JPH0435577A - peak clip circuit - Google Patents

Abstract

PURPOSE:To improve the contrast in a middle band by shunting a part of the luminance signal current to the earth at the time when the pedestal clamped luminance signal is higher than a first reference level and an ABL voltage exceeds a second reference level. CONSTITUTION:When the pedestal clamped luminance signal is inputted to an input terminal 1 and the voltage applied to the base of a transistor TR Q1 is higher than Vo, a diode D2 is turned on and a current i0 shunt to a resis tance R13 flows. When an ABL voltage Va is inputted to an input terminal 2 and is reduced, a TR Q3 is turned on and a current i2 starts to flow. Conse quently, a potential Vc of the collector of a TR Q4 starts to fall, and a TR Q5 is turned on and a current is starts to flow, and therefore, a base potential Vd of a TR Q6 is raised and the emitter voltage of a TR Q6 is raised, and the diode D2 is turned off. Thus, the contrast in the middle band is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a peak clipping circuit that clips (suppresses) the peak value of a luminance signal in video equipment to increase contrast in the intermediate range.

[Conventional technology]

In conventional video amplification circuits that handle luminance signals, contrast adjustment is performed using an ACL (automatic contrast limiter) circuit. This ACL circuit has a certain level of APL.
(average picture level) increases and ABL
(Auto brightness limiter) did not work unless it was activated.

[Problem to be solved by the invention]

For this reason, for example, if you try to increase the contrast in the intermediate level region when the APL is low, you have no choice but to increase the overall contrast, which causes white collapse (saturation phenomenon in the white image area) on the picture tube surface, reducing the image quality. The problem was that it was significantly reduced.

The present invention has been made in view of these points, and its purpose is to provide a peak clip circuit that can improve the contrast in the intermediate range without causing whitening at low APL. That's true.

[Means to solve the problem]

For this reason, the peak clip circuit of the present invention is turned on when the pedestal-clamped luminance signal becomes equal to or higher than the first reference level and the ABL voltage becomes equal to or higher than the second reference level, and a part of the luminance signal current is turned on. It consists of a switch circuit that shunts the current to ground.

〔Example〕

Examples of the present invention will be described below. FIG. 1 is a diagram showing a circuit of one embodiment.

1 is a luminance signal input terminal, and the luminance signal input here is impedance-converted by an emitter follower consisting of a transistor Q1 and a resistor R2 via a resistor R1, and then sent to the ACL.
The signal is input to a built-in video chroma IC (not shown).

2 is the ABL voltage input terminal, and the ABL voltage applied here
The voltage Va is impedance-converted by an emitter follower consisting of a transistor Q2 and a resistor R3, and is then transferred to the next stage (pnp).
Input to the base of transistor Q3. A voltage vb obtained by dividing the power supply voltage Vcc by resistors R4 and R5 is applied to the emitter of this transistor Q3, and a constant voltage (Vf) of a diode D1 is applied to the collector via a resistor R6. . R7 is a resistance for conducting the diode D1. Q4 is a transistor whose collector is connected to the power supply through a resistor R8 and whose emitter is connected to ground through a resistor R9, R10 is a bias resistor, and Q5 has an emitter connected to the power supply and a collector connected to the base of the transistor Q6. pnp) transistor. transistor Q6
functions as an emitter follower with resistor R13, and the power supply voltage Vcc is connected to resistor R11 and R1 as its base bias.
A voltage Vd divided by 2 is applied.

Under certain conditions, a part of the luminance signal applied to the base of the transistor Q1 described above flows to ground via the series circuit of the switching diode D2 and the resistor R14, and the resistor R13 at the emitter of the transistor Q6. It looks like this.

Now, in this circuit, when a pedestal-clamped luminance signal is input to input terminal 1 and the voltage applied to the base of transistor Q1 (point A) is higher than VO, diode D2 is turned on and resistor R13 is turned on. Since the shunt current 10 flows, the voltage at point A has the waveform shown by the solid line in FIG. 2(a). The waveform shown by the broken line is the waveform of the luminance signal applied to the input terminal 1. Then, the luminance signal whose peak has been suppressed in this way is input to the video chroma IC from the emitter of the transistor Q1, and if the ACL is off there and a signal with the same pp voltage as before the peak suppression is output, as shown in Fig. 2 ( As shown in b), the voltage waveform has improved contrast in the intermediate range.

On the other hand, when the APL increases and the ACL voltage Va decreases, and the ACL starts to work, the overall contrast is narrowed down, and if peak clipping continues to operate, the screen will have inconspicuous white peaks, so in this case, the diode D
It is necessary to turn off the transistor Q6 to stop peak clipping, and for this purpose, the transistor Q6 is controlled.

Therefore, the ABL voltage Va is input to the input terminal 2. When this ABL voltage Va becomes low (ABL and ACL begin to be effective), transistor Q3 turns on. Transistor Q2 is always on.

Therefore, if the emitter of transistor Q3 is set to vb, since Va<Vb, transistor Q3
turns on and current 11 begins to flow.

Therefore, the potential Vc of the collector of the transistor Q4 starts to decrease, the transistor Q5 turns on, and the current i2 starts to flow, so the base potential Vd of the transistor Q6 becomes high, the emitter voltage of the transistor Q6 becomes high, and the diode D2 becomes Turn off.

From the above, the clip level V is determined by the voltage Vd.

can be set. Furthermore, the voltage at which the peak clipping operation starts can be set using the collector voltage vb of the transistor Q3. Note that the latter voltage vb is the ABL voltage Va at which the ACL of the video chroma IC starts operating according to the ACL characteristics.
Set the peak clip operation to turn off.

Figure 2 (C) shows the output waveform of the peak clip (point A) voltage when the peak clip is off, and (d) shows the output waveform of the video chroma IC when the peak clip is off and the ACL is on. This is the waveform of the output voltage.

〔Effect of the invention〕

As mentioned above, when the APL voltage is high and the ACL does not operate, and the screen is dark with a white peak, simply increasing the contrast will cause the beam current in the picture tube to flow too much, causing the white peak to collapse. In the present invention, since peak clipping is performed at this time, the same effect as brightening the entire image and increasing contrast can be obtained without causing collapse of the white peak portion. Furthermore, on a screen where the ACL is activated and the APL (ABL) voltage is low and the entire screen is bright, peak clipping stops and white peaks become conspicuous.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a peak clip circuit according to an embodiment of the present invention, and FIG. 2 is an explanatory diagram of its operation. 1... Brightness signal input terminal, 2... ABL voltage input terminal. Agent Patent Attorney Tsuneaki Nagao 2nd

Claims (1)

[Claims] (1) A switch circuit that turns on when the pedestal-clamped luminance signal exceeds a first reference level and the ABL voltage exceeds a second reference level, and shunts a portion of the luminance signal current to ground. A peak clip circuit characterized by comprising: