JPH10270961A - Automatic gain control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an automatic gain control circuit that makes a stable automatic gain control operation in following to a rapid change in a level of a television signal. SOLUTION: The control circuit is provided with that detects a continuous synchronizing signal and provides an output of a detection voltage and a smoothing circuit 8 that has a resistor 9 and a capacitor 10 and charges the detection voltage to the capacitor 10 via the resistor 9 to smooth the detection voltage. First and second diodes 11, 12 in anti-parallel connection are connected in parallel with the resistor 9, and a voltage smoothed from the smoothing circuit 8 is given to a variable gain circuit 1 as a gain control voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a so-called head-value type automatic gain control circuit used for a head end or the like which is a transmission device of a transmission base of a television system.

[0002]

2. Description of the Related Art In a CATV system, general terrestrial television signals, satellite broadcast signals, and the like (hereinafter, simply referred to as television signals) are transmitted to a CATV subscriber via a dedicated cable. Therefore, the head end receives such a television signal, converts the frequency into an intermediate frequency band, and places the frequency-converted television signal on the frequency of the channel assigned to the CATV system again (frequency conversion) and transmits the signal. I am trying to do it.

FIG. 2 shows an automatic gain control circuit used in such a conventional head end. In FIG. 2, a television signal frequency-converted by a frequency conversion circuit (not shown) into an intermediate frequency IF (the video intermediate frequency is 45.75 MHz in the U.S. specification) is converted into a variable gain circuit Is input to the distributor 22 via the.

[0004] The television signal is divided into two by the distributor 22, one of which is converted to the frequency of the channel of the CATV system by a mixing circuit (not shown) (up-converter).
) And transmitted to the subscriber via a cable. On the other hand, the other television signal divided into two by the distributor 22 is demodulated into a video signal by a video detection circuit 23, and is appropriately amplified by a video amplifier circuit (not shown) to hold a peak value of the horizontal synchronization signal. It is input to the hold circuit 24.

The peak hold circuit 24 detects and holds the horizontal synchronizing signal, and peaks the horizontal synchronizing signal.
Outputs the gain control voltage proportional to the peak value. The gain control voltage from the peak hold circuit 24 is applied to the next operational amplifier 2
5 and is DC-amplified to an appropriate level at an amplification determined by the ratio of the input resistance 26 and the feedback resistance 27. The gain control voltage amplified by the operational amplifier circuit 25 is input to the next time constant circuit 28.

The time constant circuit 28 is constituted by an integrating circuit comprising a resistor 29 and a capacitor 30, and further smoothes a change in the gain control voltage from the peak hold circuit 24,
In addition, it has a large time constant that follows the overall level change of the gain control voltage.

The output of the time constant circuit 28 is input to the variable gain device 21 via the amplifier circuit 31 to control the gain or attenuation of the variable gain device 21.

[0008]

However, in the above-described conventional leading value type automatic gain control circuit, the peak of the horizontal synchronizing signal is not controlled.
The peak value is detected and a stable gain control voltage is obtained from the level change of the peak value.
The time constants of the hold circuit 24 and the time constant circuit 28 are increased so that the gain control voltage does not change between successive horizontal synchronization signals.

Therefore, the level of the television signal, and therefore,
When the level of the horizontal synchronizing signal suddenly changes, the gain control voltage cannot follow the sudden change of the horizontal synchronizing signal. Therefore, there has been a problem that an accurate automatic gain control operation corresponding to a rapid change of a television signal becomes impossible, resulting in deterioration of image quality. Therefore, an object of the present invention is to realize an automatic gain control circuit that can follow a rapid change in the level of a television signal and that can perform a stable automatic gain control operation.

[0010]

To solve the above-mentioned problems, an automatic gain control circuit according to the present invention detects a continuous synchronization signal and outputs a detection voltage according to the level of the synchronization signal. A first diode having a resistor and a capacitor, and a smoothing circuit for charging the capacitor with the detection voltage via the resistor to smooth the detection voltage, and connected in parallel in opposite directions to each other; A second diode was connected in parallel with the resistor, and the smoothed voltage from the smoothing circuit was input to the variable gain circuit as a gain control voltage.

Further, in the automatic gain control circuit according to the present invention, the smoothed gain control voltage from the smoothing circuit is input to the variable gain circuit via an operational amplifier circuit.

Further, in the automatic gain control circuit according to the present invention, the detection circuit is provided with a time constant circuit for holding the detection voltage generated by detecting the synchronization signal, and the time constant is set to be larger than the time constant of the time constant circuit. The time constant by the resistor and the capacitor of the smoothing circuit is increased.

[0013]

FIG. 1 shows an automatic gain control circuit according to the present invention. In FIG. 1, an intermediate frequency IF (in the US specification, the video intermediate frequency is 45.times.) By a frequency conversion circuit (not shown).
The television signal frequency-converted to 75 MHz is input to the distributor 2 via a variable gain circuit 1 such as a variable gain amplifier circuit or a variable attenuator using a pin diode.

The television signal is split into two by the splitter 2, one of which is frequency-converted (up-converted) to a channel frequency of the CATV system by a mixing circuit (not shown).
) And transmitted to the subscriber via a cable. On the other hand, the other television signal divided into two by the distributor 2 is demodulated into a video signal by the video detection circuit 3,
The signal is amplified by a video amplifier circuit (not shown) and detected by a one-stage or two-stage detection circuit 4 which detects the horizontal synchronizing signal and holds the peak value thereof.

