JPH10248023A - Doming prevention circuit for television receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent doming by extending a white level of a luminance signal so as to make an output characteristic of a white level extension circuit nearly the same as a characteristic of an electron beam. SOLUTION: A white level of a luminance signal detected by a luminance detection circuit 44 is extended by a white level extension circuit 46. Thus, an output of the white level extension circuit 46 has a gamma characteristic. That is, the electron beam shows a gamma characteristic with respect to a cathode voltage and the output characteristic of the white level extension circuit 46 is nearly equal to the gamma characteristic. Then a 2nd comparator 36 generates a high luminance detection signal based on an output of the white level extension circuit 46. Upon the receipt of the high luminance detection signal, a control signal generating 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 kind 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, the likelihood of doming is proportional to the size of the electron beam, but 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. , A gamma correction unit that applies gamma correction to a luminance signal, and a generation unit that generates a high luminance detection signal based on an output of the gamma correction unit.

[0006]

For example, the luminance signal is expanded beyond a predetermined level by a white expansion circuit. As a result, the output of the white extension circuit has substantially the same characteristics as the gamma characteristics of the electron beam. A high luminance detection signal is generated based on the output of the white extension circuit. For example, a contrast control circuit adjusts the luminance level based on the high luminance detection signal. This prevents doming.

[0007]

According to the present invention, since the output of the gamma correction means has a gamma characteristic by performing gamma correction on the luminance signal, doming can be reliably prevented. 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, and according to the beam current, each of the bright control circuit 20 and the contrast control circuit 16 controls the brightness 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-brightness detection signal is output by a brightness detection circuit 44.
The luminance signal detected by gamma correction is used as gamma correction means.
The gamma correction is performed by the long circuit 44, that is, based on the signal expanded in white.
Generated. That is, FIG.
As shown in the figure, the image containing a high-luminance part is displayed
From the white expansion circuit 46, the white expansion as shown in FIG.
A long signal is output. The high frequency component included in this white extension signal
The minute is removed by a low pass filter (LPF) 48,
As a result, a signal indicated by a solid line in FIG. 2C is obtained. That
The LPF 48 output is output from the first comparator 32 as shown in FIG.
At the first reference voltage Vf indicated by the broken line₁Is compared to
The first comparator 30 outputs LPF48 output ≧ Vf ₁High when
Level, LPF48 output <Vf₁When Lorebe
In this example, the first comparison signal is output.
The comparison signal changes as shown in FIG.

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 (E), 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 (F).

As can be seen from FIGS. 2E and 2F, the output of the charge / discharge 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 portion appears but suddenly appears when the high-luminance portion 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 white expansion circuit 46 is configured as shown in FIG. That is, the collector of the transistor T1 is connected to the resistor R
2 and the power supply _Vcc, and the emitter of the transistor T1 is grounded via the resistor R3. This resistance R
2. A current path is formed by the transistor T1 and the resistor R3, and a current according to the level of the luminance signal input from the base of the transistor T1 flows through this current path. A resistor R4, a diode D and a resistor R5 connected in series are also interposed between the emitter of the transistor T1 and the ground, thereby forming a branch current path. A connection point between the anode of the diode D and the resistor R5 is connected to a transistor T3 having a collector connected to the power supply V _CC.
Are connected. The base of the transistor T3 is grounded via a resistor R7, and the resistor R6
Connected to the collector via The base of the transistor T2 functioning as a buffer is connected to the collector of the transistor T1. The collector of the transistor T2 is connected to the power supply _Vcc , the emitter is grounded via the resistor R8, and is connected to the first comparator 32.

The transistor T3 connects the power supply V _CC to the resistor R6.
Since the current is conducted by the voltage divided by R7 and R7, a predetermined voltage is applied to the anode of the diode D. Therefore, when the emitter voltage of the transistor T1 exceeds the predetermined voltage + the forward voltage drop of the diode D, the diode D conducts, and a current flows in the branch current path. When a luminance signal having a sawtooth wave as shown in FIG. 6A is input from the base of the transistor T1, no current flows through the branch current path until the luminance signal level exceeds the threshold, and the transistor T1 The collector potential decreases in proportion to the luminance signal level. On the other hand, when the luminance signal level exceeds the threshold value, the diode D conducts, and the current flows through the branch current path, so that the collector current of the transistor T1 increases. As a result, the collector potential of the transistor T1 drops significantly. Therefore, a white extension signal whose white level is extended and whose polarity is inverted is output from the emitter of the transistor T2 as shown in FIG. 6B. That is, this white extension signal has a gamma characteristic,
It substantially matches the gamma characteristic of the electron beam shown in FIG.

According to this embodiment, the high luminance detection signal is generated based on the gamma-corrected white extension signal, so that the occurrence of doming can be reliably prevented. If the luminance signal output from the contrast control circuit 16 is input to the white extension circuit 46, the luminance detection circuit 44 can be omitted. Also,
Even if the luminance signal is compressed to generate a signal having a gamma characteristic by compressing the black level of the luminance signal instead of extending the luminance signal to white, the doming can be reliably prevented. Further, as the integrated circuit 12, an integrated circuit “CXA2” manufactured by Sony Corporation is used.
025AS "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 white extension circuit.

FIG. 6 is a timing chart showing a part of the operation of the embodiment in FIG. 5;

FIG. 7 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 27 ... Doming prevention circuit 28 ... Control signal generation circuit 30 ... White expansion circuit 32 ... First comparator 34 ... Charge / discharge circuit 36 ... Second comparator

Claims (6)

[Claims] A doming prevention circuit for a television receiver for controlling the brightness of an output video based on a high brightness detection signal indicating that a high brightness portion of the output video has a predetermined area or more, wherein a gamma correction is applied to the brightness signal. A gamma correction unit for applying a multiplication factor, and a generation unit for generating the high-brightness detection signal based on an output of the gamma correction unit. 2. The doming prevention circuit for a television receiver according to claim 1, wherein said gamma correction means includes expansion means for expanding the luminance signal by a predetermined level or more. 3. A current path for flowing a current according to a luminance signal level, and a current provided in the current path for changing a current flowing through the current path when the luminance signal level exceeds a threshold value. 3. The method according to claim 2, further comprising changing means.
A doming prevention circuit for a television receiver as described in the above. 4. The current changing means includes a branch current path connected to the current path, and a switch provided on the branch current path and turned on when the luminance signal level exceeds a threshold value. 4. The doming prevention circuit of the television receiver according to 3. 5. The doming prevention circuit for a television receiver according to claim 3, wherein said current path includes a transistor receiving said luminance signal at a base input. 6. The switch according to claim 4, wherein said switch includes a diode having a predetermined voltage corresponding to said threshold applied to an anode.
Or the doming prevention circuit of the television receiver according to 5.