JPS6362128B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit for keeping the amplitude of a video signal applied to a cathode ray tube constant regardless of the strength of broadcast waves applied to a television receiver. The purpose is to improve.

Conventionally, a peak value type automatic gain control circuit (hereinafter abbreviated as peak AGC) has been used as an automatic gain control circuit.
has been widely used. Figure 1 shows a general block diagram of peak AGC.

In the following description, a case will be described in which the control characteristic of the gain control amplifier is a reverse characteristic, that is, the control voltage increases as the input decreases. When the control characteristic is in the forward direction, it shall be rephrased as follows.

Charging current → Discharging current Discharging current → Charging current In Figure 1, the video signal applied to input terminal 1 is
The signal is amplified or amplified/detected by an amplifier circuit section 2 (hereinafter abbreviated as AGC amplifier) whose gain can be controlled, and a video signal is obtained at an output terminal 3. In order to make the video signal output a constant amplitude regardless of the strength of the input signal, a circuit as shown below is added.

First, the video signal output is applied to the comparator 4, and its peak value is compared with the comparison voltage of the comparison voltage generator 5. As a result, the output of the comparator 4 generates a signal corresponding to the potential of the peak value of the video signal. Based on the output signal of the comparator 4, a switching circuit 8 switches between a charging current generating circuit 6 and a discharging current generating circuit 7, and the charging current or discharging current is supplied to the capacitor 9. In this way, a predetermined voltage appears on the capacitor 9, and this voltage is transferred to the AGC amplifier 2.
By adding it to the gain control terminal 10 of the AGC
The gain of the amplifier 2 is controlled to keep the video signal output constant.

FIG. 2 shows a specific circuit example of the above-mentioned conventional peak AGC. The parts surrounded by dotted lines correspond to each block in FIG. The video signal output generated at the output terminal 3 is passed through a low-pass filter consisting of a resistor 11 and a capacitor 12 to remove unnecessary high-frequency components, and then sent to one input terminal of the differential amplifier that constitutes the basic part of the comparator. It is added to the base of transistor 13. This signal is the transistor 14
is compared with the voltage of a comparison voltage generator 5 connected to the base of the . This comparator output signal obtained from the collector of the transistor 14 through the collector resistor 15 at the output terminal of the comparator 4 is applied to the base of the switching transistor 16 in the switching circuit 8, and is applied to the base of the switching transistor 16 in the switching circuit 8. and a reference voltage source 18, a discharge current generating circuit 7,
Alternatively, one of the charging current generating circuits 6 including the resistor 19, the diode 20, the resistor 21, and the transistor 22 is made conductive, and the other is made non-conducting.
As a result, a discharge type or charge type voltage appears in the capacitor 9, and the gain of the amplifier circuit section 2 is controlled by this voltage as well. In FIG. 2, 23 is a circuit power supply, 24 is a transistor in the comparator 4, 25 is a base bias power supply for the same transistor 24, 26,
27 is a constant current source.

FIG. 3 shows examples of waveforms generated in each part of FIG. 2. If the video input signal at input terminal 1 in the circuit shown in Fig. 2 above is a signal with constant brightness and only a horizontal synchronization signal as a synchronization signal, as shown in 2-1 in Fig. 3, then the output terminal What appears in 3 is the waveform of the output signal as shown in 2-2 in the same figure, which is a waveform in which the leading edge of the synchronizing signal protrudes. This protruding part is generally called the "sag". In the waveform 2-2, the brightness level itself also has a slope, and this is generally called "brightness unevenness" and is a distortion caused by the same cause as the above-mentioned "sag". These distortions are necessarily inherently present in conventional peak AGC.
The dotted horizontal line in the waveform 2-2 indicates the voltage of the power supply included in the comparison voltage generator 5. The waveform 2-3 is the output waveform of the comparator 4, and is the collector waveform of the transistor 14 in FIG. In the waveforms 2-3, the dotted horizontal line indicates the voltage of the reference base power supply 18 of the transistor 17 in the discharge current generating circuit 7. The waveform of 2-4 charges the capacitor 9.
The horizontal line with an arrow indicating the discharge current and the time axis indicates the current value of zero.

