JPS5922414A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of sag, by avoiding a detected output from being inputed with a control pulse in comparing the detected output with a reference voltage and obtaining the detected output at the point of time via a time constant circuit. CONSTITUTION:The detected output Vo and the reference voltage Vr are compared at a comparison detecting circuit 13. The comparison detecting circuit 13 is controlled with the control pulse Vc and the comparison detection output is stopped for a prescribed period. An output of the comparison detection circuit 13 is inputted to an AGC voltage forming circuit 15 and outputted via a time constant circuit 18 comprising a parallel circuit of a discharge resistor 16 and a charging capacitance 17 of the AGC voltage forming circuit 15.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a video intermediate frequency amplification (hereinafter referred to as PIF) of general electronic equipment, such as a color television receiver, and an automatic column 1q control (hereinafter referred to as AGC) circuit. .) to detect the 〇〇 voltage, and is particularly suitable for an automatic column LJllI control circuit with improved charge/discharge characteristics related to its AGC time constant.

[Background technology of the invention]

In general, in color television receivers, if there is a strength or weakness in the input signal induced in the antenna, the video detection output will similarly fluctuate and the contrast of the screen will change. However, in order to obtain a constantly constant video detection output, a ΔGG circuit is provided to compensate for the above fluctuation.

As this AGC means, detection is required! 1I71! It is necessary to have high pressure, be less susceptible to external noise, and be able to follow changes in the received signal at high speed.
Generally, peak value type ΔGG, average value type AGC, and keyed type AGC are well known.

Peak value type AGC uses the fact that the synchronizing signal part of the television high-frequency signal is always sent out at a constant amplitude, regardless of the brightness or darkness of the screen, on the transmitting side, and detects changes in the amplitude of this synchronizing signal. The voltage is set to 〇〇.

This method has the characteristics that the detection output voltage is large and the 'U A G C voltage does not change depending on the brightness of the image.
If noise having an amplitude equal to or greater than that of the synchronizing signal is applied, the AGC7[f pressure will change, so it is usually necessary to insert noise filters before and after the AGC7.

Furthermore, since the amplitude of the video portion of the signal varies depending on the average brightness of the screen, the average value type AGC is used as a reference voltage for the average value level AGC of the video signal. Furthermore, the keyed AGC uses keying pulses extracted during a predetermined period to extract the synchronization signal part from the color television signal, and outputs a detected voltage corresponding to this synchronization level to the AGC.
This is used as a CW quasi-voltage. Therefore, the noise mixed into the video signal period has no relation to the AGC detection voltage and is less affected by the noise.

Among the above AGC methods, the peak value type method and the keyed type method are common, and specifically, the configuration shown in FIG. 1 is known. This is because the video signal (1) is amplified by the amplifier circuit 1, then detected by the detection circuit 2, and the JVO (for detection) is sent to the comparison detection circuit 3, while the control reference bias circuit 4
The reference voltage Vr from is also input to this comparison detection circuit 3,
Here, the output of the comparison between the two is inputted to the amplifier circuit 1 as the AGC voltage VAGC. Further, a time constant circuit including a discharge resistor R and an external capacitor C is attached to the comparison detection circuit 3, and the charging/discharging characteristics are determined by this time constant. In other words, the time constant during discharge is below the width of the horizontal synchronization pulse)! ! Then, the external capacitor C is charged during the period in which the horizontal synchronizing pulse is applied. By setting the time constant during discharging to be larger than the horizontal synchronization, the detected output Vo becomes an ABC voltage VAQC that approximately follows the level fluctuation of the synchronization signal in the video signal. The voltage waveform detected by the detection circuit 2 is as shown in FIG. 2(a), and is obtained as a constant charging/discharging output. This is compared with the reference voltage Vr in the comparison detection horn circuit 3, and AG([pressure V8GC(
Figure 2 b) is obtained.

[Problems with background technology]

In this way, in the beater value type AGC method, the synchronization signal is transferred to GC? Since this is the standard for generating the i pressure, the detection output (2) may cause a so-called sag (Saσ) during the initial transition period during charging and discharging due to the pulse nature of the synchronizing signal.

