GB2396767A

GB2396767A - Inserting positive-going pulses to back porch of a video signal for copy protection

Info

Publication number: GB2396767A
Application number: GB0407606A
Authority: GB
Inventors: Arie Wijnen; Tan Baolin
Original assignee: Dwight Cavendish Systems Ltd
Current assignee: Dwight Cavendish Systems Ltd
Priority date: 2002-02-01
Filing date: 2002-02-01
Publication date: 2004-06-30
Anticipated expiration: 2022-02-01
Also published as: GB0202384D0; GB2396767B; GB2387497B; GB2396767A8; WO2003065716A3; WO2003065716A2; GB2396767B8; GB0407606D0; GB2387497A

Abstract

An apparatus and method for applying anti-copy protection to a raster-scanned video signal are described. A positive-going pulse 44 is added to the back porch of the raster-scanned video signal such that it is substantially contiguous with the negative-going horizontal synchronisation pulse 46. The effect of the pulse is apparent following recording of the video signal by a video cassette recorder. A negative-going pulse 46 may be added after the colour burst 42 and a further positive-going pulse 48 at the start of the active line 34.

Description

ANTI-COPY PROTECTION FOR A VIDEO SIGNAL

The present application relates to a method and an apparatus for 5 modifying a raster-scanned video signal such that the modified video signal will still be viewable on a display device, such as a television screen, but that following recording of the modified video signal, playback of the recorded modified signal will be adversely affected.

The present application provides a way of preventing unauthorized 10 copying of an original video signal, and may be used, in particular, to deter copying onto tape of Pay-Per-View video signals, and video-to- video (tape-to tape) copying.

Video piracy is a significant problem for broadcasters and distributors of video signals, since the unauthorised copying and distribution of video 15 recordings by pirates can impact drastically on the revenue generated by a broadcaster or distributor through legitimate sales. It is therefore desirable to prevent video pirates from making unauthorised copies of video signals.

This is best achieved by modifying the original video signal such that when it is recorded the recorded signal cannot be satisfactorily played back.

20 In known protection techniques, the unauthorized recording of the video signal is made less enjoyable to watch by the interaction of the original signal to which the protection has been applied with the electronic components in either the video cassette recorder or the television receiver itself. For example, making an unauthorized copy of the video signal too dark to be viewed satisfactorily may 25 be achieved by adding to the original video signal a pulse which is significantly larger than that part of the signal which carries the picture information. The position at which the pulse is added depends on the way in which the circuits in the television receiver or video recorder process the signal. When the modified signal is processed by the automatic gain control circuits of a video cassette 30 recorder, the amplitude of the signal is perceived as being that of the added pulse and not that of the portion of the signal carrying the useful information.

Consequently, the video cassette recorder or television receiver amplifies the received signal by a smaller factor than if the pulse was not present. As a result of this the information-carrying portion of the signal is not therefore amplified 35 enough to be seen satisfactorily when reproduced.

- 2 An alternative protection technique described in International Patent Application WO 01/76240 involves the removal of a small number of horizontal synchronization pulses from the blanking section of the signal, so that an unauthorized recording of the signal cannot be properly synchronized by the TV 5 receiver on which it is to be played back. As a result, the resulting picture playback can be poor.

A further technique, disclosed in International Patent Application WO 96/31878, relies on inserting a pulse into the colour burst information portion of the signal section of the signal, such that automatic gain control circuits that rely 10 on the average do level of the colour burst to determine the necessary amplification of the signal, make such amplification too small. An opposing pulse signal having a magnitude sufficient to offset the change in do level of the colour burst portion caused by the pulse signal, and optionally a second pulse, are inserted somewhere from the last half of the remainder of the back porch of the 15 signal to the end of the start of the picture information portion.

We have found that this technique can be unreliable in practice, and furthermore has the disadvantage that by inserting a pulse into the colour burst part of the signal, the resulting picture quality is detrimentally affected.

Both of these techniques rely on the components employed in the video 20 cassette recorder or the television receiver, and, in some cases, certain video recorders or television receivers may have an arrangement of components that is not susceptible to the adverse picture effects caused by the modified signal. Thus, the modified video signal can still be played without significant detriment to the picture playback and the anticopy protection applied to the modified signal is 25 rendered useless.

