JPS6326167A - Picture quality correction circuit - Google Patents

Abstract

PURPOSE:To prevent malfunction such as synchornization detection or synchronization separation by detecting a black level of an input video signal so as to eliminate a picture quality correction signal component being at a black level or below except the synchronizing period. CONSTITUTION:A capacitor 1, a diode 2 and a reference power supply 3 clamp a video input signal at the synchronization tip level and a refernece power supply 4 sets a standard video input signal to a same level as the synchronization amplitude level. A diode 5 and a resistor 6 give a reference potential of a black level of a balck level detection circuit essentially comprising an operational amplfier 7, and when the input video signal level is the black level or over, the black level detection circuit gives a low potential to a base of a transistor (TR) 8, which is conducted and when the input video signal level is the black level or below, the TR 8 is turned off. The TR 8 and the diode 9 are placed in an output stage of the picture quality correction circuit and limit the signal component being at the black level potential or below given by reference power supplies 3, 4 by the turning on/off of the TR 8.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image quality correction circuit that prevents malfunctions when a subsequent stage circuit performs synchronization detection and synchronization separation.

[Conventional technology]

FIG. 5 shows a conventional image quality correction circuit, and FIG. 6(a) shows an input signal waveform of the image quality correction circuit in FIG.
FIG. 6(b) shows an example of the output signal waveform.

As shown in FIG. 5, the above-mentioned image quality correction circuit receives an input signal vl from a signal source E through a capacitor 1. An output signal vok is output from an output terminal T through a parallel circuit of a resistor, a variable resistor 11, and a coil 12, and the output terminal T is grounded via a capacitor 13.

In such an image quality correction circuit, the degree of contour correction of the video signal is adjusted by changing the frequency characteristics of the circuit using the variable resistor 11.

That is, by varying the variable resistor 11.

It is designed to change the frequency characteristics of the circuit and adjust the degree of contour correction.

[Problem that the invention seeks to solve]

Since the conventional image quality adjustment circuit is configured as described above, when a video signal with a significant change in brightness is input as shown in FIG. The following may occur.

This has a negative effect on the video signal processing circuit connected to the subsequent stage, and in particular causes malfunction in synchronization detection.

The present invention was made to solve this problem, and uses an amplitude limiter at the output stage of the image quality correction circuit to prevent the image quality correction signal component from becoming below the black level of the video signal.
The present invention aims to provide an image quality correction circuit that can prevent malfunctions due to synchronization detection and separation in a video signal processing circuit connected to a subsequent stage.

[Means for solving problems]

The image quality correction circuit according to the present invention includes a black level detection circuit provided in parallel with the image quality correction circuit.

[Effect]

In this invention, the black level detection circuit has the function of switching the amplitude limiting circuit of the output stage of the image quality correction circuit, and when the input video signal is higher than the black level, the amplitude limiting circuit operates and the output signal of the image quality correction circuit is black. It operates to limit the amplitude of signal components below the black level, and acts so that the amplitude limiting circuit does not operate when the input video signal is below the black level.

〔Example〕

Embodiments of the image quality correction circuit of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of one embodiment. In FIG. 1, when explaining the configuration,
The same parts as in FIG. 1 are given the same reference numerals, and the parts different from those in FIG. 5 will be mainly described.

As is clear from comparing Figure 1 with Figure 5,
In the figure, the resistor 10. shown in FIG. Variable resistance 11. Coil 12. A detection circuit described below is provided in parallel with the image quality correction circuit including the capacitor 13, and an amplitude limiting circuit for removing signal components below the black level is connected to the output stage of the image quality correction circuit.

In other words, the connection point between capacitor 1 and resistor 10 is resistor 2.
1 and a diode 2 to a connection point between black level reference power supplies 3 and 4.

The reference power supplies 3 and 4 for the black level are each at potential VI.

V2 (V@ > V+), connected in series,
The negative pole of the reference power source 3 is grounded, and the positive pole of the reference power source 4 is connected to a non-inverting input terminal of an operational amplifier 7 via a diode 5. This non-inverting input terminal is grounded via a resistor 6.

The inverting input terminal of the operational amplifier 7 is connected to the capacitor 1 and the resistor 10.
The output terminal of the operational amplifier 7 is connected to the connection point with the resistor 2.
2 to the pace of transistor 8.

The emitter of this transistor 8 is connected to the positive electrode of the reference power source 4, and the collector is connected to the output terminal T via a diode 9. This transistor 8 and diode 9 constitute an amplitude limiting circuit.

