal into an intermediate frequency signal.
A gain control type video intermediate frequency amplifier for amplifying a signal, 2 is a video detector with a constant gain for obtaining a video detection output based on its output, 3 is a DC level of the video detection output and compares it with a reference level. An automatic gain control circuit for controlling the output peak value to a constant value by controlling the gain of the video intermediate frequency amplifier 1 according to the comparison difference.

Reference numeral 4 is an input of the output of the video intermediate frequency amplifier 1, and automatic frequency tuning (Automatic Frequenoy Tuning:
AFT) is an automatic frequency adjustment circuit for transmitting an output (hereinafter, an automatic frequency adjustment output is referred to as an AFT output), and a video carrier amplifier 5 for amplifying the output of the video intermediate frequency amplifier 1 with a fixed gain value and its output are inputted. The phase shifter 6 that controls the phase of the input signal according to the comparison difference between the resonance frequency of the built-in LC resonance circuit and the frequency of the input signal, and the output of the video carrier amplifier 5 and the phase shifter 6 are input to them. It has a phase detector 7 that sends out an AFT output of a DC output according to the phase difference, and gives the above AFT output as a control signal to a local oscillator of a tuner (not shown).

Next, the operation of the above configuration will be described. The input video intermediate frequency signal is amplified to a constant amplitude by the gain control type video intermediate frequency amplifier 1 and the automatic gain control circuit 3, and is input to the video detector 2 having a constant gain and output as a video detection output. .

At this time, when the video intermediate frequency signal is of the negative modulation type, the output of the video intermediate frequency amplifier 1 is generally determined by the automatic gain control circuit 3 so as to keep the peak value of the amplitude of the signal constant. A method of controlling the amplitude-controlled output at the peak value is performed.

The output of the video intermediate frequency amplifier 1 is input to the automatic frequency adjustment circuit 4 and amplified by the video carrier amplifier 5, and the amplified output is directly input to the phase detector 7 and the phase shifter 6 After being phased by, it is input to the phase detector 7. The phase detector 7 outputs a DC output corresponding to the phase difference between the output of the video carrier amplifier 5 and the output of the phase shifter 6 as an AFT output. This AFT output is used as a control signal to a local oscillator of a tuner (not shown).

[0007]

By the way, in the above configuration, the video signal input to the automatic frequency adjustment circuit 4,
In other words, the video intermediate frequency signal input to the video intermediate frequency amplifier 1 is negatively modulated, and, as shown in FIG. 4, when it is controlled by the amplitude at the top and bottom pulse-shaped synchronization signals, The average value level (Av
The amplitude in the video signal portion changes due to the change of the erasure picture level (APL), and particularly when the average value level APL is high, the amplitude in the video signal portion decreases due to the negative modulation.

Therefore, the AFT detection sensitivity (A) for converting changes in the input amplitude and the input frequency of the phase detector 7 into an output voltage.
Looking at the relationship of (FT output / input frequency), there is a problem that the AFT detection sensitivity changes depending on the value of the input amplitude as shown in FIG. That is, the characteristics shown in FIG. 5 depend on the gain of the video carrier amplifier 5, but when the input levels from the video carrier amplifier 5 and the phase shifter 6 decrease, the internal transistors of the phase detector 7 cannot switch sufficiently. , At the input level above the threshold value, the internal transistor is turned on to obtain a constant AFT detection sensitivity,
At the input level below the threshold, the AFT depends on the input amplitude.
It shows that the detection sensitivity changes.

The present invention has been made in order to solve the above-mentioned problems, and has an average value level APL of a video signal.
It is an object of the present invention to obtain a processing circuit of a video intermediate frequency signal capable of suppressing the variation of the AFT detection sensitivity due to the change of

[0010]

A video intermediate frequency signal processing circuit according to claim 1 of the present invention includes a video intermediate frequency amplifier for amplifying a video intermediate frequency signal of a television broadcast signal, and video detection based on the output thereof. A video detector that obtains an output, detects the DC level of the video detection output, compares the detected DC level with a reference level, and controls the gain of the video intermediate frequency amplifier according to the comparison difference. Circuit for controlling a video intermediate frequency signal, which includes an automatic gain control circuit for controlling the frequency and an automatic frequency adjusting circuit for obtaining a control signal for automatically controlling the local oscillation frequency of the tuner based on the output of the video intermediate frequency amplifier. In the above, an average value detecting means for detecting an average value level of a video signal component amplitude-modulated into the video intermediate frequency signal based on the video detection output, The automatic frequency adjusting circuit is provided between the video intermediate frequency amplifier and the automatic frequency adjusting circuit and controls the gain based on the output of the average value detecting means to control the output level of the video intermediate frequency amplifier. And an automatic gain control amplifier provided to the adjusting circuit.

