WO1986005057A1

WO1986005057A1 - A method and an apparatus for protecting video signals

Info

Publication number: WO1986005057A1
Application number: PCT/NL1986/000006
Authority: WO
Inventors: Arie Marinus Wijnen
Original assignee: Arie Marinus Wijnen
Priority date: 1985-02-22
Filing date: 1986-02-24
Publication date: 1986-08-28
Also published as: NL8500515A; EP0214267A1

Abstract

A method and a device for protecting video signals recorded on magnetic tape against illegally copying the same, in which, when legitimately recording on a tape, in the video signal beyond the stage (13) from which the synchronisation signals are derived but before the final stage (16) connected to the recording heads (10), the negative frame synchronisation pulses (2) are each replaced by two consecutive negative pulses (2a, 2b), where the line synchronisation signals (5') are suppressed, this in such a manner that the tape and head drives of said apparatus are not influenced, and, when copying on an other tape, the disturbed synchronisation signals in the recording apparatus are sent towards the head and tape synchronisation circuits (11, 12), and will lead therein to a disturbation of the synchronisation. In particular the additional negative pulses (2b) can be extendend into the adjacent terminal portion (5) of a line signal block (3), and the length of said additional pulse (2b) can be modulated, this, if desired, in a random manner, in order to provide an additional protection.

Description

A method and an apparatus for protecting video signals.

The unauthorised recording on magnetic tape of information, and in particular video information, without paying the copyrights due, and also considerably damaging the interests of the legitimate owner in other ways, e.g. by the fact that, because of the lower price of such illegal copies, the legitimate copies are driven from the market in an increasing degree, is a gradually increasing evil against which actions have only little effect. For tracing and taking measures by the appropriate authorities can only take place intermittently and always against only a small portion of this market, and entails substantial cost for said authorities.

When trying to make copying such information impossible by means of jamming signals, care should be taken that, on the one hand, such signals do not disturb the reproduction, and, on the other hand, that these jamming signals cannot be filtered out. One has not succeeded up till now to find a satisfactory solution for this problem. Therefore there exists a very large need for an effective means for disturbing the reproduction of illegally recorded video information in such a manner that an unsaleable product is obtained. In the current picture (or video) signals, apart from video information also synchronisation information is comprised which serves for controlling the frame and time synchronisation of a reproduction apparatus (video tube). When recording the video information, said signals are also used for synchronising the tape and recording head drives present in the recording apparatus. In particular the latter drive is very sensitive, since it should take place with a very high accuracy in order to obtain a good recording and, thus, picture quality. A slight disturbance in the synchroni¬ sation can lead to a complete deregulation of the recording and re- production. It is an object of the invention to provide a solution for the above-mentioned problem, making use of the sensitivity of a re¬ cording apparatus for synchronisation deviations.

To that end the invention provides a method for protecting video information recorded on magnetic tape against illegitimate copying, which video information is comprised in a normal composite video signal consisting of blocks of line signals having positive portions which are modulated with picture information, and negative portions serving as line synchronisation signals, said blocks being mutually separated by a portion without picture information in which a negative frame synchronisation pulse is situated, and there no line synchronisation signals are present, which frame synchronisation pulses and line synchronisation signals are fed, during recording, towards the recording heads of the recording apparatus, the frame and line synchronisation signals being, furthermore, separately derived from the video signal to be recorded and being supplied to the synchronisation circuits of the head and tape drives, said method being characterised in that, when legitimately recording the video information on a tape, in the picture signal beyond the stage from which the synchronisation signals are derived, but before the final stage connected to the recording heads, the negative frame synchronisation pulses are each replaced by two consecutive negative pulses, where the line synchronisation signals are suppressed, this in such a manner that the tape and head drives of said apparatus are not influenced, and, when copying on an other tape, the dis¬ turbed synchronisation signals are sent, in the recording apparatus, towards the head and tape synchronisation circuits, and will lead to a disturbation of the synchronisation.