Next, the operation of this invention will be explained. capacitor 1
．． Diode 2. The reference power source 3 clamps the video input signal at the synchronization tip level. Further, the reference power source 4 is set to be the same as the synchronization amplitude level of the standard video input signal.

A diode 5 and a resistor 6 provide a black level reference potential for a black level detection circuit mainly including an operational amplifier 7.

When the input video signal level is higher than the black level, this black level detection circuit applies a low potential to the pace of the transistor 8 to make the transistor 8 conductive.

Also, when the input video signal level is below the black level,
Turn off transistor 8t. A transistor 8 and a diode 9 are connected to the output stage of the image quality correction circuit, and limit signal components below the black level potential given by the reference power supplies 3 and 4 by turning on and off the transistor 8.

Next, the operation of the present invention will be further explained in detail. Figure 2 (a
) When the video signal shown in ) is input, capacitor 1. Diode 2. A clamp circuit composed of a reference power supply 3 clamps the synchronization tip.

This clamped signal is then transmitted to a black level detection circuit and an image quality correction circuit.

The reference potential of the black level detection circuit is set to a potential higher than the DC potential of the synchronization tip by the synchronization level at the time of standard video input, that is, the black level (actually, a level slightly lower than the black level).

When the input signal is lower than the black level, such as during a synchronization period, the output of the black level detection circuit has a high potential, and the amplitude limiting circuit at the output stage of the first image quality correction circuit does not operate, and the output of the image quality correction circuit remains unchanged. The output is as shown in Figure (b).

Furthermore, when the input signal is below the black level, the amplitude limiting circuit operates to remove signal components below the black level from the output of the image quality correction circuit.

In the above embodiment, the simplest image quality correction circuit is shown, but the same effect can be obtained with any configuration of the image quality correction circuit.

FIG. 3 shows another embodiment of the present invention, in which the output of the operational amplifier 7 is supplied to the base of the transistor 8 through a resistor 22t-, and at the same time is supplied to the base of the transistor 140.

The emitter of this transistor 14 is grounded, and the collector is connected to the output terminal T via the capacitor 13. The transistor 14 acts as a switch, and by providing the transistor 14 in the output stage of the black level detection circuit in this way, the image quality correction circuit does not have frequency characteristics only during the synchronization period, that is, it simply passes the synchronization signal. The outline correction of the synchronization signal is not performed.

Thereby, even better effects can be achieved for the video signal processing circuit connected to the subsequent stage.

That is, when a synchronization tip flag circuit or the like is connected in the subsequent stage in addition to synchronization detection and synchronization separation, the fact that the contour correction component is not added during the synchronization period guarantees accurate rung operation.

An example of the input signal waveform at this time is shown in FIG. 4(a), and with respect to this input signal waveform, the output signal waveform is
The result will be as shown in Figure (b).

〔Effect of the invention〕

As explained above, this invention detects the black level of the input video signal and removes the image quality correction signal component below the black level excluding the synchronization period. Malfunctions such as detection and synchronization separation can be prevented. For example, if this system is used as an image quality correction circuit for a VTR, it can prevent blurring and jitter on the screen caused by out of synchronization between the horizontal period and water drop period of a TV. It has an improving effect.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of the side of the image quality correction circuit of this invention.
FIG. 2(a) is a waveform diagram showing the input signal of the above image quality correction circuit, FIG. 2(b) is a waveform diagram showing the output signal of the above image quality correction circuit, and FIG. 3 is a waveform diagram showing the output signal of the above image quality correction circuit. 4(a) is a waveform diagram of the input signal of the image quality correction circuit of FIG. 3, and FIG. 4(b) is a waveform diagram of the output signal of the image quality correction circuit of FIG. 3. Figure 5 is a circuit diagram of a conventional image quality correction circuit, Figure 6 (&) is a waveform diagram of an input signal of a conventional image quality correction circuit, and Figure 6 (b) is a waveform diagram of an output signal of a conventional image quality correction circuit. be. l, 13... Capacitor, 2, 5.9... Diode, 3.4... Reference power supply, 6, 10.22... Resistor, 7... Operational amplifier, 8.14... transistor,
11...Variable resistance, 12...Coil. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] an image quality correction means that inputs a video signal and adjusts the degree of contour correction of the video signal by changing the frequency characteristics of the circuit; a black level detection means that detects when the video signal becomes below a black level; An image quality correction circuit comprising: an amplitude limiter for preventing the output of the image quality correction means from being below the black level based on the output of the black level detection means.