JPH05145792A - Synchronizing clamp device for executing pedestal permutation processing - Google Patents

Abstract

PURPOSE:To execute a normal pedestal clamp even if an impulse noise is generated in a video signal. CONSTITUTION:A demodulating video signal 2 from a BS tuner 1 is inputted to a synchronizing separator circuit 3 and a YC separating circuit 4, and a luminance signal 5, a chroma signal 6 and a back porch signal 11 are obtained. The luminance signal 5 is processed by a synchronizing clamp circuit 7 and a synchronizing clamp luminance signal 8 is outputted to a switch circuit 10. The switch circuit 10 switches the reference potential 9 and the signal 8, based on the back porch signal 11, permutes a porch part of the signal 8 to the reference potential, eliminates an impulse noise of its part, outputs a pedestal permutation luminance signal 12 to a YC mixing circuit 13, synthesizes it with the chroma signal 6, and supplies a pedestal permutation video signal 14 to a television monitor 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention replaces a back porch portion obtained by subjecting a luminance signal after YC separation to a predetermined synchronous clamp processing with a DC signal having the same potential as a pedestal level. The present invention relates to a synchronous clamp device that removes impulse noise in a back porch portion so that a normal pedestal clamp is performed.

[0002]

2. Description of the Related Art In a television monitor, a pedestal clamp circuit is used in order to faithfully reproduce the brightness information of a video signal on a display image. This circuit reproduces the DC component lost by C coupling in order to faithfully reproduce the image. Generally, the pedestal level of the luminance signal is made uniform by using a horizontal pulse delayed by 5 μs from the horizontal synchronization signal. Is done by.

In the screen as shown in FIG. 6, when the DC component is lost due to C coupling, the zero level of the AC signal is aligned with the average level and the pedestal level which is the reference of the video signal is not aligned, as shown in (C). become. Therefore, in the screen like (a), the originally white part sinks in gray, and in the screen like (c) the originally black part floats up in gray, so that a faithful image cannot be reproduced. Before a signal is applied, it is necessary to make the pedestal level of a series of video signals uniform with a constant potential.

FIG. 5 shows an example of a conventional pedestal level clamp circuit. Tr ₁ and Tr ₂ are transistors and R _{1 to} R ₅ are transistors.
Is a resistor, VR ₁ and VR ₂ are variable resistors, and C ₁ , C ₂ and C ₃ are capacitors. In this circuit, a positive video signal is added to point A, and a phase of the horizontal pulse is delayed at point B to add a clamp pulse in phase with the back porch portion of the horizontal synchronizing signal.

The transistor Tr ₁ becomes conductive during the period when the clamp pulse is applied to the base, the collector current of Tr ₁ is saturated, and the collector voltage is fixed to the voltage at the point C of the emitter. At this time, the charging current abruptly flows through the capacitor C ₁ with a small time constant, but the potential at the point D becomes approximately the potential at the point C. Next, in the period when there is no clamp pulse, the transistor Tr ₁ is cut off, and the electric charge charged in the capacitor C ₁ is discharged. However, since the discharge time constant is selected to be very large, D remains until the next pulse arrives. The potential of the point is kept constant.

In this state, when a video signal is superimposed on the base of the transistor Tr ₁ , the amount and polarity of charging the capacitor C ₁ change according to the potential difference between the potential at point D and the pedestal level, resulting in uneven pedestal level. The DC component whose potential is clamped at a constant value is regenerated. Transistor Tr ₁
The volume VR ₁ inserted in the emitter of is a brightness adjusting volume, and the pedestal level is changed and the brightness can be adjusted by changing the potential at the point C.

[0007]

In FM transmission of a video signal represented by satellite broadcasting, if C / N is less than or equal to impulse noise is generated in the demodulated video signal. 7 (a) shows a case where C / N is good, and FIG. 7 (b) shows a case of C / N.
When the impulse noise N occurs in the back porch portion of the video signal as shown in FIG. 7B, the pedestal clamp circuit of the related art described above shows the pedestal level. However, it is not clamped at the clamp level, and there is a drawback that incorrect luminance information is displayed in the display image on the TV monitor.

An object of the present invention is to enable a normal pedestal clamp to be performed on a television monitor when impulse noise occurs in a video signal as described above.
Another object of the present invention is to propose a configuration of a synchronous clamp device that can replace the back porch portion of a video signal with a signal without impulse noise.