For synchronising the transversal displacement (rotation) of the recording heads, a tape recording apparatus utilises the frame synchronisation signals,so that, when copying the signals disturbed in this manner in a second recording apparatus, the syn¬ chronisation in said second recording apparatus will be disturbed by repeating said pulse in a composite video signal recorded on the tape, said heads, as far as the beginning of said transversal movement is concerned, then getting confused so that recording the subsequent picture information will be considerably disturbed. The synchronisation circuits of a reproduction apparatus, on the other hand, are much simpler, and are not being disturbed by the repetition of the frame synchronisation pulse. During legitimately recording, the first recording apparatus is controlled by the synchronisation signals present in the presented signal, the former being directly supplied to the respective synch-ronisation circuits so that their operation is not disturbed.

An improvement of such a protection by disturbing the synchronisation can be obtained by lengthening the additional negative pulse into the adjacent terminal portion of the line signal block. Generally no picture signal lines are present there, so that, during reproduction, the absence of the line signal pulses is not disturbing, but when recording again an additional disturbation in the synchronisation will produced in the recording apparatus, which will deteriorate the picture quality still further. Moreover a possible removal of the disturbation by means of a suitable circuit, such as a time-base recovery circuit, might be considerably hindered thereby.

A still further improvement can be obtained by modulating the length of the additional negative pulse, thereby still further hindering the possible recovery of the original condition, and, furthermore, disturbing the synchronisation of the reproducing apparatus to which a signal disturbed in this manner is supplied, so that the reproduction is deteriorated still further.

Preferably this modulation of the length of the second negative pulse is done in an arbitrary manner so as to remove any periodicity therefrom, which will make any attempt to reduce the disturbation unsuccessful.

Furthermore the invention relates to a tape recording apparatus for executing said method, said apparatus comprising a stage for deriving the head and tape drive synchronisation signals, and a final stage connected to the recording heads, the synchroni¬ sation outputs of the former stage being directly connected to the synchronisation circuits of the head and tape drives, said apparatus being characterised in that in the connection between said stages an additional stage is included which is adapted to divide at least the frame synchronisation pulses of the complete video signal each into two negative pulses where no line synchronisation pulses are present.

In particular said additional stage can be adapted for lengthening the additional negative pulse in such a manner that it will also suppress at least a part of the line synchronisation signals situated in the initial part of a line signal block. In particular said additional stage can be adapted for periodically, and in particular arbitrarily, modulating said additional pulses, to which end said circuit is connected to an auxiliary circuit which, in particular, can control the variation of the length of the- additional negative pulses on the basis of the line synchronisation frequency of the presented video signal which is derived from said first stage.

The invention will be elucidated below in more detail by reference to a drawing, showing in:

Fig. 1 a diagrammatical representation of a normal video signal suitable for being recorded on a magnetic tape;

Figs. 2A and B a composite signal according to Fig. 1 disturbed before recording on the tape and, respectively, a portion of said disturbed signal as recorded on the tape;

Figs. 3 and 4 diagrammatical representations of still further disturbed video signals; and

Fig. 5 a simplified block diagram of an apparatus for executing the method of the invention.

In Fig. 1 a composite picture (video) signal which is suitable for being recorded on magnetic tape is shown in a highly simplified manner. This signal comprises a zero level 1 on which, at the beginning of a picture frame, a negative frame synchronisation pulse 2 appears. Thereafter follows a block 3 of line signals shown as lines.

Each block 3 comprises a plurality of picture lines 4 in which the picture information is contained, and the portion 4' thereof below the zero level serves for line synchronisation. At the beginning and the end of each block 3 portions 5 are situated which, during reproduction, lie outside the limits of the screen, and which, when recording on a tape, need not to contain picture information, as is shown in the drawing by the absence of the positive line portions. On the contrary the corresponding line syn¬ chronisation portions 5' are present, which are also present in the interspaces between the blocks 3 with the exception of the location of the pulses 2.

