JPH10248022A - Doming prevention circuit for television receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent doming by generating a high luminance detection signal based on a current proportional to a strength of electron beam. SOLUTION: A beam current proportional to a strength of electron beam flows from ground to a conductive film 24a. A current extract and I-V conversion circuit 30 extracts the beam current and produces a proportional voltage corresponding to the beam current. A 2nd comparator 36 generates a high luminance detection signal based on the proportional voltage. That is, when a high luminance part of a video image outputted from a CRT 24 has a prescribed area or over, the high luminance detection signal is outputted. Upon the receipt of the high luminance detection signal, a control signal circuit 28 controls a contrast control circuit 16 so as to gradually decrease a luminance level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a doming prevention circuit for a television receiver, and more particularly to, for example, controlling the brightness of an output image based on a high-brightness detection signal indicating that a high-luminance portion of the output image exists over a predetermined area. A doming prevention circuit for a television receiver.

[0002]

2. Description of the Related Art An example of a conventional doming prevention circuit for a color television receiver of this type is disclosed in Japanese Patent Publication No. 7-85573 [H0] published on Sep. 13, 1995.
4N5 / 14, 5/16, 5/59]. This conventional technique detects a high-luminance part of an image displayed on a CRT based on a luminance signal, and gradually reduces the luminance level when the high-luminance part has a predetermined area or more.
It is intended to prevent doming.

[0003]

However, while the likelihood of doming is proportional to the size of the electron beam, the electron beam has a gamma characteristic with respect to the cathode voltage, that is, the luminance signal as shown in FIG. However, the method of detecting the luminance signal cannot surely prevent the doming.

[0004] Therefore, a main object of the present invention is to provide a doming prevention circuit for a television receiver, which can reliably prevent doming.

[0005]

SUMMARY OF THE INVENTION The present invention provides a doming prevention circuit for a television receiver which controls the luminance of an output image based on a high luminance detection signal indicating that a high luminance portion of the output image has a predetermined area or more. , Comprising a take-out means for taking out a proportional current proportional to an electron beam from a conductive film of a picture tube, and a generating means for generating a high-brightness detection signal based on the proportional current, which prevents doming of a television receiver. Circuit.

[0006]

For example, a resistor is interposed between the conductive film of the CRT and the ground, so that a proportional current proportional to the electron beam is extracted. This proportional current is converted into a proportional voltage corresponding thereto, and a high-brightness detection signal is generated based on the proportional voltage. That is, when the high-luminance portion of the video output from the CRT has a predetermined area or more, a high-luminance detection signal is obtained. For example, a contrast control circuit adjusts the luminance level according to the high luminance detection signal, thereby preventing doming.

[0007]

According to the present invention, since the high-brightness detection signal is generated based on the proportional current proportional to the electron beam, it is possible to reliably prevent doming that occurs in proportion to the size of the electron beam. it can. The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0008]

Referring to FIG. 1, a television receiver 10 of this embodiment includes an integrated circuit 12 which receives a luminance signal and a chrominance signal and generates an RGB signal. In the integrated circuit 12, the color demodulation circuit 14 demodulates the color signal to generate a color difference signal, and the contrast control circuit 16 adjusts the contrast of the luminance signal. The color difference signal and the luminance signal whose contrast has been adjusted are supplied to an RGB matrix circuit 18, which generates an RGB signal. This RG
The B signal is given to the bright control circuit 20,
As a result, an RGB signal whose brightness has been adjusted is obtained. The drive circuit 22 controls the cathode voltage in accordance with the adjusted RGB signals, and emits an electron beam proportional to the RGB signals. As a result, a desired image is displayed on the CRT 24.

At this time, a beam current proportional to the electron beam and smoothed is obtained on the secondary side of the flyback transformer 26. According to the beam current, the bright control circuit 20 and the contrast control circuit 16 respectively control the bright and the contrast. Adjust the contrast.
More specifically, when a large electron beam is emitted,
A large beam current smoothed by the capacitor C flows from the power supply V _CC to the secondary side of the flyback transformer 26. Therefore, the potential at the connection point between the resistor R1 and the flyback transformer 26 decreases, and the brightness control circuit 20 and the contrast control circuit 16 rapidly lower the brightness level and the contrast level in response to the voltage drop. This allows the so-called ABL (A
utomaticBright Level) control and ACL (Automatic Co
ntrast Level) control is executed.

