JPH021427B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal circuit that can obtain a video signal with a stabilized black level of a reproduced image by performing waveform processing on the video signal.

In a television receiver, it is desirable that the black level of the video signal component match the cutoff level of the picture tube, regardless of the content of the video signal. In particular, in a color television receiver, since color reproducibility is affected by the brightness level, it is important to supply a video signal component whose direct current component has been properly reproduced to the picture tube.

For this reason, normal television receivers reproduce DC video signals based on the level of the pedestal portion of the composite video signal. Therefore, the level difference from the black level of the video signal, that is, the setup amount (in broadcasting standards, the setup amount is the level difference between the pedestal level and the reference white level equal to 100%).
It is defined as 7.5% ± 2.5%. )
If the black level is constant, the black level can be matched with the cutoff level of the picture tube.

However, the deviation of this setup amount is
It often varies depending on the broadcast station and broadcast program.

Therefore, on the receiver side, in order to view an image in which black is correctly reproduced, the viewer has to readjust the vedestal level of the video signal in which the DC component is reproduced.

Furthermore, it is often experienced that the deviation of the set-up amount in actual broadcasting is larger than the range defined by the broadcasting standards as described above. Fluctuations in the black level due to changes in the amount of setup not only cause phenomena such as blacks appearing as white spots on the received image, or blacks becoming too deep, but also cause black With the rise and fall of color, colors become too pale or too dark, causing a significant deterioration in image quality.

The present invention eliminates the above-mentioned drawbacks of the prior art,
An object of the present invention is to provide a video signal circuit capable of reproducing a stable black level despite fluctuations in the black level of a transmitted video signal component.

In order to achieve the above object, the present invention corrects the waveform of a composite video signal twice, detects the black level if it exists in the video signal, uses the detected black level as a reference level, and In the case of a relatively bright image in which there is no black level in the signal, a pseudo black level that is slightly closer to the monochrome level than the black level is inserted into the composite video signal as a reference black level.

The present invention will be explained below with reference to FIGS. 1 and 2.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

A composite video signal is supplied to input terminal 1, amplified by transistor 1A, and a second
The signal waveform shown in Figure a is obtained. This signal waveform is supplied to transistor 2 and also to transistor 3 via resistor 4. Blanking pulses and synchronization signals are supplied to terminals 5 and 6, respectively, and these blanking pulses and synchronization signals are combined with the composite video signal of FIG. 2a, and the signal waveform of FIG. 2b is supplied to the base of transistor 3. is obtained. The reason why the blanking pulse is superimposed on the voltage divided by the resistors 8 and 9 via the capacitor 7 is to raise the level of the blanking pulse and to utilize a portion where the pulse width is as wide as possible. Through this combination, the lowest level during the blanking period becomes a pseudo black level that is somewhat closer to the white level than the black level. If a sufficient pulse width cannot be obtained and a level lower than the pseudo black level remains in a part (at the beginning of the blanking period), transistor 3, resistors 10 and 11, capacitor 1
It is necessary to make the lowest level detection circuit consisting of 2 insensitive. The reason why the synchronization signal is also added is to eliminate the influence of the equalization pulse during the vertical retrace period.

As shown in Figure 2b, the minimum level detection circuit sets this black level as the minimum level when there is a black level in the video signal that is blacker than the pseudo black level, and when there is no black level in the video signal. detects the pseudo black level as the lowest level and supplies this detected level to the base of the transistor 13. The time constant of capacitor 12 and resistor 11 is chosen to be several vertical periods, and this detection level is held until the next minimum level arrives. In addition, the detection sensitivity can be determined by the resistance value of the resistor 10, so by selecting this resistance value, the detection circuit will erroneously detect a portion lower than the pseudo black level that occurs for a very short period at the beginning of the blanking period. can be prevented from happening.

The detection level appearing at the emitter of the transistor 13 is supplied to the transistor 16 via the resistor 14 only during the period when the diode 15 is off, that is, during the blanking period. Since the higher base potential of the transistors 2 and 16 is outputted to the common emitter of the transistors 2 and 16, that is, the output terminal 17, the signal waveform shown in FIG. 2c is obtained at the output terminal 17. Note that the detection level is supplied to the transistor 16 only during the blanking period, and the detection level is not supplied to the transistor 16 during the video signal period. Even when the base potential of transistor 2 approaches the detection level, fluctuations in the output level due to nonlinearity of the differential amplifier can be prevented.

The level obtained in this way during the blanking period in Figure 2c will always match the black level if it exists in the video signal, regardless of the quality of the broadcast signal, so this level is used as the reference level. A correct black image is reproduced by performing DC reproduction. Furthermore, if the black level does not exist in the video signal, the entire image is relatively bright, so even if the image is DC-reproduced based on the pseudo-black level, almost no unnatural feeling will occur.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
Figures a to c are signal waveform diagrams of the main parts. 1A, 2, 3, 13, 16...transistor,
7, 12... Capacitor, 10, 11... Resistor.

Claims (1)

[Scope of Claims] 1. A composite video signal is input, and the black level of the composite video signal during the blanking period of the composite video signal is set to a pseudo black level that is somewhat closer to the white level than the black level of the composite video signal. a first waveform shaping means that is connected to the waveform shaping means and detects a black level that is the blackest signal level of the output of the waveform shaping means, and that the detected black level is blacker than the pseudo black level; black level detecting means for holding and outputting the detected black level when the black level is close to white, and outputting and holding the pseudo black level when the detected black level is closer to white than the pseudo black level; The output of the level detection means is inputted, and the output of the black level detection means is outputted during the blanking period of the composite video signal, and the video signal component of the composite video signal is outputted during the non-blanking period. A video signal circuit comprising: a waveform shaping means; and a means for adjusting detection sensitivity of the black level detection means. 2. The video signal circuit according to claim 1, wherein the black level detecting means includes holding means for holding the detected black level over several vertical scanning periods.