GB2267598A - Detecting copied video recordings - Google Patents

Abstract

To detect if a video recording is an original recording or a copy, detector apparatus (2) is provided for connection to a conventional video cassette recorder (VCR) (4). The detector apparatus (2) compares the amplitude of a sample of the video signal, e.g. colour burst signals (CB) at the beginning or end of a field with a reference. If the recording is a copy made on a domestically available VCR, the level of those signals will be changed from those of the original by circuitry designed to cancel out chrominance crosstalk between tracks. Any such change is indicated at (6). <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO DETECTING COPIED VIDEO RECORDINGS The present invention relates to a method and apparatus for detecting copied video recordings, and to video recording and playback machines incorporating such apparatus.

Increasingly, video recordings, particularly those on video cassette tapes, are copied. Those hiring out pre-recorded video tapes, for example, do find that sometimes the recording brought back is a copy of the original rather than the original.

There is a need for a simple way of reliably detecting whether a tape is an original or a copy, and the present invention seeks to fulfil this requirement.

According to a first aspect of the present invention there is provided a method of detecting copied video recordings, the method comprising the steps of generating a video signal from a video recording, comparing the amplitude of a sample of the video signal with a predetermined reference level, and providing an output signal dependent upon said comparison.

The output signal may be provided only when the amplitude is below the predetermined reference level, only when the amplitude is above the predetermined reference level, or distinctive outputs may be generated in each instance.

The sample of the video signal whose amplitude is compared is a signal or signals whose amplitude iseffected by the playback or recording of a video signal by a playback or recording machine. In contrast to known methods, the invention utilises the video signals normally provided, and does not require special modifications to be made thereto for the purposes of detection.

Preferably the sample of the video signal is a colour related signal. In this respect, most conventional video cassette recorders (VCRs) average the colour signals to avoid chrominance crosstalk and to maintain colour quality.

Thus, the sample of the video signal is preferably the or a part of a colour related signal.

In one embodiment, the sample of the video signal is a sample of the chrominance signal, either base band or the colour-under signal.

In an alternative embodiment, the sample of the video signal is one or more of the colour burst signals.

Where the sample of the video signal is one or more colour burst signals, it is the amplitude of the colour burst signals at the beginning or the end of a field which is to be compared with a reference level.

For example, a predetermined number of colour burst signals at the beginning of a field, or at the beginning of each field, may be detected and compared with the reference level. In this instance, the comparison is seeking to identify the presence of colour burst signals of less than normal amplitude, the presence of such signals indicating the existence of a copied recording. However, most playback machines are provided with automatic chrominance controls arranged to increase the colour burst levels to a norm. Thus, the method further comprises the steps of compensating the amplitude of the colour burst signals and/or adjusting the reference level for any such increased gain before the comparison is made.

The invention also extends to apparatus for detecting copied video recordings, said apparatus comprising an input for receiving a video signal, means for comparing the amplitude of a sample of the video signal with a predetermined reference level, and a means for generating an output signal in dependence upon that comparison.

The apparatus may further comprise indicator means responsive to the output signal. In this respect, the indicator means may be arranged to indicate either that the sampled amplitude is below the reference level, or that the amplitude is above the reference level, and/or to provide distinctive indications of each condition.

In a preferred embodiment the apparatus comprises means to gate a part of the video signal, and filter means to remove unwanted frequencies from that gated part. The apparatus further comprises means to determine the amplitude of the gated and filtered part,and comparator means arranged to compare the amplitude determined with a predetermined reference.

In a preferred embodiment, the gating means are arranged to pass the colour burst signals at the beginning of a field, or the final colour burst signals at the end of a field. Preferably, sampling means are provided for detecting the amplitude of an individual colour burst signal and applying that amplitude to the comparator.

In a preferred embodiment, in which gating means gate initial colour burst signals, the amplitude determined is adjusted by compensation means before it is applied to said comparator means to be compared with the reference level.

For example, the compensation means may reduce the detected amplitude by a value related to the normal amplitude level of colour burst signals.