It has therefore been appreciated that there is a need for an improved protection technique.

Summary of the Invention

The invention is defined by the independent claims below to which reference should now be made. Advantageous features are set forth in the appendant claims.

5 An apparatus and method for applying anti-copy protection to a raster scanned video signal embodying the invention are described in more detail below with reference to the drawings.

A positive-going pulse is added to the back porch of the raster-scanned video signal such that it is substantially contiguous with the negativegoing 10 horizontal synchronization pulse. The effect of the pulse is apparent following recording of the video signal by a video cassette recorder. A negative-going pulse 96 may be added after the colour burst 92 and a further positive-going pulse 98 at the start of the active line 84.

15 Brief Description of the Drawings

The invention in a preferred embodiment will next be described in detail, by way of example, and with reference to the drawings in which: Figure 1 is an illustration of portions of a typical video signal; Figure 2a is an illustration of a number of lines of a known unmodified 20 picture signal; Figure 2b is an illustration of a modified PAL picture signal according to the preferred embodiment of the invention; Figure 2c is an illustration of a modified NTSC signal in accordance with the preferred embodiment of the invention; and 25 Figure 3 is a schematic circuit diagram showing a preferred embodiment of apparatus for generating the signal shown in Figure 2b.

Detailed Description of Preferred Embodiment

Background

30 Figure 1 shows part of a conventional raster-scanned video signal 2 for a PAL picture signal. Two blanking regions of the signal are shown with a few picture lines on either side. The upper blanking region shown is at the end of an odd field

and the lower one is at the end of an even field.

- 4 The picture signal is comprised of 625 lines 4 containing control information 6, such as synchronisation pulses, which are used to configure the response of the television receiver to the signal and ensure generation of a good quality picture, and usually picture information 8. The line numbers 1-9,308-320, and 622-625 are 5 shown on the figure.

The synchronisation pulses are used to reset the television receiver so that it is ready to display the next line of picture information, following the completion of the previous line (horizontal synchronisation pulses), or following completion of an entire field (vertical synchronisation pulses).

10 In Figure 1, each picture information line 5, namely the lines at the left side of the figure, can be seen to comprise picture information 8, shown as a positive going waveform, and a horizontal synchronisation pulse,10, shown as a negative going pulse.

Following the lines containing picture information shown at the top-left of 15 Figure 1, is the vertical blanking region 12 of the signal. This region contains the vertical synchronisation pulses 14 used to control fly-back from the bottom of the television screen to the top once a field or raster of the picture has been displayed.

In a PAL (or NTSC) signal, the picture is generated on the screen in two rasters that are interlaced with each other, and it is therefore necessary to have 20 two blanking regions. These are both illustrated for the PAL signal in Figure 1.

The first blanking region 12 begins at line 310 of the picture signal and the second blanking region 16 begins at line 623.5. Line 623 of the picture signal therefore contains only a half line of picture information. However, the television receiver is set up such that this line lies outside of the viewable area of the 25 television screen and, like the blanking section, is therefore not viewed. It will be appreciated by those skilled in the art that the lines of the picture signal are numbered consecutively from 1 to 625, and that the signal between the two blanking regions is therefore continuous.

The blanking section itself comprises five negative-going equalization 18 30 pulses each of 2.3,s width, followed by five negative-going vertical synchronisation pulses 14 each of 27.3ps width, followed by a further five equalization pulses. The equalization pulses 18 play a similar role to the horizontal synchronisation pulses 10 and need not be discussed further here as their function is well known to those skilled in the art.

The do level 20, namely the voltage from which the line synchronization pulses extend is called the blanking level. This voltage corresponds to the colour black in the picture information.

Following the blanking region there are typically a number of lines 22 of 5 suppressed video information as shown in Figure 1. These suppressed lines allow room for the transmission of information subsequent to the video information, such as that used in teletext systems.

Preferred Embodiment of the Invention 10 A modified video signal in accordance with the preferred embodiment of the invention, which provides a protection method against video-to-video copying, will next be described with reference to Figures 2a, 2b and 2c.