The contrast control circuit 16 is also supplied with a control signal output from a control signal generation circuit 28 included in the doming prevention circuit 27. That is, when a high-luminance portion having a predetermined area or more is generated on the screen of the CRT 24, the control signal generation circuit 28 receives the high-luminance detection signal from the second comparator 36 and outputs a control signal whose level gradually decreases. . Contrast control circuit 1
6 reduces the contrast according to its control signal, thereby preventing doming.

The high luminance detection signal is transmitted to the conductive film 2 of the CRT 24.
4a and is generated based on a proportional current proportional to the electron beam. That is, when the electron beam is emitted, a proportional current flowing from the ground toward the conductive film 24a is extracted by the current extraction and IV conversion circuit 30, and the proportional current is converted into a corresponding proportional voltage. When an image including a high-luminance part is displayed on the CRT 24 as shown in FIG. 2A, the current extraction and IV conversion circuit 30 changes as shown by a solid line in FIG. 2B. A proportional voltage is output. The proportional voltage is
The first comparator 32 compares the voltage with a first reference voltage Vf ₁ indicated by a broken line in FIG. The first comparator 30
It becomes high level when proportional voltage ≧ Vf ₁ and proportional voltage <
For outputting a first comparison signal which becomes low level when vf _1, the first comparison signal is in this example changes as shown in FIG. 2 (C).

The charge / discharge circuit 34 performs gradual charging when the first comparison signal is at a high level, and rapidly discharges when the first comparison signal is at a low level. Therefore, the charge / discharge circuit 34
Outputs a voltage signal indicated by a solid line in FIG. 2 (D), which is compared by a second comparator 36 with a second reference voltage Vf ₂ indicated by a broken line in FIG. 2 (E). The second comparator 36 also outputs a second comparison signal which is at a high level when the charge / discharge circuit output ≧ Vf _{2 and is at} a low level when the charge / discharge circuit output <Vf ₂ , so that the second comparison signal is shown in FIG. It changes as shown in (E).

As can be seen from FIGS. 2D and 2E, the output of the charging / discharging circuit 34 decreases every time a high-luminance portion is detected, and when the high-luminance portion is determined to have a predetermined area or more, that is, FIG. 2 when the video signal S5 is scanned as shown in (a) and FIG. 2 (B), the output voltage of the charge-discharge circuit 34 falls below the reference voltage Vf ₂ is a second comparator 36 and the second low level The comparison signal is output as a high luminance detection signal. The control signal generating circuit 28 generates a control signal as shown in FIG. 4B with respect to the second comparison signal shown in FIG. That is, the output level is gradually reduced when the second comparison signal becomes low level, and the output level is rapidly raised when the second comparison signal returns from low level to high level.

Since the doming does not occur immediately when a high-luminance part appears but suddenly appears when the high-luminance part is present at the same position for a predetermined time or more, the contrast is gradually reduced by such a control signal. , The doming can be prevented without giving a visually unnatural change in luminance to the image. When the high-luminance portion disappears, the contents of the entire video are switched, so that even if the control signal is quickly activated, the user does not feel unnatural visually.

The vertical modulation voltage generating circuit 38 is shown in FIG.
A parabolic wave voltage as shown in FIG. 3B is generated based on a vertical pulse having a sawtooth voltage having a 1 V cycle as shown in FIG. Further, the horizontal modulation voltage generation circuit 40 is configured as shown in FIG.
As shown in FIG. 3C, a voltage obtained by slicing a predetermined level or more of a sine wave as shown in FIG. The voltages generated by each of the vertical modulation voltage generation circuit 38 and the horizontal modulation voltage generation circuit 40 are added to each other by the adder 42, whereby the first reference voltage Vf ₁ as shown in FIG.
Is obtained. As a result, it is possible to increase the detection sensitivity of the high-luminance part around the screen where doming is likely to occur.