In a steady state, the area on the + side of the waveform 2-4 and the area on the one side must always be equal when measured over a sufficiently long period of time. Therefore, in the case of a video signal in which the time width of the synchronization signal is only about 1/15 of the period, it is obvious that the maximum value of the discharging current needs to be 15 times or more the charging current. In reality, we choose 50 to 70 times more based on the variation. When the waveform 2-4 is integrated, a waveform 2-5 is obtained, which is the charge stored in the capacitor 9. The average value of charge is determined by the initial conditions. When the waveform 2-5 is divided by the capacitance value of the capacitor 9, the waveform 2-6 is obtained, which becomes the voltage of the capacitor. As is clear from the waveforms 2-6, the capacitor voltage has a ripple component, and this ripple voltage is applied to the AGC amplifier, causing distortion such as the "sag" mentioned above.

Naturally, this ripple voltage is inversely proportional to the capacitance value of the capacitor 9, so increasing the capacitance of the capacitor is a countermeasure against distortion such as "sag".

However, on the other hand, the response to rapid fluctuations in the strength of the input signal decreases in proportion to the value of the capacitor. In recent years, the number of moving large objects such as Japan National Railways Shinkansen trains and large airplanes has increased, and radio interference caused by them has become a problem. Conversely, responsiveness is also becoming important when a television receiver is placed in a car or the like and receives images while moving.

When using conventional peak AGC, that is, peak value automatic gain control circuit, "sag" and "responsiveness"
There is a contradictory relationship between these characteristics, and an attempt to improve the sag results in a worsening of the response, and an attempt to improve the response results in an increase in the sag.

This invention was made in order to eliminate the above-mentioned drawbacks that existed in conventional peak value type automatic gain control circuits. This method attempts to improve responsiveness by increasing the charging current to the capacitor only when the current is within the range.

FIG. 4 shows an embodiment of the present invention. This circuit is
In the circuit shown in Figure 3, a PNP whose base is connected to the collector of transistor 13 and whose collector is connected to the ground point
a transistor 28 and a PNP transistor 29 whose base is connected to the collector of the transistor 16 and whose collector is connected to the collector of the transistor 17;
A selection means 34 is added, consisting of a resistor 30 connected between the common emitters of the PNP transistors 28 and 29 and the power supply 23, and a resistor 31 connected between the collector of the transistor 13 and the power supply 23, and A resistor 31 is connected between the emitters of transistors 13 and 14 of comparator 4 to provide a suitable base bias for transistors 28 and 29.
~33 has been inserted.

The specific operation according to the present invention will be explained using FIG. 4. In FIG. 4, the voltage generated at the collector of the transistor 13 constituting the comparator 4 causes the collector of the transistor 29 to be activated by the selective switching function of the differential circuit consisting of the transistors 28, 29 and the resistor 30. A current flows, and the charging current to the capacitor 9 can be increased. When the differential circuit composed of transistors 13 and 14 is within the dynamic range, one of the differential circuits composed of transistors 28 and 29 and resistor 30 is also within the dynamic range. Since the base bias resistor 31 of the transistor 28 can be selected, the charging current to the capacitor 9 through the collector of the transistor 29 can be increased in this state. Specifically, such an operation increases the charging current only when the peak value of the video signal is within the dynamic range of the transistors 13 and 14, that is, close to the normal value.

As mentioned earlier, the peak as shown in Figure 2
In an AGC circuit, the value of charging current is equal to the value of discharging current.
The value must be 1/15 or less, and in reality it is about 1/70, so the deterioration of responsiveness is largely due to the lack of charging current. However, in FIG. 2, if only the charging current is increased, the sag will increase if the capacitance of the capacitor is constant. Moreover, if the charging/discharging ratio is changed to approach 1:1, the peak value cannot be maintained, so if the charging current is increased, the discharging current must also be increased by the same proportion. In other words, with conventional peak AGC, if you try to increase the charging current to improve responsiveness,
The sag worsens by exactly that percentage.

