JPH0520064Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to an AGC (automatic gain control) circuit of a video signal processing circuit in a home VTR (video tape recorder) or the like.

(B) Conventional technology Generally, in a home VTR, the luminance signal in the video signal is FM-modulated and recorded on video tape. At this time, AGC is applied so that the white peak of the luminance signal is constant, and then FM modulation is performed. I try to do it. FIG. 2 is a circuit diagram showing such an AGC circuit, in which a composite video signal from an input terminal 1 is amplified by a variable gain amplifier circuit 2 and then applied to a clamp circuit 3. The composite video signal whose leading edge of the horizontal synchronization signal has been clamped by the clamp circuit 3 is applied to an addition circuit 4 and a synchronization separation circuit 5. The horizontal synchronization signal separated by the synchronization separation circuit 5 is delayed by a delay circuit 6, further amplified by an amplifier circuit 7, and added to the composite video signal by an addition circuit 4. The delayed and amplified horizontal synchronizing signal (hereinafter referred to as key pulse) is located at the pedestal of the back porch of the horizontal synchronizing signal in the composite video signal, and its amplitude ranges from the pedestal to the white peak of the video signal. The amplitude is set to be slightly larger than the amplitude of . The composite video signal to which the keyed pulse has been added is subjected to peak detection in a peak detection circuit 8. The peak detection output is applied via a DC amplifier circuit 9 to a smoothing circuit 12 consisting of a resistor 10 and a capacitor 11, and is converted into a DC voltage. Then, the DC voltage is applied to the variable gain amplifier circuit 2 as a gain control signal. Therefore, a composite video signal with a constant white peak can be obtained at the output end of the variable gain amplifier circuit 2, and the composite video signal is transmitted to the FM modulation circuit 1.
If applied to 3, FM modulation can be performed. or,
By applying the composite video signal to the television receiver 14, the video signal to be recorded can be monitored.

The AGC circuit shown in Figure 2 is, for example,
It is described in Publication No. 58-29660.

(c) Problems to be solved by the invention By the way, the AGD circuit shown in Figure 2 stops operating before the power is turned on. At that time, the electric charges of the capacitor 16 forming the integrating circuit 15 of the peak detection circuit 8 and the capacitor 11 of the smoothing circuit 12 have become zero. If the power is turned on in this state, the upper end voltage of the capacitor 11 of the smoothing circuit 12 is zero as described above, so the gain of the variable gain amplifier circuit 2 becomes maximum at the same time as switching, and the output terminal has the set level. A composite video signal with a large amplitude exceeding . The composite video signal is then applied to a peak detection circuit 8 via a clamp circuit 3 and an adder circuit 4. The peak detection circuit 8 is set to have a short charging time constant and a long discharging time constant.
Therefore, when the composite video signal is applied, the aforementioned large amplitude peak is detected, and the detected output is applied to the smoothing circuit 12 via the DC amplifier circuit 9. The time constants of the smoothing circuit 12 are set to have equal charging and discharging time constants in view of response characteristics. Therefore, when the detection output is applied, the capacitor 1
1 is immediately charged and a high DC voltage is generated. The level of the DC voltage exceeds the range of normal gain control voltages. Therefore, the gain of the variable gain amplifier circuit 2 is abnormally reduced, and a composite video signal with an amplitude significantly lower than the set level is generated at its output terminal.
The composite video signal is applied to the peak detection circuit 8, but since the discharge time constant of the peak detection circuit 8 is set long, the capacitor 16 continues to hold a high voltage and it takes a long time to discharge. It takes. Therefore, the output DC voltage of the smoothing circuit 12 also maintains a high level, and the gain of the variable gain amplifier circuit 2 remains low. Therefore, the composite video signal from the output end of the variable gain amplifier circuit 2 will have an abnormally low amplitude for a while after switching. This problem has a particularly serious effect when the composite video signal output from the AGC circuit is monitored by the television receiver 14. That is, when monitoring, the output composite video signal of the variable gain amplifier circuit 2 is supplied to the television receiver 14 at the same time as switching.
For a while after switching, a composite video signal with an abnormally small amplitude will be reproduced, causing the problem that the reproduction screen will become dark.

(d) Means for solving the problems The present invention has been developed in view of the above points, and includes a variable gain amplifier circuit to which an input signal is applied, and a peak detection circuit for peak-detecting the output signal of the variable gain amplifier circuit. An AGC circuit comprising a detection circuit and a smoothing circuit that smoothes an output signal of the peak detection circuit and applies it as a gain control signal to the variable gain amplifier circuit,
The present invention is characterized in that a discharging means is provided for detecting that the terminal voltage of the smoothing circuit has exceeded a predetermined value and forcibly discharging a capacitor constituting the peak detection circuit.

(E) Effect According to the present invention, when it is detected that the terminal voltage of the smoothing circuit has exceeded a predetermined value, the capacitor of the peak detection circuit is forcibly discharged and the output signal of the peak detection circuit is rapidly reduced. Therefore, it is possible to rapidly reduce an excessive gain control signal applied to the variable gain amplifier circuit when the power is turned on.

