Head tracking system

Abstract

The invention relates to a head tracking system (1) for a headrest, comprising: at least one optical circuit (2) including a plurality of sensors (4), each sensor being arranged to detect light reflected from at least part of an illuminated scene, said part of the scene containing at least a part of a head to be tracked, and to provide an output signal corresponding to the amount of reflected light detected by the sensor, an acquisition circuit (6) arranged to receive a plurality of output signals from said plurality of sensors and to derive frames from said output signals, a processing unit (8) arranged to receive said frames and to derive an indication of a change in position and / or orientation of the head by comparing the received frames with one or more frames obtained at different times.

Description

Technical Field

[0001] The present invention generally relates to the field of systems for head tracking, and more specifically for tracking the movements of a person's head while seated in a vehicle. Background Art

[0002] Head tracking systems for acoustic virtualization are known in the art. Acoustic virtualization means abstracting the physical resources that produce the sound by considering logical units that together perform the same functionality as the physical resources. Various acoustic virtualization algorithms have been developed that create the illusion that the sound source is located at a specific distance and in a specific direction. Typically, the goal of these algorithms is to approximate the transfer function from the sound source (for example, two speakers in front of the user in the case of stereo audio) to the human ear. However, fixed virtualization is not enough, because human directional perception seems to be very sensitive to head movement. This is how head tracking and head tracking systems come into play.

[0003] In a head tracking system, the head position and / or orientation is measured. Through these measurements, head movements can be detected and the virtualization is adapted to the new head position. Today, a variety of head tracking systems based on various technologies are available.

[0004] In some conventional systems, head tracking is integrated into the headset. Various types of motion tracking can be applied in such systems, such as accelerometers, gyroscopes, compasses. However, such systems may produce wearing fatigue and cause discomfort. In some implementations, it may be necessary to provide an additional cable for the user. In other implementations, data transmission can be wireless, but an additional power source may be required.

[0005] Other known head tracking systems rely on the use of a front camera. Such camera trackers can be used for head detection, but also for feature detection and / or feature tracking. In addition, this type of head tracking also has several disadvantages. This may be because the position is not always available. The front camera may be located in the field of view, which may distract the user. The head may not be visible at all times (for example, because the seated user turns away). In addition, a large amount of image processing power may be required to process the received images.

[0006] An example of a camera-based head tracking system can be found in US8331614, where an image of the listener's face is acquired and then the skin tone of the image is tracked to obtain an indication of the listener's position. By triangulation, the distance between the camera and the user can be determined.

[0007] US2006 / 045294 relates to personalized headphone virtualization, the purpose of which is to allow a listener to experience the sound of virtual speakers through headphones with a realism comparable to that of real speakers. Personalized room impulse responses are collected for speaker sources across a small number of listener head positions. These responses are used to convert the speaker audio signals into virtualized outputs for the headphones. Based on changes in the listener's head position, the system can adjust the transformation so that the virtual speakers do not appear to move when the listener moves his head.

[0008] There is a need for an improved head tracking system that can be used in a flexible manner. Summary of the invention

[0009] It is an object of embodiments of the present invention to provide a head tracking system that is user-friendly in the sense that it avoids obstructions due to the presence of a wearable device and / or a front camera. Another object is to provide a headrest equipped with such a head tracking system and a seat comprising such a headrest.

[0010] The above mentioned objects are achieved by the solution according to the invention.

[0011] In a first aspect, the present invention relates to a head tracking system suitable for use in a headrest. The head tracking system comprises:

[0012] at least one optical circuit including a plurality of sensors, each sensor arranged to detect light reflected from at least a portion of an illuminated scene, said portion of the scene containing at least a portion of a head to be tracked, and to provide an output signal corresponding to an amount of reflected light detected by the sensor;

[0013] acquisition circuitry arranged to receive a plurality of output signals from the plurality of sensors and to derive a frame from the output signals; and

[0014] A processing unit is arranged to receive the frame and to derive an indication of a change in the position and / or orientation of the head by comparing the received frame with one or more frames obtained at different times.

[0015] The proposed solution does allow for head tracking. The scene part including at least a portion of the head to be tracked is illuminated by daylight and / or a light source. The reflected light is detected by a set of sensors, where each sensor converts the received signal into an electrical signal. The electrical signals are fed to an acquisition circuit, in which frames are derived and the frames are then processed to determine whether there is head movement. Decisions regarding changes in the position and / or orientation of the head are made based on a comparison of frames obtained at different times (e.g., the received frame and the previous frame, i.e., two consecutively obtained frames). The illuminated part of the head to be tracked is preferably the rear half or a part of the rear half of the head. This means that the sensed image is taken from behind the head or from the side of the head. Thus, an advantage of the proposed head tracking system is that the user is not obstructed by any wearable devices or wires as in prior art solutions. Another advantage is that the computational load for deriving an indication of any head movement in the processing unit is limited, such that an expensive processor is not required. Another advantage of the proposed head tracking system is the fact that the set-up enables the existence of an available scene including the head or at least a portion of the head, thereby enabling the head to be tracked in a continuous or quasi - continuous manner.