In the present invention, an increase in the charging current supplied by transistor 29 in FIG. 4 occurs only when the differential circuit constituted by transistors 13 and 14 is within the dynamic range. This is the basis of the present invention, and in a steady state, the fourth
The waveforms of the signals in each part of the figure are accurately represented in FIG.

That is, in steady state, it is as if the circuit of the present invention does not exist at all. Therefore, the circuit of the present invention does not increase "sag". In other words, the sag remains the same as before. The present invention improves responsiveness by increasing the charging current only during the synchronization signal period. By improving the response, it becomes possible to improve the sag by increasing the value of the capacitor.

The operation of the circuit shown in FIG. 4 when an input signal is applied will be specifically described below.

In a steady state, the synchronization head value in the video signal applied to the base of transistor 13 is approximately the same as the potential generated by comparison voltage generator 5, and transistors 13 and 14 are conductive to the same extent. In this state, if resistors 15, 31 and power supplies 18, 25 are selected so that transistors 28 and 29 and transistors 16 and 17 conduct to the same degree, then the charging current to capacitor 9 during the synchronization signal period is This means that the increase is due to the collector current of transistor 29. This increased charging current results in increased responsiveness to input changes. Moreover, it is clear that this current increase occurs only when transistors 13 and 14 are within the dynamic range.

On the other hand, during the period between the synchronization signals, the transistor 1
Since the base potential of transistor 3 is sufficiently high,
8 is fully conductive, so that transistor 29 is completely turned off and the charging current to capacitor 9 is the same as without transistor 29. For this reason, the rise in voltage of the capacitor during the period between synchronization signals is the same as in the conventional case, so sag does not worsen.

That is, in a steady state, the ratio of charging current to discharging current can be set to a value close to 1. In fact, due to the dynamic range, it is possible to reduce the value from about 1/70 to about 1/20, improving responsiveness by 3 to 4 times compared to before. It became.

Note that when extreme signal strength or weakness occurs, such as when turning on the power or switching channels, the collector current of the transistor 29 is completely stopped, so there is no fear of malfunction. Also, resistors 32, 33
It is also possible to adjust the dynamic range by changing the size of.

As described above, by implementing the present invention, it is possible to improve the responsiveness problem that occurred in the conventional peak value type automatic gain control circuit, and the effect can be said to be extremely large.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional general peak value type automatic gain control circuit, Fig. 2 is a diagram showing a specific example of a conventional peak value type automatic gain control circuit, and Fig. 3 is a block diagram showing a conventional peak value type automatic gain control circuit. FIG. 4 is an automatic gain control circuit diagram of an embodiment of the present invention. 1...Input terminal, 2...Gain control amplifier, 3...
...Output terminal, 4...Comparator, 5...Comparison voltage generator, 6...Charging current generator, 7...Discharging current generator, 8...Charging/discharging switching circuit, 9...Capacitor, 10... ...Gain control terminal, 11...Resistor, 1
2... Capacitor, 13, 14... Transistor, 15... Resistor, 16, 17... Transistor, 18... Power supply, 19... Resistor, 20... Diode, 21... Resistor, 22... Transistor,
23...power supply, 24...transistor, 25...
Power supply, 26, 27... constant current source, 28, 29...
Transistor, 30, 31, 32, 33...resistance.

Claims (1)

[Claims] 1: an amplifier circuit section that can control gain; a comparison voltage generator; a comparator that detects the output of the amplifier circuit section and compares it with the voltage from the comparison voltage generator; a charging current generating circuit that generates a current; a discharging current generating circuit that generates a discharging current in response to a signal from the comparator; and a charging current generating circuit and a discharging current generating circuit in response to a signal from the comparator. A switching circuit for switching,
A control voltage is generated by the charging current or the discharging current from the switching circuit, and this control voltage is input to the gain control terminal of the amplifier circuit only within the linear operating range of the capacitor and the comparator. An automatic gain control circuit comprising selective circuit means for increasing the charging current or the discharging current to the capacitor, the automatic gain control circuit being characterized in that the peak value of the signal is set to a constant voltage.