Additionally and/or alternatively, the reference level may be adjusted by compensation means before it is applied to said comparator means.

According to a further aspect of the present invention there is provided video playing apparatus in which detector apparatus as defined above is incorporated.

The detector apparatus may be connected to receive the video signal being played, and/or the detector apparatus may be arranged to receive the chrominance signal.

Embodiments of the present invention will hereinafter be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of detector apparatus of the invention for detecting copied video cassettes, Figure 2 shows part of four field signals of a PAL system video signal, Figures 3a to 3d show the colour burst signals of one PAL field signal, and of one PAL field signal of first, second and third generation playbacks, Figure 4 shows a block diagram of detector apparatus according to one embodiment of the invention, Figure 5 shows the end of one field signal of a PAL video signal, Figures 6a to 6d show the colour burst signals at the end of one PAL field, and at the end of a field of a first generation and subsequent playbacks, Figure 7 shows a detailed block diagram of an alternative embodiment of detector apparatus of the invention, Figure 8 shows a schematic block diagram of a video cassette player with incorporated detector apparatus, Figure 9a shows the recorded colour burst signal at the beginning of one field, Figures 9b to 9d show the first and subsequent generation colour bursts, prior to chrominance averaging, at the beginning of one field fed to the detector apparatus within the video cassette player of Figure 8, Figure 10a shows the colour burst signal at the end of one field, and Figures 10b to 10d show the first and subsequent generation colour burst, prior to chrominance averaging, at the end of one field fed to the detector apparatus within the video cassette player of Figure 8.

The present invention provides a reliable method of detecting if a video recording is an original recording or is a copy of an original recording. In one embodiment, as is illustrated in Figure 1, detector apparatus 2, is provided for connection to a conventional video cassette recorder (VCR) 4 or other playback device. An output of the detector apparatus 2 is fed to indicator means 6, arranged to provide an indication as to whether the recording being played is an original or a copy. For example, the indicator 6 may include illuminable means to indicate whether the recording is a first generation, or original recording, or is a second or multiple generation copy. In one embodiment,the indicator 6 may include a green LED which is illuminated when the recording is original and a red LED which is illuminated to signal a copy. The detector apparatus 2 may also have a second output 8 at which a control signal indicative of the generation of the recording detected is applied. Thiscontrol signal could be used to drive other devices as required. In this respect, the output of the detector apparatus 2 can be C-MOS or TTL compatible as required.

All video cassette recorders have slightly differing mechanical and/or electrical characteristics. In order to make a copy of a tape it is necessary to play the original on one machine so that it can be recorded by way of another machine. This means that the recordings do differ because of the differing characteristics of the machine and it has been proposed to look for switching points or timing differences thereby generated. However, this has not proved to be reliable.

Currently, all domestically available video cassette recorders use a delay and add method of chrominance signal averaging to cancel out chrominance crosstalk between adjacent video tracks. With PAL format recordings, there is a two line delay, whereas NTSC format recordings use a one line delay.

Figure 2 shows part of the four field signals of a video signal of the PAL format and shows the first colour burst signals in each field. Thus, Figure 2a is the first, even, field and shows the initial first and second colour burst signals CB(1) and CB(2). Similarly Figure 2b shows the second, odd, field and similiarly shows the initial colour burst signals CB. Figures 2c and 2d show the third and fourth fields.

Figure 3 shows the colour burst signals for one field only, and in particular shows their normalised amplitudes.

Figure 3a shows the recorded colour burst signals CB of an original recording and it will be seen that all of the colour burst signals CB are of the same amplitude. Figure 3b shows the colour burst signals of a first generation playback of the recorded colour burst signals CB of Figure 3a.