[0016] In a preferred embodiment, the processing unit is arranged to derive the indication by calculating the translation between the received frame and one or more frames for comparison therewith, and the indication includes one or more displacement vectors.

[0017] In an advantageous embodiment, a plurality of sensors are arranged such that they form a two - dimensional array. This provides the benefit of generating two - dimensional displacement vectors. A cost - effective implementation can also be achieved using a two - dimensional sensor array.

[0018] In a preferred embodiment, the head tracking system includes a light source for illuminating at least said part of the scene. This has the advantage that the head tracking system can also be used in the dark or when the available daylight is insufficient to obtain a sufficient amount of reflected light. In some embodiments, the intensity and / or one or more wavelengths of the light emitted by the light source can be adapted.

[0019] In embodiments, one or more wavelengths emitted by the light source are in the infrared light spectrum.

[0020] In one aspect, the invention relates to a system including the head tracking system as described above and at least two speakers. The system is arranged to adjust at least one parameter setting of the signals applied to the speakers based on an indication of changes in the position and / or orientation of the head.

[0021] In a preferred embodiment, the system is arranged to adjust and / or delay the signals applied to the speakers in amplitude and / or phase.

[0022] In another aspect, the invention relates to a headrest equipped with a head tracking system as described above. A headrest with an optical circuit of a built-in head tracking system offers the following advantages: When the headrest is used, there is always a scene containing the head or a part of the head. Due to this positioning in the headrest, the head remains visible when the system is used. It can be noted again that the head tracking system is completely out of the user's field of view and the user cannot be distracted by its presence. The processing unit can also be placed in the headrest.

[0023] In one embodiment, the optical circuitry of the head tracking system is located directly behind the user's head so as to face the back of the head.

[0024] In one embodiment, the optical circuit of the at least one optical circuit is positioned such that it faces an ear of the user.

[0025] In a preferred embodiment, the headrest further comprises at least two speakers and is arranged to adjust at least one parameter setting of said speakers based on said indication of a change in position and / or orientation of the head.

[0026] In another embodiment, the processing unit is arranged for calculating at least one parameter setting to be adjusted.

[0027] Advantageously, the headrest is arranged to detect when the total amount of reflected light captured by the sensor falls below a given threshold level.

[0028] In yet another aspect, the invention relates to a seat comprising a headrest as described above.

[0029] For the purpose of summarizing the present invention and the advantages achieved relative to the prior art, certain objects and advantages of the present invention have been described above in this document. Of course, it should be understood that not all such objects or advantages may be achieved according to any particular embodiment of the present invention. Thus, for example, those skilled in the art will recognize that the present invention can be embodied or performed in a manner that achieves or optimizes one advantage or a group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0030] The above and other aspects of the present invention will be apparent from and elucidated with reference to the embodiment(s) described herein below. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will now be described by way of example with reference to the accompanying drawings, in which like reference numerals refer to like elements throughout the various drawings.

[0032] Figure 1 An embodiment of a head tracking system according to the invention is shown.

[0033] Figure 2 Determination of the displacement vector by comparing two frames acquired at different times is shown.

[0034] Figure 3 The derivation of an indication of head movement in an embodiment of a head tracking system having an optical circuit is shown.

[0035] Figure 4 The derivation of an indication of head movement in an embodiment of a head tracking system having two optical circuits is shown.

[0036] Figure 5 An embodiment of a headrest equipped with a head tracking system and two speakers is shown.

[0037] Figure 6 It shows that Figure 5 An exemplary implementation of a headrest solution with a head tracking system and two speakers is shown.

[0038] Figure 7 An embodiment is shown in which the optical circuitry of the head tracking system is positioned to allow detection of rest or sleep patterns. DETAILED DESCRIPTION

[0039] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims.

[0040] Furthermore, the terms first, second, etc. in the specification and in the claims are used to distinguish between similar elements and are not necessarily used to describe a sequence in time, space, in ranking, or in any other manner. It is understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in a different sequence than described or illustrated herein.

[0041] It is to be noted that the term "comprising" used in the claims should not be interpreted as being limited to the means listed thereafter; it does not exclude other elements or steps. Therefore, the term should be interpreted as specifying the presence of the stated features, integers, steps or components as mentioned, but does not exclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Therefore, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that for the present invention, the only relevant components of the device are A and B.