A video intermediate frequency signal processing circuit according to a second aspect of the present invention includes a video intermediate frequency amplifier for amplifying a video intermediate frequency signal of a television broadcast signal, and a video detector for obtaining a video detection output based on the output thereof. , An automatic gain for controlling the video intermediate frequency signal to a constant amplitude by detecting the DC level of the video detection output and comparing it with a reference level and controlling the gain of the video intermediate frequency amplifier according to the comparison difference. In a video intermediate frequency signal processing circuit including a control circuit and an automatic frequency adjusting circuit for obtaining a control signal for automatically controlling the local oscillation frequency of the tuner based on the output of the video intermediate frequency amplifier, An average value detecting means for detecting the average value level of the video signal component amplitude-modulated into the video intermediate frequency signal based on the above; It is provided between the state detector that detects the no signal state or the weak signal input state of the video intermediate frequency signal based on the output value, and is provided between the video intermediate frequency amplifier and the automatic frequency adjustment circuit, and detects from the state detector. When the signal is transmitted, the gain is set to a predetermined value, and when the output of the state detector is not transmitted, the gain is controlled on the basis of the output of the average value detecting means to output the output level of the video intermediate frequency amplifier. And an automatic gain control amplifier for controlling the signal to be supplied to the automatic frequency adjusting circuit.

[0012]

In the video intermediate frequency signal processing circuit according to the first aspect of the present invention, the average value level of the video signal component amplitude-modulated into the video intermediate frequency signal based on the video detection output by the average value detecting means is changed. The detected automatic gain control amplifier controls the output level of the video intermediate frequency amplifier by controlling the gain on the basis of the output of the average value detection means, and the automatic frequency adjustment circuit outputs a signal having a necessary and sufficient input level. Give to.

In the video intermediate frequency signal processing circuit according to the second aspect, the average value detecting means detects the average value level of the video signal component amplitude-modulated into the video intermediate frequency signal based on the video detection output. Further, the state detector detects the no-signal state or weak signal input state of the video intermediate frequency signal based on the detection value of the average value detecting means, and the automatic gain control amplifier detects the detection signal from the state detector. When the output is sent, the gain is set to a predetermined value, and when the output of the state detector is not sent, the gain is controlled based on the output of the average value detecting means to control the output level of the video intermediate frequency amplifier. Then, the fluctuation of the output of the automatic frequency adjustment circuit is suppressed when the signal is not input or the weak signal is input.

[0014]

EXAMPLES Example 1. Embodiment 1 of the present invention will be described below with reference to the drawings. In FIG. 1, a portion surrounded by a broken line is a portion added to the circuit of the conventional example shown in FIG. 3, and 8 is an average for detecting an average value level of a video signal component amplitude-modulated into a video intermediate frequency signal. An integrator as a value detecting means integrates the video detection output output from the video detector 2.

Reference numeral 9 is provided between the video intermediate frequency amplifier 1 and the video carrier amplifier 5, and controls the level of the video intermediate frequency signal input to the video carrier amplifier 5 based on the output of the integrator 8. An automatic gain control amplifier for inputting a video intermediate frequency signal having an amplitude equal to or higher than the average value level to the automatic frequency adjustment circuit 4.

Next, the operation will be described. The amplitude of the input video intermediate frequency signal is controlled by the automatic gain control circuit 3 and the video intermediate frequency amplifier 1 to be controlled by the automatic gain control amplifier 9.
Entered in. At this time, the automatic gain control amplifier 9 is gain-controlled in accordance with the output of the integrator 8 which detects the average value level APL by integrating the video detection output from the video detector 2.

That is, the automatic gain control amplifier 9 is
When the average value level APL is high, the amplitude of the video signal component is reduced, so that the gain is increased based on the output of the integrator 8. On the other hand, when the average value level is low, the gain is reduced. By doing so, an output having an amplitude above a certain level is sent to the automatic frequency adjustment circuit 4.

Therefore, the video intermediate frequency signal having an amplitude of a certain level or more is always input to the automatic frequency adjusting circuit 4 regardless of the fluctuation of the average value level APL of the video signal, and the detection of the AFT output is performed. It is possible to suppress fluctuations in sensitivity and control the fluctuations at a constant level.

Example 2. Next, FIG. 2 is a block diagram showing a video intermediate frequency signal processing circuit according to the second embodiment. In the first embodiment shown in FIG. 1 described above, the input to the automatic frequency adjusting circuit 4 was controlled by controlling the gain of the automatic gain control amplifier 9 using the video detection output.
Then, similarly, the video detection output is integrated by the integrator 8 using the video detection output, and the integrated output is compared with the reference value. A comparator 10 as a state detector that detects that it is in an input state is added to the configuration of FIG.
When the detection signal is sent from the comparator 10, the automatic gain control amplifier 9 fixes the gain to a small value.

Therefore, in the second embodiment, noise and offset components are not input to the automatic frequency adjustment circuit 4 when the video intermediate frequency signal is in a no signal state or in an ultra-weak electric field state. The voltage fluctuation of the AFT output output from the automatic frequency adjustment circuit 4 can be eliminated without being affected by the above.

[0021]

As described above, according to the first aspect of the present invention, the average value detecting means for detecting the average value level of the video signal component amplitude-modulated into the video intermediate frequency signal based on the video detection output is provided. The automatic frequency adjustment is provided between the video intermediate frequency amplifier and the automatic frequency adjustment circuit, and controls the output level of the video intermediate frequency amplifier by controlling the gain based on the output of the average value detecting means. Since the automatic gain control amplifier provided to the circuit is provided, it is possible to always input the video intermediate frequency signal having the amplitude equal to or higher than a certain level to the automatic frequency adjustment circuit regardless of the fluctuation of the average value level APL of the video signal. The detection sensitivity of the automatic frequency adjustment output can be suppressed from varying and the detection sensitivity can be constantly controlled.

Further, according to claim 2, an average value detecting means for detecting an average value level of the video signal component amplitude-modulated into the video intermediate frequency signal based on the video detection output, and the detection of the average value detecting means. The detection signal is sent from the state detector, which is provided between the state detector that detects the no signal state or weak signal input state of the image intermediate frequency signal based on the value, and the image intermediate frequency amplifier and the automatic frequency adjustment circuit. If the output of the state detector is not transmitted, the gain is controlled based on the output of the average value detecting means to control the output level of the video intermediate frequency amplifier. Since the automatic gain control amplifier provided to the automatic frequency adjustment circuit described above is provided, the voltage fluctuation of the automatic frequency adjustment output when the video intermediate frequency signal is in the no signal state or the ultra weak electric field state It can be suppressed.

[Brief description of drawings]

FIG. 1 is a block configuration diagram according to a first embodiment of the present invention.

FIG. 2 is a block configuration diagram according to a second embodiment of the present invention.

FIG. 3 is a block configuration diagram according to a conventional example.

FIG. 4 is an explanatory diagram showing a relationship between a change in average value level APL of a video signal and amplitude.

FIG. 5 is a characteristic diagram of the input amplitude and AFT detection sensitivity of the automatic frequency adjustment circuit 4.

[Explanation of symbols]

 1 video intermediate frequency amplifier 2 video detector 3 automatic gain control circuit 4 automatic frequency adjustment circuit 5 video carrier amplifier 6 phaser 7 phase detector 8 integrator 9 automatic gain control amplifier 10 comparator

Claims (2)

[Claims] 1. A video intermediate frequency amplifier for amplifying a video intermediate frequency signal of a television broadcast signal, a video detector for obtaining a video detection output based on its output, and a reference level by detecting a DC level of the video detection output. And an automatic gain control circuit for controlling the video intermediate frequency signal to a constant amplitude by controlling the gain of the video intermediate frequency amplifier according to the comparison difference, and a tuner based on the output of the video intermediate frequency amplifier. A video intermediate frequency signal processing circuit having an automatic frequency adjusting circuit for obtaining a control signal for automatically controlling the local oscillation frequency of the video signal, the video signal being amplitude-modulated into the video intermediate frequency signal based on the video detection output. The mean value detecting means for detecting the mean value level of the component, and the mean value detecting means are provided between the video intermediate frequency amplifier and the automatic frequency adjusting circuit. An automatic gain control amplifier for controlling the output level of the video intermediate frequency amplifier by controlling the gain on the basis of the output of the average value detection means, and providing the automatic intermediate frequency control circuit with the automatic gain control amplifier. Signal processing circuit. 2. A video intermediate frequency amplifier for amplifying a video intermediate frequency signal of a television broadcast signal, a video detector for obtaining a video detection output based on the output, and a reference level by detecting a DC level of the video detection output. And an automatic gain control circuit for controlling the video intermediate frequency signal to a constant amplitude by controlling the gain of the video intermediate frequency amplifier according to the comparison difference, and a tuner based on the output of the video intermediate frequency amplifier. A video intermediate frequency signal processing circuit having an automatic frequency adjusting circuit for obtaining a control signal for automatically controlling the local oscillation frequency of the video signal, the video signal being amplitude-modulated into the video intermediate frequency signal based on the video detection output. A mean value detecting means for detecting the mean value level of the component, and a no signal state or weakness of the video intermediate frequency signal based on the detection value of the mean value detecting means. It is provided between the state detector for detecting the signal input state, the video intermediate frequency amplifier and the automatic frequency adjusting circuit, and when the detection signal is sent from the state detector, the gain is set to a predetermined value. When the output of the state detector is not transmitted, the gain is controlled based on the output of the average value detecting means to control the output level of the video intermediate frequency amplifier to provide the automatic frequency control circuit with the automatic gain control. A circuit for processing a video intermediate frequency signal, comprising an amplifier.