In this respect, when a conventional video cassette recorder plays back an original recording having the colour burst signals CB shown in Figure 3a, it averages the chrominance signals and the resultant colour burst amplitudes are shown in Figure 3b. Thus, if we consider the field shown in Figure 2a, the first colour burst signal CB(1) is averaged with the colour burst signal two lines previously. However, as signal CB(1) is the first colour burst in that field there is no colour burst signal two lines previously, and so CB(1) is averaged with zero. The resultant signal is a colour burst signal CB' which is half the amplitude of CB(1). Similarly, the colour burst signal CB(2) of the field shown in Figure 2a is averaged with the zero colour burst signal in the line preceding the line containing CB(1), and again the resultant is a colour burst signal CB' which is half the amplitude of CB(2). The following colour burst signals CB are averaged with colour burst signals, so that after the two reduced amplitude signals CB', the subsequent colour burst signals CB are of full amplitude.

We have seen above that when an original recording having the colour burst signals CB shown in Figure 3a is played back, the colour burst signals illustrated in Figure 3b result. Of course, the original recording may be played back in order to make a copy, and that second generation copy as recorded will include the colour burst signals as shown in Figure 3b. When the second generation copy including the colour burst signals of Figure 3b is subsequently played back, the colour burst signals shown in Figure 3c are obtained. Again, the first colour burst signals CB' are averaged with zero signals on the two preceding lines and hence quarter amplitude signals CB'' are generated. The next two colour burst signals CB are averaged with the signals CB' to provide three quarter amplitude signals Cub"'. Thereafter the colour bursts CB attain their normal amplitude. If the video signal having a characteristic as shown in Figure 3c is recorded, and that recording is played back, the colour burst signals will look as shown in Figure 3d in which the first two colour burst signals CB'V are now an eighth of the original amplitude. The next two colour burst signals CB' are half amplitude and are followed by two signals CBV' which are seven-eight amplitude signals.

By detecting the amplitude of the first and/or second colour bursts on any one, on all, or any combination of the four fields, it is clearly possible to detect if the video signal is coming from an original or a copied video recording.

The colour burst amplitudes have been shown in Figures 3a to 3d for the PAL format, but, amplitude reduction also occurs with the NTSC format. However, because NTSC averages the colour signal in one line with the immediately preceding line, the first of the first generation playback bursts would be half amplitude, but the second normal amplitude, and so on.

A circuit for a detector apparatus able to distinguish original recordings by detecting the amplitude of the colour burst signals is illustrated in Figure 4.

The circuit shown in Figure 4 has a video input 10 to which is supplied the video signal obtained by playing a recording. This signal is fed to a conventional sync separator 12 which strips the negative going sync pulses from the signal and applies them to the input of three pulse generators 14, 16 and 18. The pulse A generator 14 acts to gate the colour burst signals in the video signal.

In this respect, the pulse A generator is arranged to generate positive gating pulses of sufficient duration to enable the colour burst signals applied to an electrically operable switch 20 to pass. The pulse output of the pulse A generator may be, for example, a back porch sample pulse or a horizontal blanking pulse, which is passed to the switch 20 to periodically enable it to pass the colour burst signals, together with DC and low frequency signals to a band pass filter 22. The band pass filter 22 has a pass band centred at the colour subcarrier frequency so that its output is substantially the colour burst signal.

Thus the pass band centre frequency of the filter 22 would be 4.43MHz for PAL and 3.58MHz for NTSC.

The colour burst signals output by the filter 22 are then fed to an envelope detector 24 whose output signal is the envelopes of the filtered bursts. These resulting envelope shaped pulses are then fed to an automatic gain control (AGC) amplifier 26 arranged to automatically control the amplitude of the envelopes. In this respect, the time constant of the AGC amplifier 26 is sufficiently long so as not to control the amplitude of the individual burst envelopes, but is short enough to maintain a substantially constant burst amplitude over a number of TV fields.

The amplitude controlled burst envelopes output from the amplifier 26 are fed to a second electrically operated switch 28 which is switched by the output of the pulse B generator 16. In this respect, the pulse B generator 16 is arranged to output a line sequence gating pulse of sufficient period to let through a predetermined number of burst envelopes at the beginning of each field. For example, the first twenty colour burst signals could be input to a sample and hold circuit 30 receiving the output of the switch 28. The same burst envelopes are also fed to a peak detector circuit 32 by way of the switch 28.

