JPH05153588A - Video signal processing circuit - Google Patents

Abstract

PURPOSE:To form a video image with excellent quality by using a circuit comprising a pedestal clamp circuit, a comparator circuit, a signal changeover circuit and a clamp level setting DC voltage source to compare a pedestal potential and a white level noise and synthesizing the waveform so as to eliminate the white level noise. CONSTITUTION:A white noise gate pulse is generated from a white noise gate pulse generating circuit 4 and fed to a comparator circuit 5. The output of a pedestal clamp circuit 2, the output of the circuit 4 and a DC voltage V0 for clamp level setting are inputted to the circuit 5, only the white noise is gated and outputted from the circuit 5 and inputted to a signal changeover circuit 6. An output of the pedestal clamp circuit 2 and the DC voltage V0 are inputted to the circuit 6 and the two inputs to the circuit 5 are selected by the output of the circuit 5. Thus, the DC voltage V0 is outputted for a white noise period and a signal except the white noise period in the outputs of the circuit 2 is outputted for the other period and the original signal is outputted from the circuit 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal processing circuit of a descrambling device used in a cable television (CATV) system terminal.

[0002]

2. Description of the Related Art In recent years, the services of general terrestrial television broadcasting have come to the point that the diversification of the needs of viewers cannot be answered because the channels are limited by the width of the broadcasting band. The number of viewers who subscribe to a CATV system using a network is increasing.

The current CATV system is an urban CAT.
The market is expanding centering around a system called a V system, and a paid service is provided only to a specific contractor in this system. The problem with paid services is that they are highly confidential, and for that reason image information is scrambled. That is,
The image information of the pay service is processed by a specific means so that the image cannot be reproduced only by a normal television receiver. In order to reproduce this scrambled signal into the original television signal, descramble processing is performed based on the key signal sent together with the scrambled signal. The signal that is scrambled by the method called video synchronous compression is expanded on the intermediate video carrier by the pulse signal that matches the timing of the compressed sync signal for descrambling in the receiving terminal or the demodulated video signal. Then put it back.

The conventional descramble signal processing will be described with reference to FIGS. 3 and 4. The demodulated scrambled signal is applied to the input terminal 10 of FIG. 3 and guided to the descramble circuit 11. Descramble circuit 11
4 (1) shows the signal waveform in the vicinity of the horizontal synchronizing signal among the waveforms of the input signal of FIG. The same signal is also applied to the attenuating pulse generating circuit 12, where an attenuating pulse with a pulse width t _{2 which is} slightly narrower than the horizontal blanking period t ₁ is produced, said pulse being said descramble circuit. 1
1 to gate the scrambled signal FIG. 4 (1) at another input of the descramble circuit 11. Therefore, the signal having the waveform shown in FIG. 4C is output to the output terminal 13 of the descramble circuit 11. Pulse width t ₂
Is preferably equal to t ₁ , but when t ₂ > t ₁ ,
The portion indicated by N in FIG. 4 (3) appears downward, that is, appears on the synchronization signal side and disturbs the synchronization of the receiver even after descramble. Therefore, the attenuation pulse width t
₂ is made slightly smaller than the width t ₁ of the horizontal blanking pulse.

On the other hand, when the attenuation pulse width t ₂ is narrower than the blanking erase pulse width t ₁ , a narrow pulse having a width of the difference appears as a waveform as shown in FIG. 4 (3). This pulse appears in the direction of high brightness in the direction opposite to the sync signal, and is called white noise. Since t ₁ = t ₂ is preferable, t ₂ is designed to be as close to t ₁ as possible.
The pulse N is a very narrow pulse of about 0.5 to 1 μS.

[0006]

However, according to the above means, the difference between the width of the horizontal blanking pulse of the scramble signal and the width of the attenuating pulse for descrambling causes the front porch and the back porch of the video signal to be different. A pulsed white noise is generated on the screen. In particular, the white noise of the back porch affects the pedestal level in the television receiver and affects the brightness of the image. When the compression level of the sync signal is fixed to a certain value, it is relatively inconspicuous, but when a plurality of compression levels are randomly switched and operated in a short time, the brightness change in the image becomes flicker and becomes conspicuous.

An object of the present invention is to provide a high quality image by adding a signal processing circuit for removing the above white noise.

[0008]

In order to achieve the above object, the present invention inserts a signal processing circuit equipped with a pedestal clamp, a comparator and a signal switching circuit into the output of a conventional descrambling circuit to obtain the pulse-shaped white signal. It has a configuration for removing noise.

[0009]

The present invention can provide a high quality image by removing the pulsed white noise generated at the time of descrambling according to the above configuration.

[0010]

1 is a block diagram of an embodiment of a video signal processing circuit of the present invention, which is inserted between a descramble circuit 11 and an output terminal 13 of FIG. Figure 3
Then, the descrambled video signal (FIG. 4 (3)) having pulsed white noise is input to the input terminal 1 from the descramble circuit 11. This signal passes through the pedestal clamp circuit 2, is clamped to the DC voltage V ₀ having a pedestal level of 3, and is output, and is input to the comparator circuit 5 and the signal switching circuit 6. This signal waveform is shown in Figure 2.
As shown in (1), this differs from FIG. 4 (3) only in the amplitude and DC level. Next, the descramble gate pulse (FIG. 4 (2)) is input from the descramble circuit 11 to the input of the white noise gate pulse generation circuit 4. This pulse is also shown as (2) in FIG. The white noise gate pulse generation circuit 4 produces the white noise gate pulse shown in FIG. 2C, and this pulse is input to the comparator circuit 5. The comparator circuit 5 outputs the output of the pedestal clamp circuit 2 (FIG. 2 (1)), the output of the white noise gate pulse generation circuit 4 (FIG. 2 (3)), and the DC voltage V ₀ for setting the clamp level of 3. And are entered,
In the waveform of Fig. 2 (1), only white noise is gated,
The signal is output from the comparator circuit 5 ((4) in FIG. 2) and input to the signal switching circuit 6. The output of the pedestal clamp circuit 2 (FIG. 2 (1)) and the DC voltage V _{0 of} 3 are input to the signal switching circuit 6, and these two inputs are the output of the comparator circuit 5 (FIG. 2 (4)). It is switched and t _{3 of} FIG.
The DC voltage V _{0 of} 3 is output during the period of t _{4 and} the period of the white noise width of t ₅ to t ₆ , and during the other periods, the white noise in the output of the pedestal clamp circuit 2 (FIG. 2 (1)) is output. 2 is output to the output of the signal switching circuit 6 except the period shown in FIG.
As shown in (5), the original signal with no white noise is output, and a high quality image can be provided.

[0011]

As described above, according to the present invention, by providing the pedestal clamp, the comparator, and the signal switching circuit, white noise generated during descrambling can be removed, and a higher quality image can be obtained. ..

[Brief description of drawings]

FIG. 1 is a block diagram of a video signal processing circuit according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing a white noise removal process of the same.

FIG. 3 is a block diagram of a conventional descramble circuit.

FIG. 4 is a waveform diagram of each part.

[Explanation of symbols]

 2 Pedestal clamp circuit 5 Comparator circuit 6 Signal switching circuit

Claims (1)

[Claims] 1. A pedestal clamp for removing pulsed white noise by a method of descrambling a scrambled signal using a synchronization signal compression method on the receiving terminal side and returning it to a regular baseband video signal. A video signal processing circuit including a circuit, a comparator circuit, and a signal switching circuit for removing the pulsed white noise.