JPS6225017Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement of an automatic brightness limiting circuit (hereinafter referred to as ABL circuit) used in color television receivers, color television monitors, etc.

Generally, in color television receivers,
In view of the ratings, reliability, lifespan, etc. of the cathode ray tube, flyback transformer, and horizontal output circuit, it is necessary to suppress the average anode current of the cathode ray tube from flowing too much, and for this purpose, an ABL circuit is provided.
There are two types of ABL circuits: DC voltage type and AC voltage type. The DC voltage type ABL circuit detects and smoothes the average anode current of a cathode ray tube, and adds it to the video amplification circuit to move the DC operating point and limit the anode current. The method is shown in Figure 1. That is, in FIG. 1, the vertical axis is the cathode ray tube input voltage, and the zero point is the cut-off voltage. The horizontal axis is the modulation degree of the video signal. The solid line in FIG. 1 shows the relationship when the ABL circuit is not operating, and video signals at all modulation degrees are correctly applied to the cathode ray tube. On the other hand, when the ABL circuit operates, the DC operating point moves, so the relationship between the modulation degree and the input voltage becomes as shown by the dotted line in Figure 1, and the low modulation degree component falls below the cutoff voltage and is not reproduced correctly. It has the disadvantage that it becomes just plain black.

On the other hand, an AC voltage type ABL circuit detects the average anode current and changes the gain of the video amplification circuit according to the detected output. To explain this operation with reference to Figure 2, the solid line in Figure 2 is
This shows the relationship when the ABL circuit is not operating, and if the average anode current exceeds the set value,
The ABL circuit operates to limit the average anode current by lowering the input voltage while keeping the cutoff voltage unchanged, as shown by the dotted line in the figure. Therefore
While the ABL circuit is operating, even if you increase the gain of the contrast circuit in an attempt to further increase the contrast of the CRT image, the ABL circuit will lower the gain as described above, making it impossible to increase the contrast further, and the contrast circuit will not function properly. There is a drawback that the performance is not fully demonstrated. Further, the variable gain video amplifier circuit required in the above AC voltage type ABL circuit often uses a differential amplifier circuit in the variable gain section, and has the drawback of being complicated.

The present invention was developed to eliminate the above-mentioned drawbacks, and the linearity of the gain of the video amplifier circuit is improved so that when the average anode current exceeds a set value, the low modulation degree region of the video signal is raised above the cut-off voltage of the cathode ray tube. To provide an ABL circuit that can reproduce low modulation degree components on a cathode ray tube even if the average anode current exceeds a set value, and can correctly reproduce the gradation of high modulation degree components by changing the average anode current. .

An embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 3, 31 and 32 are video amplifying circuits, and both are cascade-connected via a coupling capacitor 33 between stages. And the subsequent video amplification circuit 3
The output of 2 is supplied to the cathode of a cathode ray tube, for example a cathode ray tube 34.

On the other hand, 35 is a flyback transformer, and one end of the high voltage winding 36 on the secondary side is connected to the anode of the cathode ray tube 34 via a high voltage rectifier diode 37, and the other end is connected to the anode of the cathode ray tube 34 via an anode current smoothing capacitor 38. It is grounded and connected to a first power source 40 via a first resistor 39 . A connection point A between the resistor 39 and the capacitor 38 is connected to a second power source 42 via a first diode 41. Further, the connection point A is connected to the input terminal of the video amplification circuit 32 via a second resistor 43.

On the other hand, 44 is a gain control circuit, the input electrode (base) of a transistor 45 is connected to the connection terminal A via a third resistor 46, and the output electrode (collector in this example) is a fourth resistor, which is an output resistor. resistance 47
It is connected to the reference potential end (ground potential in this example) via the terminal. Further, the emitter of the transistor 45 is connected to the second power supply 4 through a fifth resistor 48.
2, and its collector is connected to the input terminal of the video amplification circuit 32 via a series circuit 51 comprising a sixth resistor 49 and a second diode 50.

Next, the operation of the automatic brightness limiting circuit will be explained.
The anode current of the cathode ray tube 34 is supplied to the first power source 40.
It flows through the path of → first resistor 39 → high voltage winding 36 of flyback transformer 35 → high voltage rectifier diode 37, and is smoothed by capacitor 38. When the average anode current flowing through the resistor 39 is small, the voltage of the second power supply 42 is set in advance to be lower than the voltage at the connection point A, so the first diode 41 becomes conductive. Therefore, the connection point A
The voltage is clamped by the second power supply 42 and is constant at a voltage higher than the voltage of the second power supply 42 by the forward voltage of the diode 41. On the other hand, when the average anode current is greater than a predetermined set value, the voltage drop across the first resistor 39 caused by this current causes the voltage at the connection point A to drop until it biases the first diode 41 into a non-conducting state. It will change according to the average anode current. This voltage change is transmitted to the video amplifying circuit 32 via the second resistor 43, and by controlling its DC operating point and controlling the average brightness level, the anode current of the cathode ray tube 34 is limited to a constant value. and an automatic brightness limiting operation is performed. Note that the second resistor 43 also has the function of preventing the video signal supplied to the input terminal of the video amplifier circuit 32 via the coupling capacitor 33 from flowing out in the direction of the connection point A.