The pulse C generator 18 is arranged to generate a single pulse of restricted duration for each field enabling a selected burst to be sampled. Thus, the sample and hold circuit 30 samples and holds individual burst envelopes, and holds the sampled envelope until the next sample pulse arrives. The amplitude of the sampled burst envelope determined by the sample and hold circuit 30 is fed to one input of a comparator circuit 34. The amplitude of this selected burst envelope is then compared by the comparator 34 with a compensated reference signal applied to its second input.

It will be appreciated that the signals shown in Figure 3 are idealized. In practice, the off-tape colour bursts will be rounded. More importantly, the amplitude of the initial off-tape colour bursts will vary in dependence upon the automatic chrominance control (ACC) response in the recording and playback circuits of the VCRs used.

When faced with low amplitude colour burst signals, a normal VCR will respond by increasing their amplitude.

However, because there is some delay in its reaction, when it receives a signal as shown in Figure 3b, for example, the amplitude of the first two colour burst signals CB' may not be increased at all, but the following ones CB may be increased quite significantly. Thereafter, the gain controls will develop an even mid-field level. Because of this, it is not possible simply to look at the amplitude of the first colour burst signals and reliably establish whether or not their amplitude level signifies an original recording or a copy. Compensation for the effect of the automatic chrominance controls therefore has to be provided.

In this respect, we have already seen that the time constant of the AGC amplifier 26 is sufficiently long so as not to control the amplitude of the individual burst envelopes. It is also important that the time constant of the AGC amplifier is sufficiently long that it prevents significant gain adjustment over the vertical interval.

Thus, consider that the time constant is less than the period of the vertical interval such that during the vertical interval the AGC amplifier receives no colour burst signals. The AGC amplifier would respond to this by increasing its gain to, or near to, a maximum, and hence the first colour burst signal or signals to arrive would have their amplitude increased significantly. This is prevented by appropriate choice of the length of the time constant of the AGC amplifier 26.

We have seen above that a sequence of colour burst envelopes is selected from the beginning of each field, and that each said sequence is fed to the peak detector circuit 32. To compensate for the effect of the automatic chrominance controls the output of the peak detector circuit 32 is fed to a differential amplifier 36 where it has a reference peak threshold voltage deducted from it.

In this respect, the output of the peak detector circuit is a voltage equal to the maximum amplitude of the colour burst envelopes in a sequence. Generally, the reference voltage would be representative of, or equal to the peak amplitude of a standard sequence of colour burst envelopes produced by amplifying reference colour burst signals by the automatic chrominance control of the video recorder being used to reproduce the video signal.

The peak compensated voltage output from the differential amplifier 36 is then fed to a summing amplifier 38 where it is summed with a nominal decision voltage. This nominal decision voltage is predetermined to set the decision line between an original or a copy recording. For example, the nominal decision may be set at three eighths of the voltage resulting from a full amplitude colour burst (CB). The output from the differential amplifier 36 will generally be a positive voltage where an increased amplitude colour burst is input thereto such that the summing amplifier 38 increases the nominal decision voltage by the voltage output by amplifier 36. Thus, the comparator 34 compares the actual amplitude of a colour burst with the nominal decision voltage increased by the peak compensated detector level. If the detected amplitude is less than the comparison voltage a signal indicating a second or multiple copy can be provided. Generally, the output of the comparator 34 will be low for a first generation, or original, recording and high for a second or multiple generation copy. Of course, an inverting amplifier could be used if this logic is required to be reversed. The output of the comparator 34 is then fed to the indicator 6 and to the output terminal 8 as previously discussed.

The nominal decision voltage fed to the summing amplifier 38 and the peak threshold voltage fed to the differential amplifier 36 could be hard wired if required.

However, for reliability it is preferred that the detector apparatus can be made responsive to the particular VCR being used. Accordingly, means could be provided to input a predetermined or preselected nominal decision voltage and peak threshold voltage. Additionally and/or alternatively, a reference video tape could be provided for circuit set up.