According to the invention,_, as shown in Fig. 2A, the pulse 2 is split into two pulses 2a and 2b. If, now, these pulses are pre¬ sented to a synchronisation circuit of a recording apparatus, the first one will synchronise the frame synchronisation, i.e. the be¬ ginning of the recording head displacement. The second pulse 2b will, again, result in a head synchronisation. The head will, then, have the tendency to return to the initial position, and will get out of step thereby. In Fig. 2B it is diagram atically shown what happens when recording on a tape. As a consequence of the structure of the cir¬ cuits for signal supply towards the recording heads, the negative signal level at the pulses 2 will be quickly restored to the zero level, resulting there in a suppression of the line synchronisation signals 5' as indicated at 2a' and 2b' . This results in an additiona disturbance in the synchronisation of the recording heads.

A reproducing apparatus with simpler synchronisation cir¬ cuits has, in this respect, a certain -inertia, and. will not react to this repetition; this circuit will only react in the correct manner to the first pulse 2a. The absence of the signals 5' in the part outside the block 3 is not disturbing for the reproduction either, since, in the reproduction apparatus, the signals 4' are sufficient for the line synchronisation.

According to Fig. 3 the additional pulse 2b is lengthened in such a manner that it extends into at least a portion of the signals 5' within the block 3. When recording the disturbed signal this will lead to suppressing the signals 5' in the manner shown at 2b¹ in Fig. 2B, and to a more serious disturbation of the synchro¬ nisation when recording the signal thus disturbed. When reproducing the first recording, the reproduction apparatus will not be hindered by the absence of the pulses 5' as long as the suppression has not reached the region 4' . However, in a recording which is derived from such a signal recorded on a tape, the synchronisation is dis¬ turbed in such a manner that a picture of an unsaleable quality is obtained. The lengthening of the pulse 2b should, of course, not extend farther than the portion 5', since by suppressing the signals 4' the line synchronisation in the reproducing apparatus will be disturbed.

As a consequence of the periodicity of the pulses 2b it is, as such, feasible to suppress said pulses by means op special recovery circuits, and to replace them by line synchronisation signals 5' derived from the video signal. Although such recovery circuits will be expensive, this need not to be an objection in view of the economic interest of such illegally made copies.

In order to make such a recovery impossible, the lengthenin of the pulses 2b near the various blocks 3 can be modulated in the manner shown in Fig. 4 so that such a recovery will become virtually impossible. This will be the case indeed if such a modulation is not periodical, but is made at random. This can be obtained by means of control circuits operating according to a random function, which can, for instance, make use of a basic frequency contained in the video signal. Apart from the fact that, thereby, a recovery of the line synchronisation can be completely avoided, this has also a disturbing influence on the line synchronisation in the reproduction apparatus so that the quality of the reproduced picture will be made still worse.

In this manner it becomes possible to disturb a legitima¬ tely obtained recording in such a manner that, during reproduction, it results in a normal picture, but, when recording again on a tape, will produce such a disturbance of the synchronisation of the re- cording apparatus that the recorded picture, when reproduced, will be very seriously disturbed, e.g. by lines running through the picture, colour blots, frame skips (in particular when using the disturbance shown in Fig. 4) and the like. Since in the legitimately used recording apparatus the synchronisation of the original video signal is being used, and is led past the circuits used for dis¬ turbing towards the head and tape drives, the synchronisation will not be disturbed during said first recording.

Fig. 5 shows in a diagrammatical way the structure of a recording apparatus according to the invention. The video signal to be recorded is supplied at 3 to a normal control circuit 7. Therein the signals for synchronising the drive 8 for the tape 9 and for controlling the heads 10 are derived at 11 and 12 respectively. This synchronisation can, therefore, take place in the normal manner.

