JPH09238304A - Copy guard generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a corrected video signal with excellent guard performance without deteriorating gradation of a video signal. SOLUTION: A copy guard signal generating circuit 11 generates a pseudo synchronization pulse and an AGC(automatic gain control) pulse. A subtractor 13 subtracts a prescribed DC level from a pulse pair in a timing of the AGC pulse. Thus, a level of the AGC pulse is decreased more than a peak white level. An output of the subtractor 13 is superimposed onto digital image data by an adder 14 and converted into an analog signal by a D/A converter 16. Thus, even when the dynamic range of the D/A converter 16 is set to the peak white level, D/A conversion without distortion is attained. An adder 17 adds a DC level to an output of the D/A converter 16 and provides an output of the result. Since the peak level of the image is in matching with the dynamic range of the D/A converter 16, gradation of the image is not deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copy guard generator for preventing copying of video signals.

[0002]

2. Description of the Related Art In recent years, in order to protect the copyright of information recorded on a video tape, a copy guard device for preventing copying of the video tape has been developed. In a conventional copy guard device, a modified video signal is generated by superimposing the copy guard signal on a video signal recorded on a video tape. The modified video signal contains the original information. When the modified video signal is reproduced by a VTR (video tape recorder), the reproduced signal from the VTR is displayed on an ordinary television receiver to display the original information. Can be displayed normally as an image. However, if the modified video signal is recorded by another VTR, the original information cannot be normally reproduced from the recorded data. .

4 and 5 are explanatory views for explaining a modified video signal created by a conventional copy guard device.

In the conventional copy guard apparatus, the sync signal of the analog image signal recorded on the video tape is detected, and the copy guard signal is superimposed during the vertical blanking period of the detected sync signal. Such an apparatus is disclosed in Japanese Patent Laid-Open No. 61-288582.

FIG. 4 shows a waveform during an equalization period of a vertical blanking period in a state where a copy guard signal is superimposed on a normal video signal. In the equalization period, the equalization pulse 1 is provided every 1/2 horizontal synchronization period (about 63.55 μsec). The equalization pulse 1 is a negative pulse that falls from the blanking reference level to the sync tip level. Negative pseudo sync pulse 2 and positive AG between the equalization pulses
A plurality of pulse pairs composed of C pulse 3 are inserted.

Further, FIG. 5 shows a predetermined horizontal period within the vertical blanking period having the horizontal synchronizing pulse 5 in which the color burst 4 is superimposed on the pouch portion.

A pulse pair composed of the pseudo sync pulse 2 and the AGC pulse 3 is inserted between the horizontal sync pulses 5.

Pseudo sync pulse 2 is a negative pulse that falls from the blanking reference level to the sync chip level, and maintains a low level for at least 0.5 μsec. The positive polarity AGC pulse 3 is provided as a pair with the immediately preceding pseudo sync pulse 2, and its amplitude reaches from the blanking reference level to the peak white level.

A general VTR has an automatic gain control loop (AGC loop) circuit. The AGC loop circuit cannot distinguish between equalizing pulse 1 and pseudo sync pulse 2. Therefore, when the corrected video signal on which the copy guard signal is superimposed is input, the synchronization cannot be normally established, and the AGC loop circuit remarkably changes the recording signal standard level due to the influence of the AGC pulse 3. As a result, the video signal at the time of recording differs from the original level and becomes unstable, and normal video recording is not performed.

In order to surely prevent copying of the video tape, the AGC pulse is set to have an amplitude that reaches the clipping level of the medium for transmitting the modified video signal. Usually, the clipping level has a value of 100 to 125% of the peak white level.

As described above, according to this proposal, copy guard is performed by superimposing a pulse pair composed of a pseudo sync pulse and an AGC pulse as a copy guard signal in the vertical blanking period.

However, since the AGC pulse has a constant amplitude, the device for erasing the AGC pulse can be constructed relatively easily. Therefore, a device has been developed in which the amplitude of the AGC pulse is appropriately changed to make it impossible to erase the AGC pulse.

In the conventional copy guard apparatus that changes the amplitude of the copy guard signal as described above, it is necessary to strictly control the magnitude of the amplitude and the adjustment of the temporal change. That is, a highly accurate timer and level setting circuit are required, and it is difficult to configure these circuits with analog circuits. Therefore, in the conventional copy guard device, at least the portion that generates the copy guard signal is composed of a digital circuit.

In this case, D / is used to restore the digital video signal added with the copy guard signal to the analog signal.
A converter is required. By the way, it is conceivable to set the dynamic range of the D / A converter to the peak white level. However, as described above, the level of the AGC pulse may be set higher than the peak white level in consideration of reliable blocking of the copy. In this case, the video signal on which the AGC pulse is superimposed is accurately converted to analog. I will not be able to. So D
The dynamic range of the / A converter is set to the peak level of the AGC pulse.