The current extraction and IV conversion circuit 30 is shown in FIG.
It is configured as shown in FIG. A resistor R2 is interposed between the conductive film 24a of the CRT 24 and the ground, and a base of the transistor T1 is connected to a connection point between the conductive film 24a and the resistor R2. The collector of the transistor T1 is connected to a + 10V voltage source via a resistor R4, and the emitter is connected to a -10V voltage source via a resistor R3. Transistor T
The collector of 1 also has a +1
The emitter is connected to the base of transistor T2 which is connected to a voltage source of 0V and the ground is connected via a resistor R5. The collector of the transistor T2 is connected to the base of the transistor T3. The collector of the transistor T3 is connected to a voltage source of +10 V, and the emitter is a resistor R
7, and is directly connected to the first comparator.

When an electron beam is emitted from the CRT 24, a proportional current proportional to the electron beam is applied from the ground to the conductive film 24.
flows to a. According to this proportional current, the terminal voltage of the resistor R2, that is, the base voltage of the transistor T1 decreases, and the collector voltage of the transistor T1 increases according to the base voltage. Specifically, resistors R3 and R4 are, for example, R
3: R4 = 6: 4, and a positive voltage appears at the collector of the transistor T1 even when the proportional current does not flow through the resistor R2, that is, even when the base voltage of the transistor T1 is 0V. When a proportional current flows through the resistor R2, a base voltage of 0 V is applied to the collector of the transistor T1.
A larger positive voltage appears than at the time. That is, when the base voltage of the transistor T1 falls in a pulse form from the ground level as shown in the figure, a voltage which rises in a pulse form in a region more positive than the ground level is obtained in the collector of the transistor T1 as shown in the figure.

This collector voltage is applied to the base of the transistor T2. Since the emitter of the transistor T2 is grounded via the resistor R5, the transistor T2
In the collector 2, a voltage that falls in a pulse shape in a region more positive than the ground level appears according to a change in the base voltage. This voltage is impedance-converted by the transistor T3, and then output from the emitter of the transistor T3 to the first comparator 32 as a proportional voltage. The proportional voltage obtained by applying the predetermined bias voltage to the base voltage of the transistor T1 in this manner is used for comparison by the first comparator 32.

According to this embodiment, the high-luminance detection signal is generated based on the current proportional to the electron beam, so that the occurrence of doming can be reliably prevented. As the integrated circuit 12, an integrated circuit “CXA2025AS” manufactured by Sony Corporation can be used.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a timing chart showing a part of the operation of the embodiment in FIG. 1;

FIG. 3 is a timing chart showing another part of the operation of the embodiment in FIG. 1;

FIG. 4 is a timing chart showing another part of the operation of the embodiment in FIG. 1;

FIG. 5 is a circuit diagram showing a current extraction and IV conversion circuit.

FIG. 6 is a graph showing a beam voltage characteristic with respect to a cathode voltage.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Television receiver 16 ... Contrast control circuit 24 ... CRT 24a ... Conductive film 27 ... Doming prevention circuit 28 ... High brightness detection signal generation circuit 30 ... Current extraction and IV conversion circuit 32 ... First comparator 34 ... Charge Discharge circuit 36 ... second comparator

Claims (5)

[Claims] 1. A doming prevention circuit for a television receiver for controlling the brightness of an output image based on a high-brightness detection signal indicating that a high-brightness portion of the output image exists over a predetermined area. A take-out means for taking out a proportional current proportional to the electron beam from the device, and a generating means for generating the high brightness detection signal based on the proportional current. 2. The doming prevention circuit for a television receiver according to claim 1, wherein said extracting means includes a resistor interposed between said conductive film and ground. 3. The apparatus according to claim 2, wherein said generating means includes a proportional voltage generating means for generating a proportional voltage corresponding to said proportional current, and a signal generating means for generating said high luminance detection signal based on said proportional voltage. A doming prevention circuit for a television receiver as described in the above. 4. The proportional voltage generating means includes terminal voltage detecting means for detecting a terminal voltage of the resistor, and applying means for applying a bias voltage to the terminal voltage to generate the proportional voltage. A doming prevention circuit for a television receiver as described in the above. 5. The signal generation means according to claim 1, wherein said proportional voltage is a first signal.
First comparing means for comparing with a reference level, charging / discharging means for performing charging / discharging in accordance with the first comparison result of the first comparing means, comparing the output of the charging / discharging means with a second reference level, 5. The doming prevention circuit for a television receiver according to claim 3, further comprising a second comparing unit that sets a result of the second comparison as the high luminance detection signal.