The detection circuit 4 has a diode 4a for detecting a horizontal synchronizing signal (repetition frequency of about 15 KHz) from the video signal. Time constant circuit 4d
And a means for generating a detection voltage. Therefore, the detection circuit 4 is charged with the peak value of the synchronization signal, and the charged voltage is held at a certain level by the time constant circuit 4d between successive horizontal synchronization signals. The detection voltage generated by the detection circuit 4 is input to the next operational amplifier circuit 5 and is DC-amplified to an appropriate level with an amplification degree determined by a ratio between the input resistance 6 and the feedback resistance 7.

At this stage, the amplified detection voltage from the operational amplifier circuit 5 has not been completely smoothed yet, and has a pulsating component. The voltage is smoothed and input to the variable gain circuit 1 as a gain control voltage. The smoothing circuit 8 forms an integrating circuit with the resistor 9 and the capacitor 10, and the time constant of the resistor 9 and the capacitor 10 is set to be larger than the time constant of the detection circuit 4. In this manner, by making the time constant of the detection circuit 4 small and increasing the time constant of the smoothing circuit 8, the detection circuit 4 has a certain level of followability to the level change of the synchronizing signal, and the smoothing circuit 8 has sufficient performance. Smoothing can be performed. Further, the first diode 1 connected in parallel in the opposite direction to the first diode 1 is connected to the resistor 9 of the time constant circuit 8.
The first and second diodes 12 are connected in parallel.

Thus, the first and second diodes 1
By connecting the resistors 1 and 12 in parallel to the resistor 9, for example, when the output of the operational amplifier circuit 5 suddenly changes in response to a sudden change in the detection voltage from the detection circuit 4, these diodes are used. The capacitors 11 are turned on when the
To fast charge or fast discharge.

That is, when the detection voltage from the detection circuit 4 suddenly increases, the output voltage of the operational amplifier circuit 5 also sharply increases, and the first diode 11 conducts to charge the capacitor 10. I do. In this case, charging can be performed at a much faster speed than charging through the resistor 9. Conversely, when the detection voltage from the detection circuit 4 suddenly decreases, the output voltage of the operational amplifier circuit 5 also sharply decreases, and the capacitor 10 is charged by the conduction of the second diode 12. The discharged voltage is discharged. Also in this case, discharge can be performed at a much faster speed than discharge through the resistor 9. Therefore, with a very simple configuration, the voltage charged in the capacitor 10 faithfully responds to a change in the detection voltage from the detection circuit 4.

The output of the smoothing circuit 8 is supplied to the amplifying circuit 1
The gain or attenuation of the variable gain circuit 1 is controlled by inputting the signal to the variable gain circuit 1 via the control circuit 3. This amplifying circuit 13 is also constituted by an operational amplifying circuit, and its input impedance is extremely high, so that it does not affect the charging / discharging operation of the smoothing circuit 8. Moreover, the presence of the amplifier circuit 13 does not affect the operation of the smoothing circuit 8 even if there is a change in the input impedance due to a change in the gain of the variable gain circuit 1 or a change in the amount of attenuation.

[0020]

As described above, the automatic gain control circuit of the present invention has a detection circuit for detecting a continuous synchronization signal and outputting a detection voltage according to the level of the synchronization signal, and a resistor and a capacitor. A smoothing circuit for charging a detection voltage to a capacitor through the resistor to smooth the detection voltage, and connecting a first diode and a second diode connected in parallel in opposite directions to each other with a resistor. Connected in parallel, and the smoothed voltage from the smoothing circuit was input to the variable gain circuit as a gain control voltage, so that it can faithfully follow a sudden change in the level of the TV signal and still perform a stable automatic gain control operation. An automatic gain control circuit can be realized.

Further, in the automatic gain control circuit of the present invention, the smoothed gain control voltage from the smoothing circuit is input to the variable gain circuit via the operational amplifier circuit. It does not affect the charge / discharge operation of the circuit. Moreover, the presence of this operational amplifier circuit does not affect the operation of the smoothing circuit even if there is a change in the input impedance due to a change in the gain of the variable gain circuit or a change in the amount of attenuation.

In the automatic gain control circuit according to the present invention, a time constant circuit for holding a detection voltage generated by detecting the synchronization signal is provided in the detection circuit, and the resistance of the smoothing circuit is set to be smaller than the time constant of the time constant circuit. Since the time constant of the detection circuit and the capacitor is increased, the smoothing circuit can perform sufficient smoothing while providing the detection circuit with a certain level of followability to the level change of the synchronization signal.

[Brief description of the drawings]

FIG. 1 is an automatic gain control circuit diagram of the present invention.

FIG. 2 is a diagram of a conventional automatic gain control circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Variable gain circuit 2 Divider 3 Video detection circuit 4 Detection circuit 4a Detection diode 4b Discharge resistance 4c Charge capacitor 4d Time constant circuit 5.13 Operational amplifier circuit 6 Input resistance 7 Feedback resistance 8 Smoothing circuit 9 Resistance 10 Capacitor 11. 12 Diode

Claims (3)

[Claims] 1. A detection circuit for detecting a continuous synchronization signal and outputting a detection voltage according to the level of the synchronization signal, and a resistor and a capacitor, and the detection voltage is supplied to the capacitor via the resistor. A smoothing circuit for charging and smoothing the detection voltage; a first diode and a second diode connected in parallel in opposite directions to each other are connected in parallel to the resistor; An automatic gain control circuit, wherein the smoothed voltage is input to a variable gain circuit as a gain control voltage. 2. The automatic gain control circuit according to claim 1, wherein the smoothed gain control voltage from said smoothing circuit is input to said variable gain circuit via an operational amplifier circuit. 3. A time constant circuit for holding the detection voltage generated by detecting the synchronization signal in the detection circuit,
3. The automatic gain control circuit according to claim 1, wherein a time constant of said smoothing circuit and said resistor is larger than a time constant of said time constant circuit.