In this case, assuming that the composite video signal is D / A converted, the peak level in the video period is the sum level of the luminance signal and the chrominance signal.
The dynamic range of the A converter and the peak level of the video period substantially coincide with each other, and there is no particular problem.

However, in the S standard in which the luminance signal and the chrominance signal are separately processed, the peak level in the video period is relatively small with respect to the dynamic range of the D / A converter. That is, there is a problem that the dynamic range assigned to the video signal is small and the gradation deteriorates during D / A conversion.

[0017]

As described above, the conventional copy guard generator described above has a problem that the gradation of the video signal is deteriorated during D / A conversion.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a copy guard generator capable of improving the gradation expression of a video signal even when a copy guard signal is added. And

[0019]

A copy guard generator according to the present invention is supplied with input digital image data, detects a synchronization timing of the digital image data, and detects a pseudo synchronization pulse and an AGC pulse for copy guard. Timing generation means for detecting the timing of superimposition, and the pseudo-sync pulse and AG for copy guard based on the timing detected by the timing generation means.
A copy guard signal generating means for generating a C pulse, a limiting means for outputting the digital image data on which the pseudo sync pulse and the AGC pulse are superimposed in a limited dynamic range, and a predetermined dynamic range. A digital / analog converting means for converting the output of the limiting means into an analog signal;
A correction means for returning the output of the analog conversion means to the original image data level is provided.

According to the present invention, the timing at which the pseudo sync pulse for copy guard and the AGC pulse are superimposed is detected by the timing generating means. The limiting means limits the level within a predetermined dynamic range in a state where the pseudo sync pulse and the AGC pulse are superimposed on the digital image data. The digital / analog conversion means converts the output of the limiting means into an analog signal. Since the level of the digital image data is limited within the dynamic range of the digital / analog conversion means, digital / analog conversion can be performed without distortion, and even if the pseudo sync pulse and the AGC pulse are superposed, the digital / analog conversion is performed. In, the gradation of the image data does not deteriorate. The correction means restores the output of the digital / analog conversion means to the level of the original image data and outputs it.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a copy guard generating apparatus according to the present invention.

Digital image data is input to the input terminal 10. This digital image data is supplied to the timing generation circuit 12 and the adder 14. Timing generation circuit
Reference numeral 12 denotes a timing signal (hereinafter, referred to as an AGC timing signal) that detects a horizontal and vertical synchronization timing from image data and indicates a period in which an AGC pulse is superimposed in a horizontal period and an equalization period in the vertical blanking period and a pseudo signal. A timing signal (hereinafter referred to as a pseudo sync timing signal) indicating a period in which the sync pulse is superimposed is generated. The timing signal from the timing generation circuit 12 is supplied to the copy guard signal generation circuit 11, the selector 15 and the adder 17 as a correction means.

The copy guard signal generation circuit 11 generates a negative pseudo sync pulse at the timing of the pseudo sync timing signal and an AGC pulse at the timing of the AGC timing signal. The copy guard signal generation circuit 11 uses an AG
The amplitude of the C pulse is appropriately changed according to the time change. The peak level of the AGC pulse is set higher than the peak white level. The copy guard signal generation circuit 11 outputs a pulse pair composed of a pseudo sync pulse and an AGC pulse to the subtractor 13.

The output of the selector 15 is also given to the subtractor 13. The selector 15 is inputted with a digital value which gives a predetermined direct current (DC) level and a digital value which gives a reference potential. Based on the timing signal from the timing generation circuit 12, one of the two inputs is selected to subtract the subtractor. It is designed to output to 13. That is, the selector 15 outputs a digital value which gives a predetermined DC level to the subtractor 13 at the timing of the AGC timing signal, and outputs a digital value which gives a reference potential at other timings. The predetermined DC level is set to a value larger than the level of the difference between the peak level of the AGC pulse and the peak white level.

The subtractor 13 subtracts the output of the selector 15 from the pulse pair from the copy guard signal generation circuit 11 and adds it.
Output to The adder 15 superimposes the pulse pair from the subtracter 13 on the input digital image data to create a digital modified video signal. The output of the adder 15 is supplied to the D / A converter 16. The D / A converter 16 converts the input modified video signal into an analog signal and adds it to the adder 17
Output to The dynamic range of the D / A converter 16 is set in the range from the synchronous chip level to the peak white level.

