Method and apparatus for preventing the copying of a video program

This invention relates to a method and apparatus for prohibiting the recording of video signals.

Various apparatus and methods have been developed for modifying a video signal so that a normal colour picture may be produced by a television receiver receiving the modified video signal, but that video recording of the modified video signal is impaired. That is, a video picture produced from the video signal as recorded on a video tape recorder is altered so that it makes unacceptable viewing.

United States patent No. 4,631,603 to John O. Ryan issued December 23, 1986, entitled "Method and Apparatus for Processing a Video Signal so as to Prohibit the Making of Acceptable Video Tape Recordings Thereof", discloses such a method and apparatus. It describes adding ordered pairs of pseudo-sync pulses and white pulses to the back porch of a blanking interval. These pulse pairs act in concert to confuse the automatic gain control system of a video cassette recorder (VCR), leading to generally unviewable pictures.

US-A-4,163,253 (Morio) also discloses a method and apparatus for altering the automatic gain control of a VCR. More particularly, this US specification discloses a method of recording video signals arranged such that unmodified video signals are recorded, whereas recording of modified video signals is inhibited, the modified video signals having a selected waveform added to a blanking interval, the method comprising the steps of receiving video signals in a video signal recording device, and detecting the presence of said added selected waveform to inhibit recording.

GB-A-2,104,701 (King) discloses a similar method in which the gain of the recorder is interfered with in response to the receipt of a modified signal. This specification additionally suggests the addition of spoiler signals to the output.

US-A-3 963 865 discloses a method of modifying the vertical sync pulses such that upon playback of an unauthorized copy on a receiver the video image is eliminated or substantially distorted.

There are problems with the approach adopted by all of the identified prior art. Thus, the prior art systems seek to inhibit recording, that is, to impair the quality of the recording made of a modified video signal. However, in some instances, the impairment may be so small as to effectively provide no protection against improper copying. Furthermore, with the prior art proposals, the modified signals present on original recordings cause adjustment of the gain of the playback machine and may therefore provide problems playing the original material.

As VCR's become more sophisticated, for example, including dubbing (separate playback and recording) systems, the likelihood that video tapes will be copied increases.

The present invention seeks to provide method and apparatus for recording video signals in which recording of modified signals is inhibited. but which reduces the problems identified above.

According to the present invention a method of recording video signals as defined above is characterised in that said selected waveform comprises at least one pseudo-sync pulse extending from the blanking reference level to the sync tip level added to the vertical blanking interval of the unmodified video signal following a sync pulse, and in that the recording is disabled in response to said detection of said added selected waveform.

With a method of the invention, recording of modified video signals is not simply impaired as previously, but rather is disabled.

Thus, when modified video signals are detected, the recording device is disabled. Thus, when video tapes are originally recorded with a recognizable, copy-protecting modified waveform, the tape recording device may be completely disabled so that it cannot record the video signal at all.

In a preferred embodiment. the recording is disabled by generating a control signal indicating the presence of the added selected waveform in the received video signal, and preventing the recording device from recording the received video signal when the control signal indicates the presence of the selected waveform in the video signal.

Preferably, said method further comprises generating a control signal indicating the absence of the selected waveform in the video signal, and enabling the recording device to record the video signal when the control signal indicates the absence of the selected waveform in the video signal.

Preferably, said selected waveform comprises at least one selected pulse added to the vertical blanking interval of the video signal following a sync pulse.

In an embodiment, said selected waveform comprises a plurality of pulses added following the sync pulse, and wherein the presence of said selected waveform is detected by generating a signal having a voltage proportional to the frequency of the plurality of pulses following the sync pulse, sampling the generated signal when it has a voltage proportional to the selected-pulse frequency, and holding the value of the sample for a minimum predetermined period of time.

In an alternative embodiment, said selected waveform comprises a pseudo-sync pulse after the sync pulse and a positive pulse after each pseudo-sync pulse and within the blanking interval such that the amplitude differential between each pseudo-sync pulse and associated positive pulse is different to the normal amplitude differential between the sync pulse and the blanking interval following the sync pulse, and wherein the presence of said selected waveform is detected by detecting the amplitude differential between each pseudo-sync pulse and the associated positive pulse.

The invention also extends to tape recording apparatus for recording video signals, the apparatus being arranged such that unmodified video signals are recorded, whereas satisfactory recording of modified signals is inhibited, the modified video signals having a selected waveform added to a blanking interval, the apparatus comprising input means for receiving a video signal, recording means coupled to said input means for recording video signals on the recording medium, and means arranged to detect the presence of said added selected waveform in a received video signal and in response to inhibit recording of the modified video signal, characterised in that the selected waveform comprises at least one pseudo-sync pulse extending from the blanking reference level to the sync tip level added to the vertical blanking interval of the unmodified video signal, and in that said detecting means comprises disabling means coupled to said input means and to said recording means and arranged to disable said recording means in response to the detection of said added selected waveform.

Preferably, said disabling means is arranged to generate a control signal for disabling said recording means.

