JPS6324355B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for controlling the brightness of an image by controlling the pedestal level of a video signal in a television receiver, and an object of the present invention is to provide a device that improves the detection accuracy of the pedestal level. be.

One way to control the image brightness in a television receiver is to sample and hold the pedestal portion of the video signal, and compare the held pedestal level with a control voltage that has been manually adjusted in advance. There is a device that controls the DC level of the video signal by feeding back the comparison output so that there is a relationship (for example, the same). According to this means, it is possible to arbitrarily adjust the brightness by adjusting the comparison control voltage using a variable resistor or the like.

First, FIG. 1 shows a circuit diagram of an example of a conventional brightness adjustment device.

In the figure, terminal 1 is an input terminal for a composite video signal, and a signal like A is supplied from a video detection circuit.

Reference numeral 2 is a DC cut capacitor, and 3 and 4 are transistors constituting the amplifier circuit, and the contrast and image quality can also be adjusted here. 5 is an output terminal for the amplified composite video signal B. Reference numerals 6 and 7 indicate transistors that constitute a bidirectional switching circuit which is connected to the output side with opposite polarity and is made conductive by a pedestal clamp sampling pulse such as C applied from terminal 8; The level of the pedestal portion (back porch) of the composite video signal that appears after the signal is sampled and held in the capacitor 9. This sampling pulse C is created by differentiating the trailing edge of the horizontal synchronizing signal. Transistors 10 and 11 constitute a differential amplifier, where:
The brightness adjustment voltage applied from the variable resistor 12 to the base of the transistor 11 and the pedestal level voltage held by the capacitor 9 are compared,
The comparison output is taken out from the transistor 11 and fed back to the base of the transistor 3 via the transistor 13 as a bias for setting the DC level of the composite video signal. This controls the pedestal level of the composite video signal to be the same as the brightness adjustment voltage.

In this way, the DC level of the composite video signal B output from the output terminal 2 is determined by the voltage set by the variable resistor 12, and the brightness is adjusted.

The resistor 14 is connected in parallel to the transistors 6 and 7 in order to prevent abnormal circuit operation when the horizontal synchronizing signal is lost due to an empty channel, etc., and the sampling pulse C for pedestal clamping to the input terminal 8 is lost. It is what was done. In other words,
When the sampling pulse C for pedestal clamp disappears in an empty channel, transistors 6 and 7 are always cut off, transistor 10 is cut off, and transistor 11 is turned on, so the base of transistor 3 is biased abnormally high, and the emitter potential of transistor 4 becomes The screen becomes abnormally low and no longer appears. The resistor 14 prevents malfunction in such a case.

However, in such a conventional device, the resistance 14
By inserting transistors 6 and 7
Because complete sample and hold operation is not performed due to
is modulated at a vertical rate by the content of the image, a ripple at the vertical rate is generated at the base of the transistor 10 by the resistor 14, and even if the ripple is small, the detection sensitivity of the differential amplifier is high. However, the ripple portion is fed back to the input side, causing a sag in the vertical rate of the composite video signal B output from the output terminal 2.

Therefore, the present invention solves these conventional drawbacks, has a high DC reproduction rate, does not cause sag in the vertical rate of the output even depending on the content of the video signal, and is stable even when the circuit operation is an empty channel. The purpose of this invention is to provide a device that is

An embodiment of the present invention will be described below with reference to FIGS. 2 and 3. It should be noted that the same parts as in the prior art are designated by the same reference numerals as in FIG. 1, and the explanation thereof will be omitted.

First, in the embodiment of FIG. 3, in series with the resistor 14,
The present invention is characterized by the provision of transistors 16 and 17 which constitute a bidirectional switching circuit which is turned on by a flyback pulse D applied from a terminal 15.