We have seen that the gain control, that is, the automatic chrominance control, of a VCR increases the gain when the colour information is low, and therefore tends to counteract the averaging effect, which is being utilized to distinguish copies. Nevertheless, with a compensating circuit as illustrated in Figure 4, the detector apparatus described has been found to be extremely effective and the signals output by way of the indicator 6 and at the output 8 have been found to be very accurate.

The problem of the VCR ACC counteracting the averaging effect does not appear if the end of each field is considered. Figure 5 shows the signals at the end of the field 1 of a video signal showing the last colour burst in line 309 and then the absence of colour bursts at the end of field 1 and the start of field 2.

Figure 6 shows the colour burst signals on various generation recordings of each video signal. Figure 6a illustrates the original recording showing the colour burst signals CB of full amplitude. Figure 6b shows the result of the first playback, showing the production of half amplitude additional colour burst signals CBa on lines 310 and 311. Figure 6c shows the playback where the signal of Figure 6b had been recorded showing that colour burst signals CBb derived from the signals CBa have a three quarter amplitude, but that quarter amplitude colour burst signals CEc have now appeared on lines 312 and 313. Figure 6d shows the third generation playback and shows the appearance of one-eighth amplitude colour burst signals CBd on lines 314 and 315.

Figure 7 shows an alternative embodiment of a detector apparatus arranged to detect the amplitude of the colour burst signals at the end of each field. It will be seen that this apparatus is basically identical to that of Figure 4 except that the compensating circuits have been omitted. In Figure 7 components which are the same or similar to those of the apparatus of Figure 4 have been given the same reference numerals. Thus, in the circuit of Figure 7 the sample and hold circuit 30 establishes the amplitude of a single burst envelope and this is fed to the comparator 34 for comparison with a decision voltage.

Again the decision can be preset or determined by the use of a reference tape or the like.

In the embodiments described above, the colour burst signals themselves have been detected and compared with reference amplitudes. Of course, it will be appreciated that any part of the chrominance signal which is subject to averaging can be used in this manner. For example, it would be possible to detect chrominance in the normally "quiet" lines at the end of the field and into the start of the subsequent field.

The detector apparatus described above is a stand alone unit. It is envisaged that it will be plugged into a conventional video cassette recorder to provide an indication as to the origin of the tape being played. Such a unit will be of interest, for example to video tape hire shops so that they can quickly determine that original tapes have been returned, and not copies. However, detector apparatus for use in such a situation should be as simple as possible to use and require the minimum amount of setting up.

An alternative to the stand alone unit of Figure 1 is illustrated in Figure 8 which shows a dedicated detector apparatus 120 incorporated within a video cassette player 140. The video cassette player 140 may be a playback device only or could be a conventional video cassette recorder.

Using a dedicated detector 120 has the advantage that the nominal decision voltage and the peak threshold voltage can be preset to avoid the possibility of incorrect calibration. A dedicated detector apparatus also has the advantage that additional approaches to the detector method can be employed.

Where the detector apparatus is contained within the video cassette recorder 140 it is easy to gain access to the chrominance signal before it is mixed with the luminance signal, and it is this which can be input to the detector apparatus 120. Depending upon the tape format and the circuit design, the detector apparatus can analyse either the base band (4.43MHz or 3.58MHz) or the colour-under (usually between 600 and 750 KHz) chrominance signal.

It would also be possible to measure the chrominance signal before the chrominance averaging circuitry of the dedicated VCR 140. In this, the detector relies on the fact that second or multiple generation recordings will have previously undergone chrominance averaging.

Figure 9a shows the colour burst signals of uniform amplitude recorded as a first generation recording. If detection is made before averaging, the detector apparatus would see the playback colour burst signals of Figure 9b.

Figures 9c and 9d show the second and third generation playback burst amplitudes before chrominance averaging. As previously, colour burst amplitudes are compared with the reference level to determine if an original recording is being played. As previously, Figures 9a to 9d are idealized colour burst signals.