In an embodiment, arranged to prohibit recording of modified video signals having a selected waveform which comprises at least one pair of a pseudo-sync pulse and a positive pulse, the pseudo-sync pulse following the sync pulse and the positive pulse following the pseudo-sync pulse, the difference in amplitude between the pseudo-sync pulse and the positive pulse being different to the normal amplitude difference between the sync pulse and the blanking interval, said disabling means comprises means for detecting the amplitude difference between each pseudo-sync pulse and the associated positive pulse.

For example, said disabling means may comprise means for holding a voltage representative of the amplitude difference between each pseudo-sync pulse and the associated positive pulse a minimum predetermined period of time.

In an altemative embodiment, arranged to prohibit recording of modified video signals having a selected waveform which comprises a plurality of pseudo-sync pulses added after the sync pulse, said disabling means comprises means for detecting the frequency of the plurality of pseudo-sync pulses.

Said apparatus may further comprise means for generating a voltage representative of the frequency of the pseudo-sync pulses, and means for holding the voltage representative of the pseudo-sync pulse frequency a minimum predetermined period of time.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which:- <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 shows a simplified block diagram for preventing recording of a video signal comprising apparatus of the present invention,</li><li>Figure 2 shows a more detailed block diagram of a recording disabling apparatus usable in the system of Figure 1,</li><li>Figure 3 shows illustrations of waveforms present at selected points in the exemplary apparatus of Figure 2,</li><li>Figure 4 is an illustration of one of the waveforms shown of Figure 3 with an extended time scale, and</li><li>Figures 5 and 6 show block diagrams of alternative embodiments of a recording disabling apparatus usable in the system of Figure 1.</li></ul>

Referring initially to Figure 1, a system for preventing the copying of a video program is shown generally at 10. Initially, a conventional video signal is input to a video signal modifier 12. The modifier 12 is preferably the apparatus described in previously referenced United States Patent No. 4,631,603. The modifier adds a selected waveform to a conventional video signal which it is desired not be copied. The modified video signal is conveyed to a video tape recorder or more commonly, a video cassette recorder (VCR) 14 for recording on a magnetic tape 16 or other suitable recording medium. The video signal may be conveyed from modifier 12 by recording the modified signal on a video tape, such as a video cassette, which is played into VCR 14. Alternatively, it may be broadcast over a cable or other broadcast system and received at a tuner in the VCR.

Conventional video cassette recorders include an input 18 which may include a tuner, for receiving input video signals and a recording device 20 coupled to input 18 for recording the received video signal on tape 16. In the VCR of the present invention, what is referred to as a disabler 22 is also included. Disabler 22 receives the video signal from input 18 on a lead 24, detects whether the video signal is a modified signal and outputs a corresponding control signal on a lead 26 coupled to recording device 20. The recording device is prohibited from recording when the modified video signal is detected. Otherwise, a signal is output on lead 26 which enables recorder 20 to record the video signal.

Fig. 2 illustrates a preferred embodiment of disabler 22. However, before describing the embodiment shown in Fig. 2, it is important to understand the form of the modified video signal detected by the preferred embodiment of disabler 22. This copy-protecting signal, is one which is produced by the apparatus described in U.S. Patent 4,631,603. That system addes a plurality of ordered pairs of what are referred to as pseudo-sync pulses and white pulses to selected lines of the vertical blanking interval after the normal sync pulses. The top waveform of Fig. 3 illustrates a modified video signal having a set of five ordered pairs of pseudo-sync pulses 28 and white pulses 30 added to a line of the vertical blanking interval after a normal sync pulse 32. Generally, it is preferred that seven or more lines are modified to provide a total of about 35 or more pulse pairs per field. The pseudo-sync pulses 28 are so named because they extend from the blanking reference level to the sync tip level as do the normal sync pulses 32. They preferably have a duration of about two microseconds and are immediately followed by the white pulses 30, so named because they typically reach peak white level or beyond. The white pulses have a duration of about four microseconds. Input 18 normalizes the input video signal so that it has a 1 volt peak-to-peak amplitude. The back porch of the normalized sync pulse is 0.3 volts above sync tip. The back porches of the pseudo-sync pulses, as determined by the white pulses, are about 1 volt above sync tip.

There is thus provided a waveform which is distinctive of the conventional vertical blanking interval. It is distinctive, both as to the voltage differential between the sync tip and the peak white level as well as the frequency of the pulses. It will be appreciated that variations can be made in these waveforms or that other identifiable waveforms may be used.

Referring to Fig. 2, the input video signal on lead 24 is input to a sync tip restorer 34 which clamps the sync tips to a reference level, preferably 0 volts. The output of the sync tip restorer has the same waveform as does the output of modifier 12. The output waveform of the various circuits shown in Fig. 2 are illustrated in Fig. 3. The output of restorer 34 is connected to a sync separator 36 which produces an output of sync pulses. These sync pulses are fed to a monostable multivibrator 38. The multivibrator is triggered by the trailing edges of the input sync pulses to generate nominally three microsecond sampling pulses. These pulses activate a sample and hold circuit 40 which receives the output of sync tip restorer 34. Thus, sample and hold circuit 40 generates an output which represents the voltage level of the white pulses in the modified video signal and holds it for at least the duration of one line (63.55 microseconds) so that the pseudo-sync and white pulse pairs may be detected in the following line and a continuous voltage level held by circuit 40. The resultant output of sample and hold circuit 40 is shown more clearly in Fig. 4 which shows it having a level of 0.3 volts during normal video horizontal lines and a level of 1 volt for those lines in the vertical blanking interval having the modified signal.

