JPH0628862Y2 - Peak ABL circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a peak ABL circuit used in a television receiver, a video projector and the like.

[Outline of device]

The present invention relates to a peak ABL circuit, which is capable of improving the screen stability without changing the response characteristics by inserting a circuit having a predetermined dead zone.

[Conventional technology]

2. Description of the Related Art In a television receiver or the like, peak ABL is performed to control the gain of a video signal according to the peak value of the input video signal. That is, in FIG. 3, the video signal Vin supplied to the input terminal (31) is supplied to the gain control circuit (32), and the signal from the control circuit (32) is supplied to the cathode ray tube (CRT) through the video output circuit (33). Supplied to (34). And control circuit (32)
Is supplied to the peak ABL circuit (35), the detected gain control signal V _G is supplied to the adder (36), and the signal from the voltage source (37) is used for so-called picture control. Is supplied to the adder (36) through (38),
This added signal is supplied to the gain control circuit (32).

In this apparatus, the peak ABL circuit (35) is designed so that the control response is quick and the cancellation is delayed because of the nature of detecting and controlling the peak value. However, in that case, if the release is delayed too much, the control of the previous screen may remain at the time of signal switching and the period in which the brightness is unnatural becomes longer, and the release cannot be extremely delayed. . For this reason, the circuit is usually designed to be released within several frames, but as a result, so-called sag occurs in the detection output, and the following drawbacks occur.

That is, as shown in FIG. 4, when there is a high-intensity part in the middle of the screen, the gain of the video signal is reduced in this part, and the cancellation is advanced in the one-field period thereafter, and the gain is slightly increased again in a part. Since the gain is reduced, the brightness of the background below the high-brightness portion of the screen is displayed darker than that of the upper part as shown in the figure, causing uneven brightness in the vertical direction.

When a signal in which a peak exists only in one field of one frame, for example, VIRS is detected by the peak ABL circuit (35), a sawtooth-shaped control signal of one frame period is formed as shown in FIG. If this signal is used for control, flicker of the frame period will occur.

Therefore, in the above-mentioned device, there is a possibility that uneven brightness or fritter may occur depending on the content of the video signal.

[Problems to be solved by the invention]

As described above, the conventional technique has a problem that uneven brightness and flicker may occur on the screen when the release is accelerated.

[Means for solving problems]

The present invention detects a peak value of an input video signal and uses the detected peak value as a circuit (resistor) having a predetermined response characteristic.
(4) In a peak ABL circuit which is taken out through the operational amplifier (6) and the capacitor (8) to control the gain of the video signal, a predetermined value is provided on the output side of the circuit having the predetermined response characteristic. Circuit with dead area (diode (9) (1
0)) is inserted in the peak ABL circuit.

[Action]

According to this, the sag of the detection output is removed by inserting the circuit having the dead region, and thereby the image quality during the peak ABL operation can be stably maintained without changing the response characteristics of the start and release. .

〔Example〕

In FIG. 1, the input terminal (1) to which the input video signal Vin is supplied is connected to the base of an npn transistor (2), the collector of this transistor (2) is connected to the power supply terminal (3), and the emitter is a resistor. It is grounded through (4) and this emitter is connected to the inverting input of an operational amplifier (6) through a resistor (5). The voltage source (7) of the reference voltage Vref is connected to the non-inverting input of the amplifier (6). The output of the amplifier (6) is connected to the inverting input of the amplifier (6) through the capacitor (8). Further, the output of the amplifier (6) is connected to one end of a capacitor (11) through a parallel circuit of diodes (9) and (10) connected in opposite directions, and the other end of the capacitor (11) is grounded. And one end of this capacitor (11) is pnp
This is connected to the base of the transistor (12), the collector of the transistor (12) is grounded, and the voltage source (13) for picture control is connected to the emitter of the transistor (12) through the resistor (14). Transistor (12)
Is connected to an output terminal (15) connected to a gain control circuit (not shown).