Figures 10a to lOd show the colour burst signals where the detector apparatus is looking at the end of the field but again before averaging has taken place. Figure l0a is the recorded colour burst at the end of a field, and Figure lOb shows the first generation playback. Figures lOc and lOd show that any appearance of colour burst on lines 310 and 311 will indicate a second or multiple generation recording.

All of the techniques and circuits described above can be used with either the PAL system or the NTSC system.

It will be appreciated that other modifications and variations to the invention as described above may be made within the scope of this application.

Claims (23)

1. A method of detecting copied video recordings, the method comprising the steps of generating a video signal from a video recording, comparing the amplitude of a sample of the video signal with a predetermined reference level, and providing an output signal dependent upon said comparison.

2. A method as claimed in Claim 1, wherein said output signal is provided only when the amplitude is below the predetermined reference level.

3. A method as claimed in Claim 1, wherein said output signal is provided only when the amplitude is above the predetermined reference level.

4. A method as claimed in Claim 1, wherein a first output signal is provided when the amplitude is below the predetermined reference level, and wherein a second, distinctive, output signal is provided when the amplitude is above the predetermined reference level.

5. A method as claimed in any preceding claim, wherein the sample of the video signal whose amplitude is compared is a signal or signals whose amplitude is effected by the playback or recording of a video signal by a playback or recording machine.

6. A method as claimed in any preceding claim, wherein the sample of the video signal is the or a part of a colour related signal.

7. A method as claimed in Claim 6, wherein the sample of the video signal is a sample of the chrominance signal, either base band or the colour-under signal.

8. A method as claimed in Claim 6, wherein the sample of the video signal is one or more of the colour burst signals.

9. A method as claimed in Claim 8, wherein the amplitude of the colour burst signals at the beginning or the end of a field is compared with a reference level.

10. A method as claimed in Claim 9, further comprising the steps of compensating the amplitude of the colour burst signals and/or adjusting the reference level for any increased gain before the comparison is made.

11. Apparatus for detecting copied video recordings, said apparatus comprising an input for receiving a video signal, means for comparing the amplitude of a sample of the video signal with a predetermined reference level, and a means for generating an output signal in dependence upon that comparison.

12. Apparatus as claimed in Claim 11, further comprising indicator means responsive to the output signal.

13. Apparatus as claimed in Claim 12, wherein said indicator means is arranged to indicate either that the sampled amplitude is below the reference level, or that the amplitude is above the reference level, and/or to provide distinctive indications of each condition.

14. Apparatus as claimed in any of Claims 11 to 13, further comprising means to gate a part of the video signal, and filter means to remove unwanted frequencies from that gated part.

15. Apparatus as claimed in Claim 14, further comprising means to determine the amplitude of the gated and filtered part, and comparator means arranged to compare the amplitude determined with a predetermined reference.

16. Apparatus as claimed in Claim 15, wherein said gating means are arranged to pass the colour burst signals at the beginning of a field, or the final colour burst signals at the end of a field.

17. Apparatus as claimed in Claim 16, wherein sampling means are provided for detecting the amplitude of an individual colour burst signal and applying that amplitude to the comparator means.

18. Apparatus as claimed in any of Claims 15 to 17, in which said gating means gate initial colour burst signals, wherein the amplitude determined is adjusted by compensation means before it is applied to said comparator means to be compared with the reference level.

19. Apparatus as claimed in any of claims 15 to 18, wherein the reference level is adjusted by compensation means before it is applied to said comparator means.

20. Video playing apparatus in which detector apparatus as claimed in any of Claims 11 to 19 is incorporated.

21. A method of detecting copied video recordings substantially as hereinbefore described with reference to the accompanying drawings.

22. Apparatus for detecting copied video recordings substantially as hereinbefore described with reference to the accompanying drawings.

23. Video playing apparatus substantially as hereinbefore described with reference to the accompanying drawings.