Method of controlling a video capture device and video capture device

METHOD OF CONTROL OF A VIDEO ACQUISITION DEVICE AND VIDEO ACQUISITION DEVICE

The invention relates to a method for controlling a video acquisition device and a video acquisition device implementing this method.

Visual content, whether still or moving images, are generally creations that benefit from exclusivity guarantees linked to copyright. Their reproduction is generally authorized only within a strictly defined framework which allows the remuneration of the authors and their beneficiaries.

In order to ensure that these legal rules are correctly observed, many systems have been developed to prevent illegal copies or to deteriorate their quality sufficiently to render them unusable. Methods have in particular been developed to combat the copying of images by shooting during their display, for example by a camcorder in a cinema room. Such a method consists for example in modulating the intensity of the pixels of a pattern around the value to be displayed with a high frequency which makes the pattern invisible to the human eye but which generates artefacts on the sequence filmed by the camcorder. All these methods are commonly implemented in the image display device, for example in the projector of the cinema room.

The present invention proposes a method of combating the copying of images by shooting during their display which is implemented, not in the device for displaying the images, but in the device for acquiring the images. According to the invention, when the image acquisition device detects that the scene being acquired is a film, that is to say a scene having periodic content with a frequency close to 24 or 25 images per second (or a multiple of this frequency) for cinema or a frequency close to 50 or 60 images per second (or a multiple of this frequency) for television, correct acquisition of the images is prevented.

The invention relates more particularly to a method for controlling a video acquisition device filming a scene, comprising the following steps:

- if the brightness of the filmed scene varies periodically, measure the frequency corresponding to this periodic variation,

- compare the measured frequency with a determined frequency or a set of predetermined frequencies; and

- if the measured frequency is equal to one or one of the determined frequencies, modify the operation of the video acquisition device.

The operation of the video acquisition device is modified for example by blocking the generation of video images by the video acquisition device or by encrypting the images generated. In the latter case, the images are then rendered unusable for the user of the device.

The video acquisition device can be a camcorder, a digital camera or any device capable of filming video scenes.

In our application, the frequencies for which the acquisition is disturbed are, in particular:

- frequencies between 23.5 Hz and 24.5 Hz, and the corresponding multiple frequencies, to prevent the acquisition of films projected with a cinema projector operating at 24 frames per second; and or

- frequencies between 24.5 Hz and 25.5 Hz, and the corresponding multiple frequencies, to prevent the acquisition of films projected with a cinema projector operating at 25 images per second; and or

- frequencies between 49 Hz and 51 Hz, and the corresponding multiple frequencies, to prevent the acquisition of films broadcast on televisions using image frequencies close to 50 Hz; and or

- frequencies between 59 Hz and 61 Hz, and the corresponding multiple frequencies, to prevent the acquisition of films broadcast on televisions using image frequencies close to 60Hz.

The invention also relates to a video acquisition device implementing this method.

More particularly, it relates to a video acquisition device capable of filming a scene, comprising

- an image sensor for capturing video images of said scene,

- control means for controlling video acquisition,

- measuring means for measuring, when the brightness of the filmed scene varies periodically, the frequency corresponding to said periodic variation, and

a comparator for comparing the measured frequency with at least one determined frequency and sending, if the measured frequency is a determined frequency, a signal to said control means to modify the operation of the device.

The invention will be better understood on reading the description which will follow, given by way of non-limiting example, and with reference to the appended drawings, among which:

- Figure 1 illustrates the different steps of the method of the invention,

FIG. 2 is a diagram of a device capable of implementing the method of the invention;

FIG. 3 illustrates a first position of the light sensor of the device of FIG. 2 in the plane of the image sensor of the camcorder;

- Figure 4 illustrates a second position of the light sensor of the device of Figure 2 on the front face of a camcorder;

- Figure 5 is a diagram of a camcorder implementing the method of the invention, and

- Figure 6 is the diagram of another camcorder implementing the method of the invention.

In the remainder of the description, the invention is described in the context of video acquisition by a camcorder.

FIG. 1 represents the different steps of the method of the invention.

According to a first step 100, if the brightness of the scene filmed by the camcorder varies periodically, the frequency corresponding to this periodic variation is measured. Such periodic variation exists when filming the screen of a cinema while showing a movie or the screen of a television set while playing a movie.

According to a second step 110, the measured frequency is compared with a determined frequency or a set of determined frequencies. These determined frequencies are frequencies which have been previously declared illegal or prohibited for video acquisition. These illegal frequencies are preferably, for the cinema, the frequencies between 23.5 Hz and 24.5 Hz and their multiples, or the frequencies between 24.5 Hz and 25.5 Hz, and their multiples. Indeed, the display frequency for analog cinema is 24 (or in some infrequent cases 25) images per second with a shutter operating at 48 (respectively 50) images per second. For digital cinema, it is 48 with frame refresh at 24.

If one also wishes to prevent the acquisition of images broadcast by television-type display systems, the frequencies to be prohibited are 49 Hz to 51 Hz and their multiples for countries based on frequencies close to 50 Hz and, frequencies between 59 Hz and 61 Hz and their multiples for countries based on frequencies close to 60Hz. These frequencies are those which are conventionally used for film projection in cinemas and for SD or HD television.

According to a third step 120, if the detected frequency is one of the so-called illegal frequencies, the operation of the camcorder is disturbed, for example by blocking the video acquisition, by encrypting the images produced by the camcorder or by embedding in the images acquired. a warning message such as copy prohibited or illegal copy.