Figure 2a shows two adjacent active lines 30 of an unmodified video signal 32. The video signal could be either a PAL or an NTSC signal.

15 Video information 34 is represented by the stepped waveform ascending from the blanking or black level at its bottom to the white peak level at its top. It will be appreciated that this waveform if viewed on a television screen would be seen as colour bars.

Between the active lines are negative-going horizontal synchronization 20 pulses 36 which control flyback in the television receiver between successive lines of the video signal. The horizontal region on the left of a horizontal synchronization pulse 36 is known as the front porch 38, while that on the right is known as the back porch 40. Colour burst information 42, used by the television receiver during demodulation of the chrominance part of the video signal is located on the back 25 porch of the signal on each line and is represented purely diagrammatically by a rhombus or diamond shape.

The video signal in Figure 2b has been modified according to the invention in a preferred embodiment so that it is compatible with PAL television receivers. A signal suitable for NTSC television receivers is described below in relation to 30 Figure 2c.

The modified PAL signal includes a first additional pulse 44 with a magnitude that is approximately equal to the peak-white level; typically at a level of 1 to 1.2 volts. The first additional pulse is substantially contiguous with the horizontal synchronization pulse. As shown it is situated directly adjacent to the 35 horizontal line synchronization pulse. In fact the ascending or right-hand edge of

- 6 the horizontal synchronisation pulse meets the ascending or left-hand edge of the additional pulse smoothly to form a continuous slope, that is to say that there is substantially no space between the edges of the two pulses.

The presence of the pulse at this location has been found to have an 5 adverse effect on the Automatic Gain Control (AGC) circuits of video recorders.

These circuits detect horizontal synchronisation pulses and, based on a determination of their amplitude, amplify the video signal such that it is suitable for recording. The combination of the negative-going horizontal synchronisation pulse 36 and the positive-going additional pulse 44 appears to the automatic gain control 10 circuit as a larger than normal horizontal synchronisation pulse. As a result the amplification provided by the automatic gain control circuit is less than it should be and is insufficient in respect of the rest of the video signal. The recorded signal, when played back later on a television receiver, will be too dark to be satisfactorily viewed. Furthermore it may exhibit stability problems, as the horizontal l S synchronisation pulses and vertical synchronisation pulses have not been amplified enough to be reliably detected by the circuits of the television receiver.

As mentioned above, it is important for the implementation of this feature that the additional pulse be placed contiguous with the horizontal synchronisation pulse. In particular, the pulse is not placed in the colour burst part of the signal, as 20 this has been found to have an adverse effect on the picture quality while having little or no effect on the automatic gain control circuits.

The amplitude of the pulse 44 is approximately white level, as shown, though it may be that an amplitude above 30% of white level will be sufficient in certain circumstances. The duration of the pulse 44 is between 0.5 and 21ls and is 25 preferably about ups. If the duration of the pulse 44 is at the larger end of the range, the horizontal synchronizing pulse 36 can be moved slightly earlier (to the left) to provide sufficient space.

The presence of the first pulse 44 has however been found to have a detrimental effect on the playability of the modified signal even before 30 unauthorized recording has occurred. This is because the pulse interferes with the detection stage circuits in the television receiver causing the receiver to display the modified signal as a picture that is darker than it should be. In order to compensate for this effect, a second, negative going pulse 46, substantially equal in magnitude to a normal horizontal synchronisation pulse, is preferably added to

- 7 the signal directly after the colour burst. This additional negativegoing pulse has been found to reverse the ill-effect of first pulse 44 on legitimate playback of the original modified signal. The duration of pulse 46 is in the range 1 to Bus, and is preferably 1.81ls.

5 The presence of second pulse 46 however also makes possible the addition of a third, positive-going pulse 48, located just prior to or at the start of the active video information contained in the signal, and directly next to the negative going pulse 46. The magnitude of the third pulse is about the same as that of the peak white level, and its duration is in the range 1 to Ups, preferably Ups.