(F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
17 is a signal processing circuit consisting of the clamp circuit 3, adder circuit 4, synchronous separation circuit 5, delay circuit 6 and amplifier circuit 7 shown in FIG. 2; 18 is a base to which the terminal voltage of the smoothing circuit 12 is applied;
A transistor 19 is connected to the upper end of the transistor 18 and operates as a discharging means, and a diode 19 is connected between the emitter of the transistor 18 and the ground for setting a reference voltage. In addition, in Figure 1, the second
Circuit elements that are the same as those in the figures are given the same reference numerals and their explanations will be omitted.

Next, the operation will be explained. First, when the power is not turned on, the charges on the capacitor 16 of the peak detection circuit 8 and the capacitor 11 of the smoothing circuit 12 are zero. Assume that the power is turned on from this state. Immediately after switching, the terminal voltage of the capacitor 11 is zero, so the gain of the variable gain amplifier circuit 2 becomes maximum. Therefore, if a composite video signal is applied to the input terminal 1, the composite video signal is excessively amplified by the variable gain amplifier circuit 2,
At its output end, a composite video signal with a large amplitude exceeding the set level is generated. The composite video signal is applied to the signal processing circuit 17, subjected to signal processing similar to that in FIG. 2, and then applied to the peak detection circuit 8. Then, the peak detection circuit 8
A high voltage higher than the set level is generated at the upper end of the capacitor 16, and the output signal corresponding to the high voltage is
After being amplified by the DC amplifier circuit 9, it is applied to the smoothing circuit 12 . Therefore, a high DC voltage higher than a set level, that is, a large gain control signal is generated at the upper end of the capacitor 11 of the smoothing circuit 12 , and the gain control signal is applied to the variable gain amplifier circuit 2. Meanwhile, the DC voltage is applied to the base of the transistor 18. Here, the number of diodes 19 connected between the emitter of the transistor 18 and the ground is appropriately set so that abnormal control of the variable gain amplifier circuit 2 is detected, and the transistor 18 is connected to the DC voltage at the time of the abnormal control. Turns on depending on the situation. Then,
The excessive charge stored in the capacitor 16 is
It is rapidly discharged via the collector-emitter path of transistor 18 and diode 19 . Therefore, the upper end voltage of the capacitor 16 is nV _D +
V _CE (however, V _D is the rising voltage of diode 19 ,
V _CE rapidly decreases to the collector-emitter saturation voltage of the transistor 18 (n is the number of diodes 19 ), and the output signal of the peak detection circuit 8 also decreases accordingly. Then, the level of the output signal of the DC amplifier circuit 9 also decreases, so that the terminal voltage of the capacitor 11 of the smoothing circuit 12 becomes a predetermined value. Therefore, the upper end voltage of the capacitor 11 decreases, and the level of the gain control signal applied to the variable gain amplifier circuit 2 decreases. As a result, the gain of the variable gain amplifier circuit 2 increases rapidly, and the composite video signal from the input terminal 1 is amplified with a predetermined gain. Note that as the upper end voltage of the capacitor 11 decreases, the transistor 18 eventually turns off and the discharge of the capacitor 16 stops.

Therefore, a composite video signal of a predetermined amplitude can be obtained at the output terminal of the variable gain amplifier circuit 1 immediately after the power is turned on, and the composite video signal is transmitted to the FM modulation circuit 13.
If applied to the television receiver 14, correct FM modulation can be performed immediately after switching, and if applied to the television receiver 14, normal monitoring can be performed immediately after switching.

(g) Effects of the invention As described above, according to the invention, in an AGC circuit comprising a peak detection circuit and a smoothing circuit, when it is detected that the output DC voltage of the smoothing circuit has exceeded a predetermined level, the peak detection circuit Since the internal capacitor is rapidly forced to discharge, an output signal of a predetermined level can be obtained even immediately after power is turned on. Therefore, as in the example, AGC
To prevent abnormal images from appearing on the screen when monitoring the output composite video signal of the circuit with a television receiver.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional AGC circuit. 2...Variable gain amplifier circuit, 8...Peak detection circuit, 12 ...Smoothing circuit, 15 Integrating circuit, 18...
Transistor, 19 ...diode.

Claims (1)

[Scope of utility model registration request] A variable gain amplifier circuit to which an input signal is applied; a peak detection circuit that peak-detects the output signal of the variable gain amplifier circuit; an amplifier circuit that amplifies the output signal of the peak detection circuit; and a smoothing circuit that smooths and applies a gain control signal to the variable gain amplifier circuit, the base of which is connected to one end of the capacitor that constitutes the smoothing circuit, and the collector of which is connected to one end of the capacitor that constitutes the peak detection circuit. A discharge circuit consisting of a detection transistor connected to one end and a plurality of diodes connected between the emitter of the detection transistor and ground is provided, and the terminal voltage of the capacitor constituting the smoothing circuit is equal to or higher than a predetermined value. The AGC circuit is characterized in that the transistor is turned on when the peak detection circuit is turned on, and a capacitor constituting the peak detection circuit is forcedly discharged.