The output of sample and hold circuit 40 is applied to the positive input of a positive peak detector 42 which holds its most positive value for the duration of several fields. This assures that the output will have a value throughout the receipt of the modified video signal so that no part of the video signal can be recorded. The output of peak detector 42 is thus a constant voltage of about +0.3 volts in the case of a normal video signal and about +1.0 volt in the case of a copy-protecting modified video signal in the embodiment illustrated herein.

The output of detector 42 is joined to the positive input of a differential comparator 44. The negative input of comparator 44 is preferably connected to a positive 0.5 volt reference voltage source. Comparator 44 thereby produces an output on lead 26 having a logic level which is high in the case of a modified or copy-protected video signal and low in the case of a normal video signal. As described previously with reference to Fig. 1, the logic signal at the output of comparator 44 is coupled to the record enable system of the VCR in such a manner as to prohibit or disable recording when in the high state, and enable recording when in the low state. In its simplest form, such an enable system is in the form of a switch connected in line with the recording device.

Each of the circuit blocks of Figs. 1 and 2 other than the modifier, which is described in U.S. Patent 4,631,603, are well known to persons of ordinary skill in the art and do not form an aspect of the present invention, other than in combination with the other circuits as described. Therefore, they are not discussed in further detail.

Referring now to Fig. 5, an alternative embodiment of disabler 22, shown as 22', is illustrated. The method of this embodiment relies on detecting the presence of the tightly grouped pseudo-sync pulses. Thus, although it is operable on the modified video signal waveform shown in Fig. 3, it will be appreciated that the white pulses are not utilized specifically, and therefore could be deleted. Further, other forms of waveform could be used which also produce a frequency distinguishable from the frequency of normal sync pulses.

In particular, the video signal received on lead 24 is input to a sync separator 46 which produces an output similar to that of sync separator 36. The resulting series of pulses is input to a frequency-to-voltage convertor 48. Convertor 48 generates an output voltage proportional to the frequency of its input pulses. The output of convertor 48 will thus have the general characteristics of the waveform of Fig. 4 in that it will be most positive during the vertical interval region corresponding to the increased sync frequency. The output of convertor 48 is coupled to a positive peak detector 42' and differential comparator 44' in the same arrangement as peak detector 42 and comparator 44 of Fig. 2. This gives rise to the same logic level indication of the presence or absence of the modified or copy-protected video signal as was discussed with reference to the embodiment of Fig. 2.

Fig. 6 shows yet a third exemplary embodiment of a disabler, shown as 22". Disabler 22" functions similar to disabler 22 in that it detects the presence of the pseudo-sync and white pulse pairs by determining the voltage differential between each pseudo-sync pulse and paired white pulse.

Specifically, the video signal is input from lead 24 to a sync tip restorer 50, the output of which is applied to a positive peak detector 52 having a fast charge time constant (less than 1 microsecond) and a long discharge time (about 100 microseconds). The output of sync tip restorer 50 is also connected to a sync separator 54 for generating an output waveform similar to that of sync separator 36 described with reference to Figs. 2 and 3. The output of separator 54 is coupled to a vertical sample pulse generator 56. Generator 56 produces a series of sampling pulses for activating a sample and hold circuit 58. Circuit 58 receives as input the output of peak detector 52. Thus, the output of the positive peak detector is sampled in sample and hold circuit 58 during the vertical interval at a time which coincides with the occurrence of the anti-copying waveform.

It can be seen that the output of circuit 58 will be at +0.3 volts in the case of a normal video signal and at about +1.0 volt in the case of a video signal modified or copy-protected as described herein. The output of sample and hold circuit 58 is converted to logic levels as described with reference to the embodiment of Fig. 2 by a differential comparator 44".

It can be seen that the outputs of all three embodiments of the disabler illustrated in Figs. 2, 5 and 6 correspond to those of the waveform shown in Fig. 3 for differential comparator 44. A system is thus described wherein a video signal, modified as shown in United States Patent No. 4,631,603 is positively prohibited from being recorded. By incorporating detection circuitry or disablers in commercial VCRs, particularly in more sophisticated recorders which include dubbing decks, it will be possible for recorded video tape producers to produce copy-protected video signals which may be viewed without impairment on a television receiver but which cannot be copied on the video recorder.

The foregoing describes a method for copy protecting a video signal. With this method, a television receiver produces a normal color picture from the modified signal whereas a video tape recorder having a recording disabling circuit according to the present invention prevents the recording of the modified video signal. Many different forms of video-signal-modifying waveforms and disabler circuits are possible for practicing the method of this invention. The particular circuitry shown is by way of example only. All such modifications and variations which would occur to one of the ordinary skill in the art are included within the scope of the present invention as defined in the attached claims.