10 The third pulse acts in the same way as first pulse 44, by interfering with the operation of the automatic gain control of the video recorder. The combination of the pulses 46 and 48 again appear like a horizontal synchronization pulse of larger magnitude than an ordinary synchronization pulse, and therefore interferes with the automatic gain control circuits of the video recorder to add to the effect 15 caused by the first pulse 46. Without second pulse 46, the presence of third pulse 98 is not enough to cause sufficient instability in the recorded signal.

Although in Figure 2a and 2b, only active lines of the video signal have been shown, the pulse could be inserted throughout the entirety of the video signal, including the blanking section for example.

20 Figure 2c shows a video signal modified according to the invention in a second aspect for use with NTSC television receivers. The signal is similar to that shown in Figure 2b for PAL receivers except that the third pulse 48 is missing and the second pulse is narrower and is located before the colour burst rather than after it. In Figure 2c, the second pulse is given the reference numeral 49 to make it 25 clear that it is different to the second pulse 46 in the case of PAL transmission.

The second pulse 49 has a duration in the range 0.5 to 1.21ls, and is preferably O.9,as. The horizontal synchronization pulse 36 can be moved slightly earlier if necessary to provide sufficient space for the second pulse 49.

The combination of first pulse 44 and second pulse 49, shown in Figure 2c, 30 has been found in the case of NTSC receivers to produce a signal that, once recorded by a video cassette recorder, cannot be viewed on a television receiver, but that before recording can be viewed at or near to the picture quality intended by the originator of the video signal. Once again, the presence of second pulse 49

- 8 is to undo the darkening effect that the pulse 44 has on playback of the television signal. The two different signals illustrated in Figures 2b and 2c are required to implement the invention because of the different operation of television receivers 5 set up to receive PAL signals and NTSC signals respectively.

The preferred apparatus for adding pulses 44, 46 and 48 to the video signal is shown in Figure 3 to which reference should next be made. Preferably this apparatus is made use of by video distributors to add the anti-copy protection described above to the video signal. Both however include the first additional 10 pulses 44.

The apparatus 50 comprises a housing 52 in which an external input 54 is mounted for receiving an unmodified video signal. The signal is passed from the input to a digital video processor 56 controlled by control software 58. The digital processor analyses the received unmodified signal, and under the control of the 15 software 58, adds pulses to the signal in the manner described above. The digital video processor outputs a signal containing the original signal and the additional pulses to video amplifier 60. This amplifies the signal for output to a video output 62 external to the apparatus. The video output thereby supplies a video signal that has been protected against unauthorized copying according to the second aspect 20 of the invention described above. The video distributor can then record this onto a video cassette using a professional video recorder in which the automatic gain control circuit is turned off.

Although the preferred apparatus has been described with reference to a digital video processor and control software, it will be appreciated that both the 25 control software and the digital control software could be implemented in purpose-

built equivalent electronic circuits. Also, although reference has been made throughout this application to a television receiver, it will be understood that any display device on which video signals can be viewed and which operates in a way equivalent to a television receiver to display a raster-scanned signal is included 30 within the scope of the invention. Furthermore, the operation of any apparatus described in this application could be implemented partly or wholly in software as appropriate.

Claims

- 9 - CLAIMS

1. An apparatus for adding anti-copy-protection to a raster-scanned video signal to produce a modified video signal, the apparatus comprising: 5 an input for receiving a video signal which is to be modified, the video signal having horizontal synchronization pulses; modifying means for so modifying the video signal received at the input to produce a modified video signal, such that the modified video signal can be viewed on a display device at or near the quality intended by the originator of the video 10 signal, but that recordings of the modified video signal made by a video cassette recorder with automatic gain control exhibit a reduction in picture quality when played back and viewed on a display device; wherein the modifying means is operable to add at least a first pulse to the video signal in the vicinity of one or more horizontal synchronization pulses, the 15 first pulse being a positive-going pulse added to the back porch of the horizontal synchronization pulse such that it is substantially contiguous with the horizontal synchronization pulse; and an output for supplying the modified video signal as an output.

20 2. The apparatus according to claim 1 wherein the modifying means is operable to add the first pulse to the video signal such that the ascending edge of the horizontal synchronization pulse and the ascending edge of the first pulse form a substantially continuous ascending edge.

