JPS5922488A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of sag, by providing plural comparison detecting circuits having different time constants and switching the circuits with an external control pulse for selecting a comparison detecting circuit having a long time constant for a specific period only. CONSTITUTION:An input signal Vi is inputted to a front stage amplifier circuit 11, an amplified output is inputted to a video detecting circuit 12 for detection and a detected output Vo is outputted to a video amplifier stage of the rear stage. The detected output Vo is inputted to the comparison detecting circuits 13-1 and 13-2. The output of the comparison detecting circuits 13-1, 13-2 is connected to terminals 15-1, 15-2 in the switching circuit 15, and the output of the two comparison detecting circuits is switched with a control pulse Vc for a limited period and the selected output is applied to the front stage amplifier circuit 11 as an AGC voltage output. The control pulse Vc is obtained so that the synchronizing signal when a sag takes place is delayed and the pulse is given in one horizontal period.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention adds automatic gain control (hereinafter referred to as AGC) to a video intermediate frequency amplification (hereinafter referred to as PIF) circuit of general electronic equipment, such as color television receiver equipment. Therefore, the present invention relates to an automatic gain control circuit that detects an AGO voltage, and in particular has improved charge/discharge characteristics related to the AGC time constant.

[Background technology of the invention]

In general, in color television receivers, if the input signal induced in the antenna is strong or weak, the video detection output will also fluctuate and the contrast of the screen will change. In order to obtain the detection output, an AGC circuit is used to compensate for the above fluctuations.

This AGC means requires a large detection control voltage;
It is necessary to be resistant to the influence of external noise and to be able to follow changes in the received signal at high speed. Generally, peak value type AGC, average value type AGC, and keyed type AGC are used.
C is well known.

Peak value type AGC utilizes the fact that the synchronization 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 synchronization signal. This is the AGC voltage.

This method has the feature that the detection output voltage is large and the AGC voltage does not change depending on the brightness or darkness of the image, but it has the disadvantage that the AGC voltage changes when noise with an amplitude equal to or higher than that of the synchronization signal is applied, so it is usually filtered using a noise filter. It is necessary to intervene before and after that.

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 the detected voltage according to this synchronization level to the AGC.
This is used as a C reference voltage. Therefore, 1. Noise mixed into the video signal period has nothing to do with 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 Vi is amplified by the amplifier circuit 1, then detected by the detection circuit 2, and the detected output ``0'' is input to the comparison detection circuit 3, while the reference voltage ``0'' from the control reference bias circuit 4 is also input. This is input to the comparison detection circuit 3, and the output of the comparison between the two is input to the amplifier circuit 1 as the AGC voltage VAGC.The comparison detection circuit 3 also includes a time constant circuit of a discharge resistor R and an external capacitor C. The charging and discharging characteristics are determined by this time constant.In other words, the time constant during discharging is selected to be equal to or less than the width of the horizontal synchronizing pulse, and the external capacitor C is charged during the period in which the horizontal synchronizing pulse is applied.Then, when discharging By selecting a time constant larger than the horizontal synchronization pulse, the detection output VO becomes the AGC voltage VAGC that approximately follows the level fluctuation of the synchronization signal in the video signal.The voltage waveform detected by the detection circuit 2 is shown in FIG. (a,), which is obtained as a constant charging/discharging output. This is compared with the reference voltage ■r in the comparison detection power circuit 3, and the AGCI pressure VA(,C(second
Figure b) is obtained.

[Problems with background technology]

In this way, in the beater value type AGC method, the synchronization signal is
Since this is used as a standard for generating a GC voltage, the detected output V0 may cause a so-called sag in the initial stage (transient 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 detection output output from the detection circuit 2 during this period. vO is charged with an equalization pulse of 0°04H (period A), discharged during the following slit period B, further charged by applying a vertical synchronization pulse of 0.46H (period D), and then discharged during the following slit period (E). It becomes a discharge waveform. At the beginning of this charging 1fi electric power, a-1, a-, 2
, a-3, and a-4. However, each waveform is a detected voltage waveform with the time constant circuit connected. If this sag occurs in a large amount, the AGC voltage VA
Large fluctuations in GC can cause synchronization instability and brightness gradients, and even if the amount generated is small, it is extremely harmful to checking and correcting broadcasting station transmission conditions and radio wave conditions due to disturbances such as ghosts. However, there was a problem in that effective suppression measures could not be taken.