On the other hand, in the gain control circuit 44, when the average anode current is low and the connection point A is clamped by the diode 41, the transistor 45 is reverse biased between the base and emitter and becomes non-conductive, and the collector potential is zero. V, and the second diode 50 is in a non-conducting state, so
The video amplification circuit 32 is not affected in any way.

On the other hand, when the average anode current increases and the first diode 41 becomes non-conductive as described above and automatic brightness limiting operation starts, the potential at the connection point A becomes lower than the potential at the second power supply 42. Therefore, transistor 45 is given a forward bias and becomes conductive. Therefore, the collector potential of the transistor 45 increases as the potential at the connection point A decreases. FIG. 4 shows the relationship between each of the above potentials, the average anode current, and the average anode current when the ABL operation is not performed. In FIG. 4, the vertical axis shows the average anode current and each potential when an ABL circuit is provided, and the horizontal axis shows the magnitude of the average anode current when there is no ABL circuit. In other words, Fig. 4 shows that when the average anode current tries to increase linearly,
It shows how the average anode current and each potential change by providing an ABL circuit.

By the way, a video signal (the synchronization signal level is lower than the video signal level) is superimposed on the input potential of the video amplification circuit 32. Therefore, when the automatic brightness limiting operation starts, the potential difference between the input potential of the video amplifier circuit 32 and the collector potential of the transistor 45 is reduced, and the second diode 50 becomes conductive in the low modulation degree region of the video signal.
The input potential is applied to the second diode 50 and the sixth diode 50.
is suppressed to the collector potential of the transistor 45 through the resistor 49. As a result, the signal level in the low modulation degree region, which tends to fall below the cutoff voltage of the cathode ray tube, is shifted up by the gain control circuit 44 and raised above the cutoff voltage.

This will be explained below with reference to FIGS. 7a and 7b and FIG. 8. Note that FIGS. 7a and 7b show a staircase wave as a video signal. During normal operation, the potential at point A shown in FIG. 3 is high, the ABL circuit does not operate, and the diode 41 is turned on. As a result, the DC operating point of the video amplification circuit 32 is clamped to the output voltage L1 of the power supply 42, which is indicated by a dashed line in FIG. 7a.

From this state, when the anode current increases, A
The potential at the point drops, and the DC operating point of the video amplification circuit 32 drops to, for example, L2 shown by the dashed line in FIG. 7a. Therefore, if the gain control circuit 44 is ignored, the video signal will change from the solid line waveform X to the one-dot chain waveform Y in FIG. 7a. As a result, the low modulation degree region of the video signal (portions of staircase portions 1, 2, and 3 close to the pedestal level) falls below the cutoff voltage L3 shown by the dashed line in FIG. 7a, and the black level is distorted. In other words, the blackout occurs due to the movement of the DC operating point as explained in FIG.

On the other hand, if the gain control circuit 44 is provided, the operation will be as follows. In other words, the video signal is
When the waveform is as shown by the dashed line Y in FIG. 7A, the voltage at point A decreases, the transistor 45 is turned on, and a voltage L4 (see the dashed dot line in FIG. 7A) is generated at its collector. As a result, when the signal level on the cathode side of the diode 50 is lower than the collector voltage L4, the diode 50 is turned on.

When this diode 50 is conductive, resistors 49 and 47 are connected between the input terminal of the video amplifier circuit 32 and the ground terminal.
are connected in series. As a result, the video amplification circuit 3
When viewed from the video amplifying circuit 31 in the previous stage of 2, an impedance circuit as shown in FIG. 8 is established. In this FIG. 8, Z1 is the output impedance of the video amplification circuit 31, and Z2 is the resistance
This is the impedance due to

By forming the impedance circuit as described above, the video signal output from the video amplification circuit 31 is attenuated at a ratio of Z2/Z1+Z2 and supplied to the video amplification circuit 32. That is, when the diode 41 is turned on, the amplitude of the video signal becomes smaller.