In this circuit, when the potential of the emitter of the transistor (2) corresponding to the input signal Vin exceeds Vref, the current I ₁ flows as shown by the arrow. Is the resistance value of the resistor (5), and the potential V ₁ of the output of the amplifier (6) begins to drop. Then, the potential at one end of the capacitor (11) is V
₂ , when the forward voltage drop of the diode (10) is V _D2 , when V ₁ <V ₂ −V _D2 , V ₂ also begins to drop (V ₂ = V ₁ + V _D2 ), and this voltage The gain of the video signal is controlled by a control voltage V _G , which will be described later, corresponding to V ₂ and becomes stable when V _in = Vref.

When the input signal Vin decreases, the transistor (2) turns off and the current I ₂ flows as shown by the arrow. Is the resistance value of the resistor (4), and V ₁ starts to rise. When the forward voltage drop of the diode (9) is V _D1 ,
When V ₁ > V ₂ + V _D1 is reached, V ₂ also begins to rise (V ₂ = V ₁ −V _D1 ), and the gain of the video signal is controlled by the control voltage V _G corresponding to this voltage V ₂ to Vin. =
It becomes Vref.

When the voltage of the voltage source (13) is Vc and the base-emitter drop voltage of the transistor (12) is V _BE , V ₂ > V _c
When it becomes −V _BE , the transistor (12) is turned off, and the control voltage V _G which is the emitter voltage of this transistor (12) is the maximum V
It is limited to c, above which the peak ABL is rendered inoperative.

Therefore, in this circuit, the voltage of each part is as shown in FIG. 2, and when the input signal Vin becomes large, the control voltage V _G is lowered as shown in the range a. After that, the voltage V ₁ is changed as shown in the range b by charging / discharging the capacitor (8), but in the portion where this change does not exceed V _D1 + V _D2 , the voltage V ₂ is not changed and the control voltage V _G is fixed. When the voltage V ₁ is further changed to V _D2 or more, the range c
The voltages V ₂ and V _G are also increased, as shown at. Further, the voltage V _G is limited by the maximum voltage Vc as shown in the range d.

That is, in this circuit, the voltage V
_The voltage V ₂ does not fluctuate (is insensitive) to a fluctuation within ₁ V _D1 + V _D2 , and this voltage V _D1 + V _D2 (insensitive region) is changed in one frame period by releasing the voltage V ₁ for example. By setting the voltage to be set, it is possible to remove the sag described above and prevent uneven brightness and flicker.

The configuration for obtaining the dead region in the above circuit is not limited to the configuration of the diodes (9) and (10), and other configurations may be used.

[Advantage of the Invention] According to the present invention, the sag of the detection output is removed by inserting the circuit having the dead zone, and thereby the peak AB is maintained without changing the response characteristics of the start and release.
It is now possible to maintain stable image quality during L operation.

[Brief description of drawings]

FIG. 1 is a block diagram of an example of the present invention, FIG. 2 is a diagram for explaining the same, and FIGS. 3 to 5 are diagrams for explaining a conventional technique. (1) is an input terminal, (2) (12) is a transistor, (3) is a power supply terminal, (4) (5) (14) is a resistor, (6) is an operational amplifier, (7) (13) is Voltage source, (8) (11) capacitors, (9) (10) diodes,
(15) is an output terminal.

Claims (1)

[Scope of utility model registration request] 1. A peak for detecting a peak value of an input video signal, and when the detected peak value is a predetermined value or more, the peak value is taken out through a circuit having a predetermined response characteristic to control the gain of the video signal. In the ABL circuit, the input video signal is supplied to a gain control circuit, the output is supplied to an operational amplifier, the output of the gain control circuit is compared with a reference voltage value serving as a reference, and the comparison output having a predetermined width is used. A peak ABL circuit, characterized in that it is processed through a circuit having a dead zone, and this output is fed back to the control terminal of the gain control circuit.