25 3. The apparatus according to claims 1 or 2 wherein the modifying means is operable to add the first pulse such that its magnitude is substantially equal to the peak white level.

4. The apparatus according to claim 1 or 2 wherein the modifying means is 30 operable to add a second pulse to the back porch, the second pulse being a negative-going pulse added adjacent to the colour burst portion of the signal.

- 1 0 5. The apparatus according to claim 4 wherein the modifying means is operable to add the second pulse directly after the colour burst portion of the signal. 5 6. The apparatus according to claim 4 wherein the modifying means is operable to add the second pulse before the colour burst portion of the signal but after the first pulse.

7. The apparatus according to any of claims 4, 5, or 6 wherein the modifying 10 means is operable to add the second pulse such that its magnitude is substantially equal to the voltage level of the horizontal synchronization pulse.

8. The apparatus according to any of claims 4 to 7 wherein the modifying means is operable to add a third pulse to the back porch of the video signal, the 15 third pulse being a positive-going pulse added such that it is substantially contiguous with the second pulse.

9. The apparatus according to claim 8 wherein the modifying means is operable to add the third pulse to the video signal such that the ascending edge of 20 the horizontal synchronization pulse and the ascending edge of the second pulse form a substantially continuous ascending edge.

10. The apparatus according to claims 8 or 9 wherein the modifying means is operable to add the third pulse such that its magnitude is substantially at the peak 25 white level.

11. A method for adding anti-copy-protection to a raster-scanned video signal to produce a modified video signal, the method comprising: receiving a video signal which is to be modified, the video signal having 30 horizontal synchronization pulses; modifying the video signal received at the input by adding to it at least a first pulse in the vicinity of one or more horizontal synchronization pulses, to produce a modified video signal, such that the modified video signal can be viewed on a display device at or near the quality intended by the originator of the video 35 signal, but such that recordings of the modified video signal made by a video

- 1 1 cassette recorder with automatic gain control exhibit a reduction in picture quality when played back and viewed on a display device; the first pulse being a positive-going pulse added to the back porch of the horizontal synchronization pulse such that it is substantially contiguous with the 5 horizontal synchronization pulse; and supplying the modified video signal as an output.

12. The method according to claim 11 wherein modifying the video signal includes adding the first pulse to the video signal such that the ascending edge of 10 the horizontal synchronization pulse and the ascending edge of the first pulse form a substantially continuous ascending edge.

13. The method according to claim 1 1 or 12 wherein modifying the video signal includes adding the first pulse such that its magnitude is substantially equal to the l S peak white level.

14. The method according to claim 1 1, 12 or 13 wherein modifying the video signal includes adding a second pulse to the back porch, the second pulse being a negative-going pulse added adjacent to the colour burst portion of the signal.

15. The method according to claim 14 wherein modifying the video signal includes adding the second pulse directly after the colour burst portion of the signal. 25 16. The method according to claim 14 wherein modifying the video signal includes adding the second pulse before the colour burst portion of the signal but after the first pulse.

17. The method according to claims 14, 15 or 16 wherein modifying the video 30 signal includes adding the second pulse such that its magnitude is substantially equal to the voltage level of the horizontal synchronization pulse.

18. The method according to any of claims 14 to 17 wherein modifying the video signal includes adding a third pulse to the back porch of the video signal, the

- 12 third pulse being a positive-going pulse added such that it is substantially contiguous with the second pulse.

19. The method according to claim 18 wherein modifying the video signal S includes adding the third pulse to the video signal such that the ascending edge of the second pulse and the ascending edge of the third pulse form a substantially continuous ascending edge.

20. The method according to claims 18 or 19 wherein modifying the video 10 signal includes adding the third pulse such that its magnitude is substantially the peak white level.

21. The method according to any of claims 11 to 20 comprising recording the outputted signal onto a video tape using a video cassette recorder in which the 15 automatic gain control has been deactivated.

24. Apparatus substantially as described herein with reference to Figures 2 and 3 of the drawings.

20 25. A method substantially as described herein with reference to Figures 2 and 3 of the drawings.