The complete video signal at the output 13 is led towards a circuit 14 wherein the splitting of the frame synchronisation signal 2 and, as the case may be, the suppression of the synchroni¬ sation signals 5' takes place. The output 15 thereof is connected to the normal frequency modulation circuit 16, and the output 17 thereof is connected to the heads 10. The picture signal sent towards the heads 10 is disturbed in the manner described above, but this will not influence the synchronisation of the drive 8 of the tape 9 and of the heads 10, so that a normal recording of the disturbed signal on the tape 9 takes place .

In order to produce, in the manner of Fig. 4, an additional disturbance, an auxiliary stage 18 is used which is adapted to control the stage 14 in such a manner that the length of the pulses 2b is modulated in the desired manner. This stage 18 can comprise an associated oscillator, but can also be connected to an additional output 19 of the stage 7, from which a frequency present in the video signal, e.g. that of the line synchronisation signals 4' and 5', can be derived, which frequency can be lowered in a suitable manner by means of frequency sealers. This stage can also be con¬ structed in such a manner that the scaling factor is modulated in turn„ and this, as the case may be, in a completely arbitrary way.

Claims

C l a i m s

1. A method for protecting video information recorded on magnetic tape against illegitimate copying, which information is comprised in a normal composite video signal, consisting of blocks of line signals having positive portions which are modulated with picture information, and negative portions serving as line syn¬ chronisation signals, said blocks being mutually separated by a portion without picture information in which a negative frame syn¬ chronisation pulse is situated, and there no line synchronisation signals are present, which frame synchronisation pulses and line synchronisation signals are fed, during recording, towards the recording heads of the recording apparatus, the frame and line syn¬ chronisation signals being, furthermore, separately derived from the video signal to be recorded and being supplied to the syn¬ chronisation circuits of the head and tape drives, characterise in that, when legitimately recording the video information on a tape, in th^*e video signal beyond the stage (13) from which the synchroni¬ sation signals are derived, but before che final stage (16) con¬ nected to the recording heads (10), the negative frame synchroni¬ sation pulses (2) are each replaced by two consecutive negative pulses (2a, 2b), where the line synchronisation signals (5¹) are suppressed, this in such a manner that the tape and head drives of said apparatus are not influenced, and, when copying on an other tape, the disturbed synchronisation signals are sent, in the re¬ cording apparatus, towards the head and tape synchronisation cir- cuits (11, 12), and will lead to a disturbation of the synchroni¬ sation.

2. The method of claim 1, characterised in that the additional negative pulses (2b) are lengthened into the adjacent terminal portion (5) of the adjacent line signal block (3).

3. The method of claim 1 or 2, characterised in that the length of the additional negative pulse (2b) is modulated.

4. The method of claim 3,. characterised in that the modulation of the length of the additional negative pulse (2b) is done in an arbitrary manner.

5. A tape recording apparatus for executing the method of any one of claims 1..4, comprising a stage for deriving the head and tape drive synchronisation signals and a final stage connected to the recording heads, the synchronisation outputs of the former stage being directly connected to the synchronisation circuits of the head and tape drives, ch aracteri sed in that, in the connection (13, 15) between said stages (12, 16) an additional stage (14) is included which is adapted to divide at least the frame synchronisation pulses (2) of the complete video signal each into two negative pulses (2a, 2b) where no line synchronisation pulses (5¹) are present.

6. The apparatus of claim 5, characterised in that the additional stage (14) is adapted for lengthening the additional negative pulse (2b) in such a manner that it will also suppress at least a part of the line synchronisation signals "(5' ) situated in the initial part (5) of a line signal block (3).

7. The apparatus of claim 5 or 6, characterised in that the additional stage (14) is adapted for periodically modulating the length of the additional pulses (2b).

8. The apparatus of claim 7, characterised in that the additional stage (14) is adapted to modulate the length of the additional pulses (2b) in an arbitrary manner.

9. The apparatus of claim 8, ch aracterised in that the additional stage (14) is connected to an auxiliary circuit (18), adapted to control the variation of the length of the additional negative pulses (2b) on the basis of the line synchronisation fre- quency of the presented video signal.