That is, FIG. 2 shows the AGC voltage formed in one horizontal period at the boundary between the equalization pulse period and the vertical synchronization pulse period in the vertical retrace period, and the detected voltage output from the detection circuit 2 during this period. The horn ■0 is charged with the equalization pulse of 004H (period A), discharged during the following slit period B, and further charged by applying the vertical synchronization pulse 0.461-f (period D), and the subsequent slit period B. ]・The waveform is discharged during period (E). At the beginning of this charging and discharging, a-1, a-2
・, sag as shown in a-3 and a-4 occurs. however,
Each waveform is a detected voltage waveform with a time constant circuit connected. If the amount of sag generated is large, the AGC voltage V
Large fluctuations in AC and C can cause synchronization instability and brightness gradients, and even if the amount of occurrence is small, check the transmission status of the broadcasting station and the radio wave status due to disturbances such as ghosts.
Although extremely harmful to b correction, effective suppression reduction tI
There was a problem with not being able to find a suitable place.

In order to eliminate this sag, it is preferable to have a long charging time constant and a long discharging time constant, but since the period of the synchronization signal is shorter than the video signal period, the best 1q solution is to shorten it during charging and lengthen it during discharging. there were.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and is designed to prevent unnecessary noise from being superimposed on the AGC voltage when detecting pressure in the AGC 71 using the peak value type AGC method, thereby obtaining an image with uniform brightness. The purpose of the present invention is to provide an automatic gain control circuit that achieves the following. , Tatami [Summary of the Invention] The present invention amplifies and detects an incoming saw signal, inputs the detected output to a comparison detection circuit 13, and compares the detected output with a reference voltage Vr corresponding to a synchronizing signal level. When doing so, the control pulse generated for a limited period of time forces the 0! ! Optionally, the detection aperture j is not input. The AGC 7 which is the output of the comparison detection circuit 13
(The f pressure is for detection) The detection output at the time when j is not input is obtained via a time constant circuit. By cutting off the input of the detection output in this way, the cause of sag generation can be cut off.

[Embodiments of the invention]

Hereinafter, an embodiment of the present invention will be described based on the drawing of FIG.

FIG. 3 is a block diagram of this embodiment, and FIG. 4 is a specific circuit diagram. Input signal 1 is the previous stage layer width circuit 11
After inputting the amplified output to the detection circuit 12 and detecting it, the detected output ■0 is derived to the next stage, and the detected output ■0 is inputted to the comparison detection circuit 13.

Further, the output J of the control reference bias circuit 14, that is, the reference voltage ■r to which a voltage is set according to the synchronizing signal level, is also input to the comparison detection circuit 13.

Further, the output of the comparison detection circuit 13 is sent to the AGC voltage formation circuit 15.
This AGO voltage forming circuit 15 has the following steps:
A time constant circuit 18 composed of a parallel circuit of a discharging resistor 16 and a charging capacitor j117 is attached.

The comparison detection circuit 13 is controlled by a commi-roll pulse VC, which stops the comparison detection output for a predetermined period. This control pulse VC can be obtained, for example, by delaying a synchronizing signal when a sag occurs, so that it appears as a pulse during, for example, a horizontal period.

The specific configuration of this comparison detection circuit 13 is shown in diagram 1T4. In FIG. 4, the comparison detection circuit 13 includes a first transistor 21 to which the detection signal JVO is inputted to the base;
A voltage bias 23 is provided between the base of the second transistor 22 forming a differential pair with the 11th transistor to supply the DC voltage setting (IIIVo-1-VF) higher than the detection output 0 to the ground. The third and fourth transistors 25 and 26, which are used to select and supply the current source 24 to the first and twenty-first transistors 21 and 22, are connected to each collector.
The emitter paths are connected in series, and a voltage source Vp is attached to the base of the 31st transistor 25 between it and ground, and 70 control pulses are applied to the base of the 4th transistor 26. Furthermore, the first. The first transistor has its cathode connected between the emitters of the third transistor.
A second diode 28 whose cathode is connected between the diode 27 and the emitter-collector of the second and fourth transistors.
and the anode of each diode 27, 28 and the power supply terminal 2
A current source 30 is interposed between 9 and 9.

Next, a current source 3 is connected to the emitter side of the v1 transistor 31 whose base is connected to the anodes of both diodes 27 and 28.
2 is inserted, and the output j of the comparison detection circuit 13 is derived to the AGC7rA pressure forming circuit 15 at the next stage. The AGC voltage forming circuit 15 includes a fifth transistor 33 and a sixth transistor 34 forming a differential pair, and a load resistor 35 connected to the collector side of the 51st transistor 33. and protrusion terminal 3
Diode 37 inserted between 6 and 6. It is composed of a current source 38° on the common emitter side and a time constant circuit 18 provided between the output terminal 36 and ground. Further, the reference voltage Vr is applied from the control reference bias 14 to the bases of the 61st to transistors 34. In this case, the control pulse VO, which is the main means of the present invention, is applied to the bases of the fourth transistor It is applied to