For example, if the values of amplitudes P1 and P2 of the video signal shown by the dashed-dotted line waveform Y in FIG. It is attenuated to 10×3/4=7.5 (attenuation amount 2.5), and the amplitude P2 part is attenuated to P2=5×3/4=3.75 (attenuation amount 1.75). In this way, the amount of attenuation increases as the amplitude increases, and decreases as the amplitude decreases. As a result, a broken line characteristic as shown by the dotted line in FIG. 5 is obtained. As a result, the step portions 1 to 5 below the voltage L4 in the video signal shown by the dashed-dotted line waveform Y in FIG.
Among the stair portions 1 to 3 which were below the cut-off voltage L3, for example, the stair portions 2 and 3 become above the cut-off voltage L3, as shown in FIG. 7b, and the blackout is improved. In this case, there is no effect on the gain characteristics for high modulation degree components (components of the staircase portions 5, 6, 7, . . . close to the white level), and contrast adjustment by the contrast circuit is also possible. The relationship between this degree of modulation and the input voltage of the cathode ray tube 34 is shown by the dotted line in FIG. second diode 5
0 indicates that even if the level suppressed by the collector potential of the transistor 45 is different, the low modulation degree component does not fall below the cutoff voltage of the cathode ray tube brightness input.

Note that the resistor 43 and the gain control circuit 44 act in opposite directions with respect to changes in the input potential of the video amplification circuit 32 due to changes in the potential at point A, but by appropriately setting circuit constants, the video amplification circuit 32
As shown in FIG. 4, the input potential can be changed to follow the potential at point A in the figure.

Also, when the ABL circuit is operating, the video amplification circuit 32
The input potential of the ABL circuit is not fixed to a certain level, but is suppressed to the collector potential of the transistor 45, which has a value according to the signal level (modulation degree) of the video signal, so even when this ABL circuit is operating, Can maintain gradation.

Note that the third resistor 46 in the gain control circuit 44 is inserted to increase the input impedance of the transistor 45, and the fifth resistor 46 is inserted to increase the input impedance of the transistor 45.
8 is for protection of the transistor 45.

FIG. 6 shows a main part of another embodiment of the present invention, in which a series circuit of a resistor 61 and an inductance 62 is used as an impedance element in place of the sixth resistor 49 in FIG. That is,
Generally, in a video amplification system, the video amplification circuit 32 at the output stage has a large aperture in order to compensate for deterioration in frequency characteristics by providing sufficient gain in the video amplification circuit 32 at the output stage. Therefore, the input video signal of the video amplification circuit 32 contains a large amount of high frequency components. Therefore, the inductance 62 allows the video amplification circuit 32 to
By reducing the amount of suppression of the input potential, the high frequency component superimposed on the low modulation degree component is no longer impaired, resulting in the effect that the resolution of the dark part of the image on the cathode ray tube surface does not deteriorate.

As mentioned above, the present invention changes the linearity of the gain of the video amplifier circuit so that when the average anode current exceeds a set value, the low modulation degree region of the video signal is raised above the cutoff voltage of the cathode ray tube. It is possible to provide an ABL circuit that can reproduce low modulation degree components on a cathode ray tube even if the average anode current exceeds a set value, and can correctly reproduce the gradation of high modulation degree components.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the operation of the conventional DC voltage type ABL circuit, and Figure 2 is the conventional AC voltage type ABL circuit.
A characteristic diagram showing the operation of the ABL circuit; FIG. 3 is a circuit diagram showing an embodiment of the ABL circuit according to the present invention;
5 and 5 are characteristic diagrams showing the operation of FIG. 3, FIG. 6 is a circuit diagram showing a modified example of the resistor 49 in FIG. 3, and FIG. 7 is a characteristic diagram for explaining the operation of FIG. 3. The signal waveform diagram in FIG. 8 is a circuit diagram for explaining the operation of FIG. 3. 32...Video amplification circuit, 34...Cathode ray tube,
35... flyback transformer, 36... high voltage winding, 39... first resistor, 40... first power supply,
41...First diode, 42...Second power supply, 43...Second resistor, 45...Transistor, 46...Third resistor, 47...Output resistor, 4
9... Impedance element, 50... Second diode.

Claims (1)

[Claims for Utility Model Registration] (1) A first power supply connected in series to a high voltage winding of a flyback transformer via a first resistor, and a connection point between the first resistor and the high voltage winding. a second power supply that is connected via the first diode and makes the first diode conductive and clamps the connection point to a predetermined voltage value when the anode current of the cathode ray tube is low; a second resistor connected between the input terminals; an input electrode connected to the connection point via a third resistor; an output electrode connected to the reference potential terminal via the output resistor; A series circuit including a transistor controlled by the voltage at the connection point when conductive, and an impedance element and a second diode connected between the output electrode of the transistor and the input terminal of the video amplification circuit. Automatic brightness limiting circuit featuring. (2) The automatic brightness limiting circuit according to claim 1, wherein the impedance element is constituted by a series circuit of a resistor and an inductance.