JPS5830282A - Video signal circuit - Google Patents

Abstract

PURPOSE:To reproduce a stable black level, by performing two times waveform modifications on a composite video signal, by using the black level detected in the picture signals as a reference level and inserting a false black level into the composite picture signal as the reference level when the black level does not exist. CONSTITUTION:A composite video signal supplied to an input terminal 1 is derived from the emitter of a transistor 1A after it is amplified by the transistor 1A and supplied to transistors 2 and 3. A blanking pulse and a synchronizing signal are supplied to terminals 5 and 6 and added to the base of the transistor 3 after they are combined with the composite video signal. A minimum level detecting circuit composed of the transistor 3, etc., detects the black level as the minimum level when the black level exists in the video signal and detects a false black level as the minimum level when the black level does not exist in the video signal, and supplies the black level or the false black level to the base of a transistor 13.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a four-television receiver related to a video signal enclosure that can obtain a video signal with a stabilized black level of a reproduced image by performing waveform processing on the video signal. In this case, 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 color television receivers, color reproducibility depends on the brightness level, so
It is important to supply a video signal component whose DC 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 set-up amount (in broadcasting standards, the set-up amount is 7.5% ± 25 when the level difference between the pedestal level and the reference white level is 100-). ) is constant, it is possible to match the black level with the cut-off level of the picture tube.

However, this deviation in set-up amount often differs depending on the broadcast station or broadcast program.

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

Moreover, it is often experienced that the difference in the set-up amount in actual broadcasting is larger than the range defined by the broadcasting standards as described above. Black level fluctuations due to changes in the set-up amount not only cause blacks to appear whitish on the received image, or blacks to appear too deep, but also to cause problems in color television receivers. As the black level rises and falls, the colors become too pale or too dark, causing a significant deterioration in image quality.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and 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 where there is no black level in the signal, a pseudo black level that is slightly closer to the white level than the black level is inserted into the composite video signal as a reference level.

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

FIG. 1 is a circuit diagram showing seven embodiments of the present invention.

A composite video signal is supplied to input terminal 1, and transistor 1
It is amplified by a person, and the signal waveform shown in FIG. 2 (1) is obtained at its emitter. This signal waveform is supplied to transistor 2 and also to transistor 3 via resistor 4. A blanking pulse and a synchronizing signal are supplied to the terminals 6, respectively, and these blanking pulses and synchronizing signals are combined with the composite video signal shown in FIG. 2(a). A signal waveform of is obtained. The reason why the blanking pulse is superimposed on the voltage divided by the resistor 8.9 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 (in the beginning of the blanking period), the transistor 4 resistor 10, 11 . It is necessary to make the lowest level detection circuit consisting of the capacitor 12 insensitive.

The reason why the synchronizing signal is also added is to eliminate the influence of the equalization pulse during the vertical retrace period.

As shown in Fig. 2(b), when a black level exists in the video signal that is blacker than the pseudo black level, the minimum level detection circuit determines this black level as the minimum level, and detects the existence of a black level in the video signal. If not, the pseudo black level is detected as the lowest level, and this detected level is supplied to the base of the transistor 15. The capacitor 12, resistor 110 time constant is chosen for several vertical periods, and this detection level is held until the next minimum level is reached. In addition, since the detection sensitivity can be determined by the resistance value of the resistor 10,
By selecting this resistance value, it is possible to prevent the detection circuit from erroneously detecting a portion lower than the pseudo black level that occurs for a very short period at the beginning of the plunging period.

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. Common emitter of transistor 2.16 or output terminal 1
Since one of the base potentials of the transistor 216, which is higher, is outputted to the output terminal 17, the signal waveform shown in FIG. 2(C) is obtained at the output terminal 17. Note that the detection level is supplied to the transistor 16 only during the pre-ranking period, and the detection level is not supplied to the transistor 16 during the Eirin signal period. Even if the pace potential of the transistor 2 is detected during the period and approaches the pel level, it is possible to prevent fluctuations in the output level due to the nonlinearity of the differential amplifier.

The level obtained in this way during the blanking period in Figure 2 (C) will always match the black level if it exists in the video signal, regardless of the quality of the broadcast signal. A correct black image can be reproduced by performing DC reproduction based on . Also, if the black level does not exist in the video signal, the entire image is relatively bright, so
Even when an image is reproduced using DC current based on a pseudo black level, almost no unnatural feeling occurs.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 (1)
-(C) are signal waveform diagrams of the main parts thereof. 1 person 2. & 1,16. ...transistor 7.12
．． ...Capacitor 10.11...Resistance

Claims (1)

[Claims] A first modified image that receives the composite video signal and has a pseudo level somewhat closer to the white level than the black level during the blanking period, and has a video signal component in the received composite video signal during the non-blanking period. The first one that generates the signal
a waveform shaping means for detecting the blackest level from the first modified video signal and holding the detected level; and second waveform shaping means for generating a second modified video signal having the video signal component in the composite video signal received during the non-blanking period.