These transistors 16 and 17 act greatly when the sampling pulse C for pedestal clamping from the terminal 8 is lost due to an empty channel, etc.
When the channel is empty, transistors 16 and 17 are turned on by this flyback pulse D, and resistor 14 is turned on.
The voltage is sampled and held in the capacitor 9 only during the horizontal retrace period, and compared with the voltage from the variable resistor 12 (brightness adjustment voltage), negative feedback is applied, and the circuit operates stably. On the other hand, when the sampling pulse C is applied normally, the sampling pulse C for the pedestal clamp causes the transistor 6 to
7 becomes conductive, and the pedestal portion of the composite video signal is sampled and held in the capacitor 9. At the same time, flyback pulse D causes transistor 1 to
6 and 17 conduct, and the horizontal retrace period of the composite video signal is sampled and held in the capacitor 9. At this time, since the resistor 14 is present, the influence of the horizontal retrace period is much smaller than that of the pedestal portion.
Normally, it can be considered that only the pedestal portion formed by the transistors 6 and 7 is sampled and held by the capacitor 9.

On the other hand, when there is no signal such as in an empty channel, the sampling pulse C for pedestal clamping disappears, and the transistors 6 and 7 are cut off. but,
Transistors 16 and 17 are made conductive by the flyback pulse D that is still being generated, and the noise signal appearing at the output side of transistor 4 is charged to capacitor 9 via transistors 16 and 17 and resistor 14, and is charged to the base of transistor 11. It is compared with the voltage and negative feedback is applied.

In this way, the brightness is adjusted by sampling and holding the pedestal portion using the bidirectional switch circuit which is turned on by the pedestal clamp sampling pulse. At this time, the transistors 16 and 17, which are turned on by the flyback pulse D, operate independently of the composite video signal to sample the horizontal retrace period via the resistor 14, thereby increasing the DC reproduction rate to 100%.
It can be raised up to.

In addition, as mentioned above, since the operation is independent of the video signal, there is no vertical rate sag even if the composite video signal is vertical rate and there is modulation in the video part, and it remains stable even when an empty channel is used as before. take action.

In addition, in the embodiment shown in FIG. 3, a series circuit of transistors 16 and 17 and a resistor 14 constituting a bidirectional switching circuit that conducts with a flyback pulse is connected between the bases of transistors 10 and 11 of the differential amplifier. This is a case where a constant DC reference potential is applied to one end of a series circuit by inserting it, and the same effect as the circuit shown in FIG. 2 can be obtained. To explain briefly, normally transistors 6,
The pedestal portion is sampled and held by the capacitor 9 by the capacitor 7. When the channel becomes empty, transistors 6 and 7 are turned off, and only transistors 16 and 17 are made conductive by the flyback pulse D, and the same base potential as that of transistor 6 is applied to the base of transistor 10 via resistor 14, and the transistor Equal currents flow through transistors 10 and 11 to provide an appropriate bias from the collector of transistor 11 through transistor 13 to the base of transistor 3. This operating state is almost the same as that of the embodiment shown in FIG. 2, and similar effects can be obtained.

As described above, according to the present invention, it is possible to obtain a good device that has a high DC reproduction rate, does not cause sag due to the content of the video signal, and operates stably even when an empty channel is used.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional brightness adjustment device, and FIGS. 2 and 3 are circuit diagrams of a brightness adjustment device in an embodiment of the present invention. 1... Input terminal for composite video signal, 3, 4... Transistor forming an amplifier circuit, 5... Output terminal for complex video signal, 6, 7... Transistor forming bidirectional switching circuit, 8... Sampling pulse input terminal, 9...Holding capacitor, 10, 11...Transistor forming a differential amplifier, 12...Transistor for brightness adjustment, 13...Transistor forming a feedback circuit, 14...Resistor , 15... flyback pulse input terminal, 16, 17... transistors constituting a bidirectional switching circuit.

Claims (1)

[Claims] 1 an amplifier circuit for amplifying a composite video signal; a first bidirectional switching circuit connected to the output side of this amplifier circuit and made conductive by a sampling pulse for pedestal clamping created by a horizontal synchronizing signal; A capacitor that holds the pedestal level of the composite video signal sampled by the bidirectional switching circuit, a differential amplifier that compares the voltage held in this capacitor with a reference voltage for brightness adjustment, and A feedback circuit is provided which feeds back the comparison output voltage to the composite video signal amplification circuit as a bias for setting the DC level of the composite video signal, and a horizontal feedback circuit is provided in parallel with the first bidirectional switching circuit using a flyback pulse. A brightness adjustment device comprising a series circuit of a second bidirectional switching circuit that is conductive during a line period and a resistor.