FIG. 2 represents a device 200 capable of implementing the method of the invention.

A light sensor 210, such as for example a photodiode, captures the light of the scene filmed by the camcorder and generates an analog signal representative of the presence or absence of light in the scene.

When the sensor 210 detects the light present on a cinema screen during the projection of a film, it generates a rectangular signal whose pulses correspond to the light of each of the images of the film and the valleys correspond to the periods of suppression between 2 images. (intervals during which the shutter of the cinema projector is in the closed position or blanking intervals for televisions).

A conditioning circuit 220, such as a hysteresis circuit, is advantageously provided at the output of the sensor 210 to accentuate, if necessary, the slope of the edges of the signal supplied by the sensor 210 and to obtain a rectangular signal with well edges. marked which is easier to use by the rest of the device.

The frequency of this rectangular signal is then measured by a circuit 230. This circuit is synchronized on a clock signal H. Circuit 230 is typically a counter which counts the number of pulses of the clock signal H between 2 rising edges (or descendants) of the signal supplied by the sensor 210 or, as the case may be, by the conditioning 220. The clock signal H can be supplied by a clock circuit specific to said device, by a clock circuit already present in the camcorder or by the camcorder control circuit. It can also be derived from a clock signal already used in the camcorder.

A circuit 240 then compares this number of pulses to reference numbers corresponding to frequencies of 24, 25 or 30 pulses per second (equivalent to frequencies of 24, 25 or 30 frames per second) or to a multiple thereof. ci (48, 50, 60, 72, 75 ...). As a precaution, we widen this frequency interval to all the frequencies between 23.5 Hz and 24.5 Hz (respectively, ...) and their multiples. If this number of pulses corresponds to one of the reference numbers, operation of the camcorder is blocked or the video images it generates are encrypted. To this end, a signal is transmitted to the control circuit of the camcorder.

The light sensor 210 can be placed either near the image sensor (CCD or CMOS) of the camcorder or elsewhere on the front of the camcorder.

FIG. 3 illustrates the case where the light sensor is positioned near the image sensor of the camcorder. It can be placed next to the image sensor in the same plane or at any other place where the light optical signal can be observed directly or indirectly. For example, it can be placed in the optical chamber of the camcorder which already contains the image sensor. In this configuration, it is very difficult to mask the light sensor 1 to prevent it from detecting the light of the filmed images. If the fraudster wishes to do so, he must disassemble the camcorder. The manufacturer can advantageously ensure that access to the sensor chamber is impossible or that it cannot be done without damage. Furthermore, the signals exchanged between the different elements of the device of FIG. 2 are advantageously made inaccessible by the use of integrated circuits, of routing of tracks in internal layers of the printed circuits or of the use of multiple and redundant electrical paths. to prevent the potential fraudster from deleting or modifying them. The presence of these signals may also be made necessary for the proper functioning of the camcorder in order, in the event of deletion, to block the camcorder.

FIG. 4 illustrates the case where the light sensor is not positioned near the image sensor of the camcorder. This configuration has the advantage that it allows the use of a light sensor already present in the camcorder, for example the ambient light sensor of the camcorder. The sensor may be easier to hide, but this may interfere with the operation of the camcorder (detection of ambient light inoperative).

In the example of Figure 2, the light sensor 210 is a photodiode. It is also possible to envisage using a resistor whose ohmic value varies as a function of the light present, for example a cadmium sulphide resistor. This type of resistor is conventionally used in a camcorder to detect whether the presence of a flash is necessary for video acquisition. However, this type of sensor has a response time of between 3 and 20 ms depending on the light conditions. It is therefore recommended instead to use a photodiode as a light sensor.

FIG. 5 illustrates a possible implementation of the device of FIG. 2 in a conventional camcorder. The camcorder 400 comprises a CCD or CMOS sensor 410, a circuit 420 for applying an analog processing (filtering, amplification, etc.) to the signal delivered by the image sensor 410, an analog / digital converter 430 to transform the analog signal as a digital signal and a digital video processing and control circuit 440 for processing the video data of the signals picked up and generating signals such as for example the control of the autofocus, the camcorder's shutter opening time control or the video data recording control in a camcorder's memory. According to the invention, the camcorder is completed by the device 200 of FIG. 2. The signal delivered by the frequency comparator 240 is transmitted to the circuit 440 to block the operation of the camcorder.

FIG. 6 illustrates another implantation of the device 200 of FIG. 2 in a camcorder, no longer requiring the presence of a photodiode. This particular implementation concerns camcorders with a variable scan function (variable scan in English). This function allows the user to synchronize the video acquisition to the frequency of the film being projected. This variable scanning function is implemented by a variable timing circuit 450 which sends control signals to the image sensor 410 and to the circuit 420 based on a user command. This circuit 450 is able to deliver a signal Vsync representative of the acquisition frequency at which it controls the image sensor 410. As soon as the user has set the acquisition frequency of the sensor 410 to that of the projected film, the signal frequency

Vsync corresponds to that of the projected film. The acquisition frequency can be adjusted manually using a potentiometer or automatically using a phase locking loop synchronizing the acquisition frequency of the camcorder with that of the perceived signal. The Vsync signal is transmitted to the circuit 200 which is then limited to the frequency measurement circuit 230 and to the frequency comparator 240.

Of course, the invention is not limited to the embodiments described above.

In particular, those skilled in the art could use other light sensors or other frequency measurement circuits than those presented here, and possibly at other frequencies which would be declared as illegal or prohibited.