JPH05274787A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To prevent the output of an excessive signal at the time of conducting a switch in an automatic gain control circuit containing a switch circuit for connecting/disconnecting an output signal directly in a loop. CONSTITUTION:A mute pulse is inputted to a mute switch 7 and a current source 21 in common through a mute pulse input terminal 20. In the state of that the mute switch 7 is turned off by the mute pulse, a current whose absolute value is larger and whose polarity is opposite to the current outputted from a comparator 16 is outputted by the current source 21. Thus, a voltage generated in a hold capacitor C2 is set so that the gain of a first voltage control amplifier circuit 2 becomes a minimum. In the transitional state of that the mute pulse disappears, since the gain of the first voltage control amplifier circuit is converged to a setting level applied to a first control signal input terminal gradually from the minimum, an excessive recording current is not outputted to a recording current output terminal 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control circuit, and more particularly to an automatic gain control circuit for a recording / amplifying circuit of a magnetic recording / reproducing apparatus.

[0002]

2. Description of the Related Art FIG. 3 shows an automatic gain control circuit of a recording amplifier circuit of a conventional magnetic recording / reproducing apparatus. The first voltage control amplifier circuit 2 amplifies the frequency-modulated luminance signal provided via the first input terminal 1 and outputs it to the adder circuit 3.
The second voltage control amplifier circuit 5 amplifies the low-frequency conversion color signal given through the second input terminal 4 with an amplification factor according to the control voltage given to the second control signal input terminal 6, Output to the adder circuit 3. The adder circuit 3 adds the output of the first voltage-controlled amplifier circuit 2 and the output of the second voltage-controlled amplifier circuit 5, and via the mute switch 7 controlled by the mute pulse applied to the mute pulse input terminal 20. Output to terminal 8. Terminal 8 is the first capacitor C1
And a first resistor R1 are connected to a terminal 10 via a voltage-current conversion circuit 9 formed of a series circuit. Here, the terminal 10
Is connected to the input of the current amplifying circuit 11, but the input impedance of the current amplifying circuit 11 is set so low as to be on the order of 10% of the impedance of the voltage-current converting circuit 9, so that the terminal 8 The voltage output to is converted into a current substantially equal to a value obtained by dividing the voltage by the impedance of the voltage-current conversion circuit 9, and is supplied to the current amplification circuit 11 via the terminal 10. Current amplifier circuit 1
1 amplifies the current flowing into the terminal 10 and outputs it to the recording head L1 via the recording current output terminal 12. The damping resistor R2 reduces the variation in the recording current with respect to the frequency due to the resonance of the output capacitance of the current amplification circuit 11, the stray capacitance of the wiring to the recording head L1, and the inductance of the recording head L1. It is connected between the current output terminal 12 and the ground. The current flowing through the recording head L1 is supplied to the recording current detection circuit 14 via the recording current detection terminal 13, converted into a voltage, detected by the detection circuit 15 and supplied to the comparison circuit 16.

The comparison circuit 16 compares the control voltage applied via the first control signal input terminal 17 with the output voltage of the detection circuit 15 and outputs the horizontal sync signal applied via the horizontal sync signal input terminal 18. Only during the period that there is. The difference generated by the comparison is output as a current to the hold capacitor C2 via the control voltage output terminal 19. The voltage generated by charging and discharging the output current of the comparison circuit 16 in the hold capacitor is the first voltage control amplifier circuit 2
It is fed back as the gain control voltage.

Next, the operation will be described. A current obtained by superimposing a frequency-modulated luminance signal applied to the first input terminal 1 and a current proportional to the low-frequency conversion color signal applied to the second input terminal 4 flows through the recording head L1. The amplitude of the recording current is the recording current detection circuit 1
4, detected by the detection circuit 15 and compared with the control signal applied to the first control signal input terminal 17 by the comparison circuit 16. The comparison circuit 16 has a horizontal synchronization signal input terminal 18
The comparison operation is performed only during the period when the horizontal synchronizing signal is applied to the detection circuit 15. However, during the period when the horizontal synchronizing signal is present, the low-pass conversion chrominance signal is not present, so the detection circuit 15 is composed of only the frequency-modulated luminance signal component. The amplitude of the recording current is detected. Therefore, the comparison circuit 16 detects the difference between the amplitude of only the frequency-modulated luminance signal and the control signal applied to the first control signal input terminal without interfering with the low-frequency conversion color signal, and the difference is detected. Is output as a charge / discharge current of the hold capacitor C2. If the detected recording current amplitude is smaller than the voltage applied to the first control signal input terminal 17, the gain of the first voltage control amplifier circuit 2 increases,
On the contrary, if the detected recording current amplitude is larger than the voltage applied to the first control signal input terminal 17, the output of the comparison circuit 16 is set so that the gain of the first voltage control amplifier circuit 2 decreases. The current is set. Therefore, a feedback circuit is formed in which the gain of the first voltage control amplifier circuit 2 is controlled so that the recording current amplitude and the control signal of the first control signal input terminal 17 always match. As a result, automatic gain control is performed so that a frequency-modulated luminance signal having a recording current amplitude corresponding to the control voltage applied to the first control signal input terminal always flows through the recording head L1.

Next, the operation of the mute switch 7 will be described. The mute switch 7 is controlled by a mute pulse applied to the mute pulse input terminal 20 so as to obtain a state in which no signal component is output to the recording current output terminal 12 when the recording amplifier circuit shown in FIG. 3 is in operation. .. This is because the recording current output is performed in order to perform recording, such as joint recording, in which the recorded signal is temporarily overwritten while maintaining continuity with the already recorded signal. The purpose is to interrupt the recording current flowing through the terminal 12 at high speed.