In the above circuit, first, the controller l~rollno<)less VC
When there is no control, that is, the control 1 to roll pulse Vchi is off.
! ji 3! ! Reference voltage VDJ for selection: Since the third transistor 25 is conductive, the detection voltage VDJ is low.
O remains the first transistor 21. Dia j −1:
27 to the base 1 of the buffer I/transistor 3'1.
(supplied. Also, when the control pulse VC > base I ■ lightning voltage p (when there is a control pulse, HcL, the second
The base setting voltage VO+VF of the transistor 22 is superimposed on the detection output VO and is supplied to the buffer i to silance 31. Therefore, when there is CON 1 HERO NO HEALTH 0, the detection ratio]jVo becomes the corrected output to be detected from which the sag during the transition period of charging and discharging is removed.

Figure 5(a) shows the detected voltage waveform at this time.
When the roll pulse VC is 0, the sags a-1, a-2h< shown in FIG. 2(a) occur, but
When control pulse (2) 0 is added, the sag a-3 of 1, t which appears at the beginning of charging during the next vertical synchronizing pulse period is shaped by the set voltage Vo-1-VF. In the next period E, V gummy -4 is also erased.

In the fifth and sixty-first transistors 33 and 34 of the next stage, the corrected detection horn (FIG. 5a) and the reference voltage V
The comparison result is Vo (or VO+V
F)>Vr, the time constant circuit 18. Δ to the charging capacity fi17
The GC voltage VAGC is charged continuously (period A). This charging voltage is discharged with a long time constant of .degree. C. when the diode 37 is turned off at the beginning of period B when the detected output Vo (or VO+VF) is inverted. Then, when the control pulse Vc is applied, VC=Vll +VF >Vll Tect5V
), detection output when Vc<Vp(7) ■Vo for 0
+VF > Vr (1- transistor 33 is V o
+VF is only led out to the anode of the diode 37) while keeping the diode 37 off. Therefore, since the voltage applied to the charging capacity ffi 1 V is further discharged via the discharge resistor 16, the AGC detection operation is stopped while the controller 1 to roll pulse VO is output. Become.

As a result, the AGC voltage waveform shown in FIG. 5(b') is Vc
<Va) The charging voltage accumulated in the capacitor 1-17 is discharged through the resistor 16, and the charging waveform I)-1 and the discharging waveform I)-2 become a continuous waveform. By this, A.G.
An AGC voltage VAGC that eliminates the fluctuation caused by the sag that occurs in the 'C voltage VAGC and reduces the brightness slope.
You can get 1.

〔Effect of the invention〕

As described above, according to the present invention, the GC is controlled for a predetermined period by a control pulse generated with a specified period.
Since the formation of voltage is stopped, the AGC voltage before the stop is maintained, making it possible to eliminate some of the sag.
This has the effect of flattening the charge/discharge characteristics of the C voltage. Therefore, there is an effect that synchronization stability and l1w degree inclination 1mi1 can be reliably compensated for by IR.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional GC circuit, jl'!
2 is a waveform diagram showing the operating waveforms of FIG. 1, FIG. 3 is a block diagram of the A G and C circuit according to the embodiment of the present invention, FIG. 4 is a specific circuit diagram of FIG. 3, and FIG. 5 15(a) is a waveform diagram showing the detected output waveform, FIG. 15(b) is a waveform diagram showing the AGC voltage, and FIG. 5(0) is a waveform diagram showing the control pulse voltage waveform. 11... Pre-stage amplifier circuit, 12... Detection circuit, 13.
... Comparison detection circuit, 14 ... Control reference bias circuit, 15 ... AGO voltage formation circuit, 18 ... Time constant circuit (16 ... resistor, 17 ... capacitor), 21.
'2.2゜25.26.31.33.34...]・Lansistor, 27.28.47...Diode, V r
...Reference voltage, VO...Detection no.j, VO+VF・
...Setting voltage for detection output, VC...Control voltage, Vp...Basic I$ lightning voltage for Bontrolumin pressure

Claims (1)

[Claims] a video amplification circuit, a detection circuit that detects this output, and an AGC voltage that inputs the output of this detection circuit and compares it with a reference voltage corresponding to a preset synchronization signal level to form an AGC voltage that controls the gain of the video amplification circuit. a comparison detection circuit, a switching means for switching the comparison target of the detection circuit to a separately set set voltage instead of the detection output using a control pulse generated in a specified period; and a switching means provided at a subsequent stage of the comparison detection circuit. AGC before switching? ! ! An automatic column 19 control circuit comprising a time constant circuit for maintaining pressure.