[0009]

In order to achieve the above object, the synchronous clamp device of the present invention comprises a luminance component extracting means for obtaining a luminance component signal from a video signal containing an impulse noise component, and a synchronization from the video signal. Synchronous component signal extracting means for extracting a component signal, and synchronous clamp means for performing clamp control at a level such that the potential difference between the negative peak of the luminance component signal and the pedestal level becomes a predetermined value to obtain a luminance component signal of a constant pedestal level. And a reference signal generated at substantially the same level as the pedestal level, and the clamp-controlled luminance component signal, based on the synchronization component signal, to switch the porch portion of the luminance component signal to the reference. Pedestal replacement means for replacing the impulse noise component existing in the pouch portion with the signal level. And butterflies.

[0010]

The luminance component signal and the synchronization component signal are extracted from the video signal containing the impulse noise component. A luminance component signal of a constant pedestal level obtained by clamp control at a level such that the potential difference between the negative peak of the luminance component signal and the pedestal level becomes a predetermined value, and a reference generated at substantially the same level as the pedestal level. And a signal based on the synchronization component signal. As a result, the porch portion of the luminance component signal is replaced with the level of the reference signal, and the impulse noise component existing in the porch portion is removed.

[0011]

Embodiments of the present invention shown in the drawings will be described below.
FIG. 1 shows an embodiment of a synchronous clamp device for pedestal replacement processing according to the present invention. Reference numeral 1 is a BS tuner, 2 is a demodulated video signal, 3 is a sync separation circuit, and constitutes the sync component signal extracting means. Reference numeral 4 is a YC separation circuit, which constitutes the luminance component extraction means. Reference numeral 5 is a luminance signal, 6 is a chroma signal, and 7 is a synchronous clamp circuit, which constitutes the synchronous clamp means.
Reference numeral 8 is a synchronous clamp luminance signal, 9 is a reference potential, and 10 is a switch circuit, which constitutes the pedestal replacement means. 11
Is a back porch signal, 12 is a pedestal replacement luminance signal,
Reference numeral 13 is a YC mix circuit, 14 is a pedestal-replacement video signal, and 15 is a television monitor. The demodulated video signal 2 output from the BS tuner 1 is separated into a luminance signal 5 and a chroma signal 6 by a sync separation circuit 3 and a YC separation circuit 4, and then the brightness signal 5 is input to a sync clamp circuit 7.

FIG. 2 shows an example of the structure of the synchronous clamp circuit 7. In the figure, 31 is an input video signal, R ₁ and R ₂ are bias resistors, and 32 is a first NPN transistor, which constitutes the video signal input means. 35 is a charging capacitor,
Reference numeral 38 is a discharge resistor, which constitutes the charging / discharging means. Reference numeral 40 is an amplifier, 42 is a low pass filter (LPF), and 44 is a low frequency component video signal.

Reference numeral 46 is a fourth NPN transistor, 48 is an output signal thereof, 50 is a fifth NPN transistor, 52 is a switching current, 54 is a load resistance, and 56 is a switching signal (control signal). Reference numeral 58 is a third PNP transistor, and 60 is a video output signal. Further, RV is a variable resistor for generating the clamp voltage 25 (clamp reference signal).

In the synchronous clamp circuit of FIG. 2, the input video signal 31 is biased by the power supply voltage + V and the resistors R ₁ and R ₂ and then reduced in impedance by the first transistor 32 connected to the emitter follower and then charged. The clamp operation is performed by charging and discharging the capacitor for use 35.

Charging in this clamp operation is performed when the potential of the switching signal 56 input to the base of the third transistor 58 becomes lower than the emitter voltage of the third transistor 58 (that is, power supply voltage + V).
8 flows into the charging capacitor 35. Regarding the discharge, by setting the input resistance value r of the amplifier 40 to a negligibly smaller value than the resistance value R of the discharge resistor 38, the above-mentioned discharge time constant T = RC.
By obtaining the discharge current 29

Next, the operation until the switching signal 56 for flowing the charging current 28 is generated will be described. The video output signal 60 is output as a video signal that is synchronously clamped as shown in FIG. 3, and is also input to the low-pass filter 42 via the amplifier 40, and its output is a low-frequency component video signal 44 with impulse noise smoothed. Is extracted. At this time, the level reduction of the signal by the low-pass filter 42 is amplified in advance by the amplifier 40 (or the clamp voltage may be adjusted in consideration of the level reduction). Use one that has a characteristic that can sufficiently smooth impulse noise without damaging the signal. For example, with respect to impulse noise generated when the FM modulation method is used to modulate a video signal such as satellite broadcasting, the FM is used in this embodiment.
When the PLL demodulation method is used as the demodulation method, the cutoff frequency fc of the low-pass filter 42 is about 0.1 to 0.
It is more effective if set to about 5 MHz.