[0006]

In the conventional automatic gain control circuit of the recording amplification circuit, the mute switch for performing joint photographing or the like is placed in the feedback loop of the automatic gain control circuit. As a result, when the frequency-modulated luminance signal is input and the mute switch is turned off, no recording current appears in the recording head.
The control voltage fed back to the voltage control amplifier circuit that performs automatic gain control is set so that the gain becomes maximum. From this state, in a transitional state in which the mute switch is in a conductive state, the frequency-modulated luminance signal input is amplified with the maximum gain, so that an excessive recording current temporarily flows in the recording head. Such an excessive mustache-shaped recording current
The value is different from the value suitable for recording, and a streak-shaped white line appears on the screen at the switching point of the mute switch during playback. Therefore, the image recorded on the tape by the joint shooting has a drawback that the image quality at the time of starting the joint shooting is deteriorated and becomes very unsightly when reproduced.

[0007]

SUMMARY OF THE INVENTION An automatic gain control circuit according to the present invention comprises a voltage control amplifier circuit for amplifying an input signal and a switch means for receiving an output of the voltage control amplifier circuit as a first control signal input. A detection circuit that receives the output of the switch means, a capacitor that holds the output of the detection circuit, a feedback path that returns the voltage generated in the capacitor to the voltage control amplifier circuit, and an output to the capacitor. Connect the first
Control signal input and a current source having a second control signal input commonly connected.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of the first embodiment of the present invention. In addition,
Parts having the same functions as those of the conventional example shown in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted.

The current source 21 has a mute pulse input terminal 2
A mute pulse given to 0 is given,
The output is connected to the hold capacitor C2 via the control voltage output terminal 19 in common with the output of the comparison circuit 16.
The output current of the current source 21 is output when a mute pulse is applied, and its value is larger than the absolute value of the current value output by the comparison circuit 16 when the mute pulse is input, and its sign is set to be opposite. Accordingly, when the mute pulse is input, the comparison circuit 16 causes the first voltage control amplifier circuit 2
The output current is canceled to maximize the gain and the voltage generated in the hold capacitor C2 is set to minimize the gain of the first voltage control amplifier circuit. Immediately after the mute pulse input disappears, the gain of the first voltage control amplifier circuit 2 is minimum, so that the frequency-modulated luminance signal component does not flow excessively in the recording head L1.

Further, the current source 21 in this embodiment will be described in detail with reference to FIG. The base of the NPN transistor Q1 is connected to the mute pulse input terminal 20, the collector is connected to the control voltage output terminal 19, and the emitter is connected to the ground via the resistor R3. The mute voltage applied to the mute pulse input terminal 20 is E, the NPN transistor Q
Assuming that the voltage between the base and the emitter of 1 is V _BE , a current of approximately (E−V _BE ) / R3 is output to the collector of the NPN transistor Q1 during mute. Therefore, by selecting the value of R3, the output current of the current source can be easily set so as to cancel the current output by the comparison circuit 16 shown in FIG. 1 during mute.

Next, a second embodiment of the present invention will be described with reference to the drawings. FIG. 2 is a block diagram of the second embodiment of the present invention. In addition, parts having the same functions as those of the first embodiment shown in FIG.
The description is omitted.

The blanking circuit 22 includes an inverter 23 whose input is connected to the mute pulse input terminal 20,
The output of the inverter 23 and the horizontal sync signal input terminal are connected to the input, and the output is connected to the comparison circuit 16 AND
Composed of circuits. Since the blanking circuit 22 prohibits the horizontal synchronizing signal from being input to the comparison circuit 16 when the mute pulse is input, only the current source 21 supplies current to the hold capacitor C2 during mute. That is, in the second embodiment, the output current of the current source 21 is set to an appropriate value within the range in which the gain of the first voltage control amplifier 2 can be kept to a minimum independently of the output current of the comparison circuit 16. Can be set. Therefore, the present embodiment has an advantage that the degree of freedom in setting the output current value of the current source 21 can be increased.

[0013]

As described above, according to the present invention, the holding capacitor for holding the gain control voltage in the automatic gain control circuit is supplied with the current by the current source working in conjunction with the mute switch. Therefore, even when the mute switch is off, it is possible to prevent the gain control voltage held in the hold capacitor from changing so as to increase the gain. There is an effect that there is no. In particular, when it is used as an automatic gain control amplifier circuit of a recording amplifier circuit of a magnetic recording / reproducing apparatus, it is possible to prevent deterioration of an image when a joint shooting or the like is performed.

[Brief description of drawings]

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

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

FIG. 3 is a block diagram of a conventional example.

FIG. 4 is a circuit diagram of a current source 21 shown in FIG.

[Explanation of symbols]

 1 1st input terminal 2 1st voltage control amplification circuit 3 addition circuit 4 2nd input terminal 5 2nd voltage control amplification circuit 6 2nd control signal input terminal 7 mute switch 8 terminal 9 voltage-current conversion circuit 10 terminals 11 current amplification circuit 12 recording current output terminal 13 recording current detection terminal 14 recording current detection circuit 15 detection circuit 16 comparison circuit 17 first control signal input terminal 18 horizontal synchronization signal input terminal 19 control voltage output terminal 20 hold capacitor 21 Current source

Claims (1)

[Claims] 1. A voltage-controlled amplifier circuit, a switch means having a first control signal input and having an output of the voltage-controlled amplifier circuit as an input, a detection circuit having an output of the switch means as an input, In an automatic gain control circuit, which is connected to the output of the detection circuit and holds the output of the detection circuit, and a feedback path for returning the voltage generated in the capacity to the voltage control amplifier circuit, the output is connected to the capacity. An automatic gain control circuit comprising a current source having a second control signal input commonly connected to the first control signal input.