In order to eliminate this sag, it is preferable to have a long charging time constant and a long discharge specific number, but since the synchronization signal period is shorter than the video signal period, the best solution is to shorten charging time and lengthen discharging time. Ta.

(Object of the Invention) The present invention has been made in view of the above-mentioned points, and when detecting an AGC voltage using a peak value type AGC method, the unnecessary sag superimposed on the AGC voltage is reduced and brightness is improved. The present invention provides an automatic gain control circuit capable of obtaining a uniform image.

(Summary of the Invention) That is, in the present invention, a plurality of comparison detection circuits 13-1 and 13-2 each having an independent time constant are provided, and the comparison detection circuit 1 is controlled by an external control pulse Vc.
3-1.13-2 can be freely switched, and a comparison detection circuit with a sufficiently longer time constant is selected for a specific period than for other periods, thereby eliminating the sag that occurs transiently at the initial stage of each charge/discharge described above. reduced.

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described based on the drawings from FIG. 3 onwards.

FIG. 3 is a block diagram of an embodiment showing the present invention, and FIG. 4 shows a circuit for obtaining an AGC voltage from the detection output used in the embodiment of FIG.

To explain the configuration of FIG. 3, the input signal V+ is input to the preamplifier circuit 11, the output amplified here is input to the video detection circuit 12 for detection, and the detected output VO is sent to the next stage video. While outputting to the amplification stage, this detection output VO is output to a plurality of, in this case, the first and second comparison detection circuits 13-1.13.
-2.

In addition, a control reference bias voltage 14, ie, a reference voltage (r), which is a voltage corresponding to the synchronization signal level (or pedestal level), is applied to each of the comparison detection circuits 13-1 and 13-2. 1st. Second comparison detection circuit 13-1.1
The output of 3-2 is connected to each terminal 15 in the switching circuit 15.
-1.15-2, the output of the first or second comparison detection circuit 13-1.13-2 is switched by the limited time control pulse Vc (hereinafter referred to as control pulse), and the selected output is connected to the previous stage. A to amplifier circuit 11
GC voltage output is supplied as VAGC.

This control pulse Vc can be obtained, for example, by delaying a synchronizing signal when a sag occurs so as to exhibit a pulse in one horizontal period, for example.

13-1°13-2 forming the above AGC voltage VAGC
The specific structure of the first . A second transistor 21,22, a detection diode 23 connected to the collector of the first transistor 21, and a first detection diode 23 connected to the cathode of this diode 23.
It is composed of a time constant circuit 24, and the first time constant circuit 24 is a parallel circuit of a discharge resistor 25 of a predetermined value and a charging capacitor 26.

Further, the second comparison detection circuit 13-2 has the same configuration as the first comparison detection circuit 13-1, and includes third and fourth transistors 27, 28, a detection diode 29, and a second time constant circuit 30. The second time constant circuit 30 includes a discharge resistor 3 having a time constant longer than the time constant of the first time constant circuit 24.
1 and a charging capacity 32 in parallel.

The detection output Vo is input to each base of the first and third transistors 21 and 27 of the comparison and detection circuits 13-1 and 13-2, while the second and third transistors 21 and 27 of the comparison and detection circuits 13-1 and 13-2 receive the detection output Vo at their respective bases. 41st - Control reference bias voltage (DC constant power supply circuit) 1 for each base of transistor 22.28
4, the reference voltage Vr is applied. In addition, the first pair
．． the second transistor 2'1.22 and the third transistor 2'1.22; The fourth transistor 27, 28 is supplied with a current source 33, 34 from its emitter.