A video output signal 60 is shown in FIGS. 3 (b) and 3 (c).
And an example of the low frequency component video signal 44. When the low-frequency component video signal 44 has its impedance lowered by the fourth transistor 46 and its output signal is applied to the emitter of the fifth transistor 50, the potential (synchronization signal level) of the output signal of the fourth transistor 46 is clamped by the clamp voltage 25. In the lower case, the switching current 52 will flow and the load resistor 54 will generate the switching signal 56. The switching signal 56 drives the third transistor 58 so that the negative peak of the synchronizing signal of the output signal of the fourth transistor 46 is kept constant so that it becomes the potential of the clamp voltage 25. That is, regardless of the presence or absence of impulse noise in the input video signal 31, the negative peak of the sync signal of the video output signal 60 is set to the clamp voltage 2
It can be clamped to a potential of 5.

As described above, the sync clamp circuit 7 outputs the sync clamp luminance signal 8 in which the negative peak of the sync signal is clamped to a constant potential without being affected by the impulse noise. At this time, the potential difference between the negative peak of the sync signal of the sync clamp luminance signal 8 and the pedestal level (potential of the back porch portion) is always a substantially constant value. The reason is firstly, for example, the journal of the Television Society of Japan, Vol. Four
4, No. 2 As disclosed on pages 169 to 172, since the video signal transmitted from the broadcasting station complies with the RS170A standard of the US EIA, the level difference between the negative peak of the synchronization signal and the pedestal level is 40IRE. Secondly, the level of the demodulated video signal 2 demodulated by the BS tuner 1 is kept substantially constant by the AGC. Since the negative peak of the sync signal is clamped to a constant potential and the potential difference between the negative peak of the sync signal and the pedestal level is almost constant, the pedestal level of the sync clamp luminance signal 8 depends on the presence or absence of impulse noise. Regardless of this, a constant potential is maintained.

Next, this will be described with reference to FIG. For example, if the potential of the sync clamp level is e _c and the potential difference between the negative peak of the sync signal and the pedestal level is e _o ,
The pedestal level of the synchronous clamp luminance signal 8 is e _c + e _o
Kept at the potential of. Here, as shown in FIG. 1, e _c +
When a reference potential 9 having a DC potential of e _o is prepared and only the back porch portion of the synchronous clamp luminance signal 8 is replaced with the reference potential 9 by the switch circuit 10, pedestal replacement in which impulse noise is removed from the back porch portion. The luminance signal 12 can be obtained. At this time, the back porch signal 11 for controlling the switch circuit 10 can be easily obtained from the sync separation circuit 3. The pedestal replacement luminance signal 12 and the chroma signal 6 are combined by the YC mix circuit 13 to obtain a pedestal replacement video signal 14. When the pedestal-replaced video signal 14 is supplied to the television monitor 15,
Normal luminance information is displayed without being affected by impulse noise.

[0020]

As described above, according to the present invention, B
In FM transmission of video signals represented by S broadcasting, C /
Even if impulse noise occurs in the demodulated video signal because N is low, normal pedestal clamping is performed without being affected by impulse noise, so that the luminance information can be faithfully reproduced in the display image on the TV monitor. it can.

[Brief description of drawings]

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

FIG. 2 is a block diagram showing a configuration example of a synchronous clamp circuit.

FIG. 3 is a waveform diagram for explaining the operation of the synchronous clamp circuit.

FIG. 4 is a waveform diagram for explaining the operation of the embodiment.

FIG. 5 is a circuit diagram showing a conventional pedestal clamp circuit.

FIG. 6 is a waveform diagram showing a relationship between a screen and an average level of a video signal.

FIG. 7 is a waveform diagram of a pedestal clamp when impulse noise occurs.

[Explanation of symbols]

 3 sync separation circuit 4 YC separation circuit 7 sync clamp circuit 10 switch circuit 13 YC mix circuit 15 TV monitor

Claims (1)

[Claims] 1. A brightness component extracting means for obtaining a brightness component signal from a video signal containing an impulse noise component, a sync component signal extracting means for extracting a sync component signal from the video signal, and a negative peak of the brightness component signal. And a pedestal level are clamped at a level such that the potential difference between the pedestal level and the pedestal level is a predetermined value, and a synchronous clamp means for obtaining a luminance component signal of a constant pedestal level; A pedestal for switching the controlled luminance component signal based on the synchronization component signal, replacing the porch portion of the luminance component signal with the level of the reference signal, and removing the impulse noise component existing in the porch portion. A synchronous clamp device for pedestal replacement processing, comprising: a replacement means.