The adder 17 generates an analog signal of a predetermined DC level corresponding to the digital value supplied to the selector 15, and the D / A converter 16 at the timing of the AGC timing signal from the timing generation circuit 12. The analog signal generated is added to the output of and the corrected video signal is output.

Next, the operation of the embodiment thus constructed will be described with reference to the timing chart of FIG. 2A shows an AGC pulse generated by the copy guard signal generating circuit 11, FIG. 2B shows an AGC pulse from the subtractor 13, and FIG. 2C shows a pseudo sync pulse. 2D shows the output of the adder 14, FIG. 2E shows an analog signal of a predetermined DC level, and FIG. 2F shows the modified video signal from the adder 17.

Digital image data input via the input terminal 10 is supplied to the timing generation circuit 12 and the adder 14. The timing generation circuit 12 detects the synchronization timing of the digital image data, and in the horizontal period and the equalization period within the vertical blanking period, the pseudo synchronization pulse and the AGC.
The pseudo sync timing signal and the AGC timing signal indicating the period in which the pulses are superimposed are output.

The copy guard signal generating circuit 11 is based on the timing signal from the timing generating circuit 12
The AGC pulse and the pseudo sync pulse shown in FIGS. 2A and 2C are generated. The pseudo sync pulse is a negative pulse that falls from the blanking erase reference level to the sync chip level, as in the conventional case (FIG. 2C). Also,
As shown in FIG. 2A, the AGC pulse is a positive pulse that rises from the blanking blanking reference level to a level higher than the peak white. Furthermore, the amplitude of the AGC pulse is changed in order to improve the copy guard performance. The copy guard signal generation circuit 11 outputs the generated pulse pair to the subtractor 13.

On the other hand, the timing generation circuit 12 also supplies the AGC timing signal to the selector 15. Selector 15 is A
A digital value giving a predetermined DC level is output to the subtractor 13 at the timing of the GC timing signal. At other timings, the selector 15 outputs a digital value (for example, “0”) giving the reference potential to the subtractor 13.

The subtractor 13 subtracts the output of the selector 15 from the pulse pair. Since the predetermined DC level generated at the AGC timing is set to a value larger than the difference between the peak level and the peak white level of the AGC pulse, as shown in FIG. To
The peak level of the AGC pulse portion becomes lower than the peak white level.

The pulse pair from the subtractor 13 is added to the digital image data input by the adder 14. Thus, the pulse pair shown in FIG. 2D is obtained. FIG.
(D) shows the waveform during the equalization period within the vertical blanking period. As shown in FIG. 2D, during the equalization period,
Between the equalization pulses 1, a pseudo sync pulse 2 having a negative polarity that falls from the blanking reference of the digital image data to the sync chip and a positive polarity that rises to a level below the peak white level immediately after the pseudo sync pulse 2 And a plurality of AGC pulses 6 are formed.

This digital image data is converted into an analog signal by the D / A converter 16. The peak level of the output of the subtractor 14 is below the peak white level. Therefore, the dynamic range of the D / A converter 16 can be set to the peak white level which is the peak level of the video signal. Since the dynamic range of the D / A converter 16 is set to the peak white level, the gradation of the video signal is not deteriorated in the digital / analog conversion by the D / A converter 16.
Further, since the peak level of the AGC pulse included in the output of the subtractor 14 is within the dynamic range of the D / A converter 16, the digital image data on which the AGC pulse is superimposed by the digital / analog conversion by the D / A converter 16. Does not deteriorate.

The output of the D / A converter 16 is supplied to the adder 17. The adder 17 generates an analog signal of a predetermined DC level shown in FIG. 2E at the timing of the AGC timing signal. The adder 17 adds the generated analog signal to the output of the D / A converter 16 and outputs it. Thus, the corrected video signal shown in FIG. 2 (f) is output from the adder 17. The adder 17 adds the potential corresponding to the value subtracted by the subtractor 13 to the AGC pulse portion, and the original A
It is returned to the level of the GC pulse.

As described above, in the present embodiment, after the level exceeding the peak white level is subtracted from the AGC pulse as an offset amount, it is superimposed on the image data to obtain a digital modified video signal. After the corrected video signal is converted into an analog signal, the analog value of the potential corresponding to the offset is added to obtain the corrected video signal on which the regular AGC pulse is superimposed. The peak level of the modified video signal input to the D / A converter is less than or equal to the peak white level.
The dynamic range of the converter can be set to the peak white level regardless of the AGC pulse level. That is, a modified video signal having a level exceeding the dynamic range of the D / A converter can be generated without increasing the number of bits of the D / A converter, and the gradation of the image is deteriorated in the digital / analog conversion. Can be prevented.

FIG. 3 is a block diagram showing another embodiment of the present invention. In FIG. 3, the same components as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

The digital image data input via the input terminal 10 is supplied to the timing generation circuit 12 and the subtractor 22. The subtractor 22 receives a digital value corresponding to the DC level from the blanking reference level to the sync chip level, subtracts this DC level from the input digital image data, and outputs it to the adder 23. That is,
The subtractor 22 outputs the digital value of the image data whose level is lowered by the level from the blanking reference level of the image data to the sync chip level.

The adder 23 includes a copy guard signal generation circuit 11
Also gives a pulse pair. The AGC pulse from the copy guard signal generating circuit 11 is set such that the level of the difference between the peak level and the peak white level is smaller than the DC level from the blanking reference level to the synchronous chip level. . The adder 23 adds the input image data and the pulse pair and outputs the result to the D / A converter 16. The D / A converter 16 converts the input image data into an analog signal and outputs it to the subtractor 24.

A timing signal indicating the synchronization signal timing is input from the timing generation circuit 12 to the subtractor 24. The subtractor 24 generates a DC level analog value from the blanking reference level to the sync chip level, and subtracts the DC level analog value generated from the output of the D / A converter 16 at the sync signal timing. And output as a modified video signal.

Next, the operation of the embodiment thus configured will be described.

The digital image data input through the input terminal 10 is given to the subtractor 22 and subtracted by the DC level from the blanking reference level to the sync chip level. As a result, the subtractor 22 outputs the image data whose level is lowered by the level from the blanking reference level to the sync chip level. The adder 23 adds the pulse pair from the copy guard signal generation circuit 11 to this image data. The output of the adder 23 is converted into an analog signal by the D / A converter 16.

The DC level of the image data is lowered in the subtractor 22, and the peak level of the AGC pulse is below the peak white level. Therefore, even when the dynamic range of the D / A converter 16 is set in the range from the synchronous chip level to the peak white level, the image data and the AGC pulse are converted into analog signals without distortion.

The output of the D / A converter 16 is given to the subtractor 24, a DC level from the blanking reference level to the sync chip level is added at the sync signal timing, and the potential of the sync signal portion reaches the sync chip level. It is lowered and returned to the original image signal. The output of the subtractor 24 is output as a modified video signal.

As described above, in the present embodiment, the fact that the range from the blanking reference level to the synchronous chip level is irrelevant to the image data is used to input digital image data within this range. By lowering only the level, it is possible to convert the digital image data on which the pulse pairs are superimposed into an analog signal without distortion by the D / A converter having a dynamic range from the synchronous chip level to the peak white level. As a result, even in the present embodiment, it is possible to prevent the gradation of the image from deteriorating.

[0045]

As described above, according to the present invention, it is possible to improve the gradation expression of a video signal even when a copy guard signal is added.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a copy guard generator according to the present invention.

FIG. 2 is a timing chart for explaining the operation of the embodiment.

FIG. 3 is a block diagram showing another embodiment of the present invention.

FIG. 4 is a waveform diagram for explaining a copy guard signal.

FIG. 5 is a waveform diagram for explaining a copy guard signal.

[Explanation of symbols]

11 ... Copy guard signal generating circuit, 12 ... Timing generating circuit, 13 ... Subtractor, 14 ... Adder, 15 ... Selector, 16 ... D / A
Converter, 17 ... Adder

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location H04N 5/92 H04N 5/92 F

Claims (3)

[Claims] 1. Timing generation means for receiving input digital image data, detecting a synchronization timing of the digital image data, and detecting a timing for superimposing a pseudo synchronization pulse and an AGC pulse for copy guard, and the timing generation means. Copy guard signal generating means for generating the pseudo sync pulse and AGC pulse for the copy guard based on the timing detected by the means, and the pseudo sync pulse and the AGC pulse are superimposed in a state of being limited to a predetermined dynamic range. Limiting means for outputting the digital image data, digital / analog converting means for converting the output of the limiting means into an analog signal in the predetermined dynamic range, and output of the digital / analog converting means for the original image data. Compensation means for returning to the level Copy protection generating apparatus characterized by the. 2. The limiting means limits the level of the AGC pulse from the copy guard signal generating means to a level equal to or lower than a peak white level and then superimposes it on the digital image data, and the correcting means comprises the digital / analog. 2. The copy guard generator according to claim 1, wherein the level of the AGC pulse included in the output of the converting means is returned to the original level. 3. The limiting means suppresses the level of the synchronous portion of the input digital image data, and then superimposes the AGC pulse from the copy guard signal generating means on the digital image data, and the correcting means 2. The copy guard generator according to claim 1, wherein the level of the suppressed synchronous portion of the output of the digital / analog converting means is returned to the original level.