Furthermore, the first. The collector output output by the load resistor 35.36 of the third transistor 21.27 is input to the next stage switching circuit 15 via the diode 23.29. This switching circuit 15 includes a pair of transistors 37 and 38 to which the collector outputs are applied to their bases, a diode 39 and 40 connected to their emitters, and a current source 4.
1, and a switching transistor 44 whose base is connected to the anode of the diode 39, 40, and the control pulse Vc is applied to the base of the transistor 43. However, the reference bias Vrs is applied to the base of the other transistor 42. Also,
A current source 46° is provided between the power supply terminal 45 and the anodes of the diodes 39 and 40, and a current source 47 is provided between the emitter of the transistor 44 and ground.

In the above configuration, the relationship between the value of the control pulse VC and the reference bias Vrs is determined by the first time constant circuit 24, which is an external circuit of the first comparison detection circuit 13-1, when VC<VrS. The AGC output voltage is obtained and VC>
When the voltage is VrS, the AGC output voltage determined by the second time constant circuit 30 is obtained by being selectively selected.

That is, in FIG. 4, the detection output VS 1+V''2 waveform of FIG. 5(C) and FIG. 5(d) derived to each cathode of the diode 23. In this case, when the transistor 43 causes the collector current to flow due to the voltage of the control pulse VC, the detected voltage of the diode 29 (FIG. 5d) applied to the transistor 38 is selected.
→Output through diode 40 →transistor 44.

During the period when there is no control pulse VC, the transistor 42
The current from the current source 41 is supplied to the transistor 37, and the detection output VS+ (FIG. 5C) of the diode 23 is selected.
The ΔGG voltage is derived as VAGC. The composite detection voltage waveform, that is, the AGC voltage VAGC (Fig. 5a) selected in this way is selected by the control pulse VC from the discharge time constant of the resistor 25° and capacitor 26, which produces, for example, sag a-2 (see Fig. 2). Since the discharge time constant of the detection output VS2 is long, the level during the discharge period B is made uniform. Also,
During the next charging period, since the charging time constant is lengthened by the capacitor 32, the sag a-3 during the transition period does not occur, and the voltage becomes smooth.

Such a detection output VS1. The AGC voltage VA(,C (Figure 5a)) output by switching VS2 has little fluctuation, and -1 is sufficient to detect the synchronizing signal level at all times.
The waveform is such that the C operation can be performed. This provides images with even brightness. Incidentally, Fig. 5 shows the operating waveforms during the same period as Fig. 2, and Fig. 5 (1)
) indicates a control pulse.

〔Effect of the invention〕

As described above, according to the present invention, by applying a control pulse VC to select a plurality of independent time constant circuits corresponding to each predetermined period, an AGC detection voltage that greatly reduces sag can be obtained. , since it does not affect the characteristics of the period without sag, which occupies most of the period,
You can get ideal images.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional AGC circuit, and Figure 2 (a
) is a waveform diagram showing the detected output waveform obtained by the circuit in FIG. 1, FIG. 2(b) is a waveform diagram showing the waveform of the AGC voltage, and FIG. Block Diagram,
Figure 4 is a circuit diagram showing the specific circuit of Figure 3, and Figure 5 (a
) is a waveform diagram showing the AGC voltage waveform obtained by the present invention, FIG. 5(b)' is a waveform diagram showing the control pulse,
FIGS. 5(c) and 5(d) are waveform diagrams showing detected voltage waveforms that are selectively switched. 11... Pre-stage amplifier circuit, 12... Detection circuit, 13-
1. 13-2... Comparison detection circuit, 14... Control reference bias circuit, 15... Switching circuit, 21, 22.
27.28.37.38.42. j3...Transistor, 23,29.39.40...Diode, 24.
...First time constant circuit, 25.31...Resistance, 26.3
2... Capacity, 30... Second time constant circuit. Agent jo! J! J-Kenyu Norichika (1 person) 0
,O-523-

Claims (1)

[Claims] A video amplification circuit, a detection circuit that detects this output, first and second comparison detection circuits that input the outputs of this detection circuit and compare them with a reference voltage corresponding to a preset synchronization signal level, and each comparison detection circuit. a switching circuit that switches and selects the output of the circuit using a control pulse generated in a specified period;
and means for applying the output of the switching circuit to the gain control section of the video amplification circuit, the first. An automatic gain control circuit characterized in that the second comparison detection circuit includes time constant circuits each having a different time constant.