[0042] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification do not necessarily all refer to the same embodiment, but may refer to the same embodiment. Furthermore, in one or more embodiments, as will be apparent to one of ordinary skill in the art from this disclosure, the particular features, structures, or characteristics may be combined in any suitable manner.

[0043] Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. However, this disclosure method should not be interpreted as reflecting an intention that the claimed invention requires more features than those expressly recited in each claim. On the contrary, as reflected in the appended claims, inventive aspects reside in fewer features than all of the features of a single preceding disclosed embodiment. Therefore, the claims appended after the detailed description are hereby expressly incorporated into this specific implementation, with each claim itself representing a separate embodiment of the invention.

[0044] In addition, although some embodiments described herein include some features included in other embodiments but do not include other features included in other embodiments, as will be understood by those skilled in the art, the combination of features of different embodiments is intended to fall within the scope of the present invention and form different embodiments. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

[0045] It should be noted that the use of a particular term in describing certain features or aspects of the present invention should not be taken as implying that the term is redefined herein to be limited to including any specific characteristics of the feature or aspect of the present invention with which the term is associated.

[0046] In the description provided herein, numerous specific details are set forth. However, it should be understood that embodiments of the present invention may be practiced without these specific details. In other examples, well-known methods, structures, and techniques are not shown in detail to avoid confusing the understanding of this description.

[0047] In a first aspect, the invention proposes a head tracking system, wherein no wearable (eg headphones) or physical connection to the user is required. The solution according to the invention requires only lightweight processing, which can be easily embedded in, for example, a digital signal processor.

[0048] In embodiments of the present invention, a head tracking system is disclosed, the system comprising one or more optical circuits, an acquisition unit and a processing unit. In one embodiment, daylight is used to illuminate a scene or at least a portion thereof including a head to be tracked. In other embodiments, at least one optical circuit comprises a light source for illuminating the scene of interest. A possible implementation of such an embodiment of the head tracking system (1) is as follows Figure 1 As shown, it includes a single optical circuit (2). In the illustrated embodiment, the optical circuit of the head tracking system includes a light source (5). The light source is positioned so that a scene including the head of the person to be tracked or at least a portion of the head (e.g. the back of the head or the side of the head or a portion thereof) is illuminated. In either case, the scene includes at least a portion of the back half of the head. Note that in this specification, the terms 'user', 'person' and 'listener' are used interchangeably in relation to the head to be tracked. In a preferred embodiment, the light source is positioned so that at least a portion of the back half of the person's head is illuminated. In another embodiment, a side view of the head including one of the person's ears can be illuminated with a single optical circuit. In a specific embodiment, the light source can be arranged to track a portion of the head.

[0049] The light output by the light source may be visible light, or may be light in the invisible part of the spectrum, such as infrared light. For example, in some embodiments, an infrared light emitting diode (LED) may be used as a light source. The light may then contain one or more wavelengths in the range of 650 to 1000 nm. If visible light is used, light of a specific wavelength may be selected, so a specific color (e.g., blue or green) may be selected or light containing a range of wavelengths or light containing various specific selected wavelengths may be selected. In some embodiments, the intensity and / or one or more wavelengths of the light emitted by the light source of the head tracking system may be adapted. The wavelength may be selected according to the environmental conditions at hand, for example, based on the intensity of the light captured by the sensor. However, as previously described, in one embodiment of the system there is no light source, and daylight and / or ambient light (e.g., from one or more light sources already present in the facility) is used to illuminate the scene.

[0050] The light reflected by the head is focused by a lens (3) and then reaches a plurality of sensors (4). Each sensor detects the amount of reflected light and accordingly converts the physical quantity into an analog electrical output signal corresponding to the amount of reflected light detected by the sensor. For example, the output signal is a voltage signal. The amplitude of the voltage signal of each sensor corresponds to the amount of reflected light captured by the sensor.

[0051] In one embodiment, the sensors are aligned to form a one-dimensional array. If the sensors are aligned in a given direction (e.g., horizontally), for example, the sensors can be used to track the movement of the head in the given direction. Advantageously, the aligned sensors are equally spaced so as to obtain meaningful displacement vectors. In other embodiments, multiple sensors form a two-dimensional array.

[0052] In some embodiments, the sensor may be a photoelectric sensor, i.e., an electronic component for detecting the presence of visible light, infrared light, or ultraviolet (UV) light. The photoelectric sensor may be part of a charge coupled device (CCD). As is known in the art, a CCD is a light sensitive integrated circuit in which the light sensitive sensor records the amount of radiation incident on the sensor.

[0053] Each sensor output is connected to an input of an acquisition circuit, which in some embodiments may be integrated into a processing unit. The acquisition circuit receives an output electrical signal from each sensor, such as an A / D (analog to digital) converted voltage signal. From the digitized electrical signal, a frame is derived.

[0054] An embodiment of an acquisition circuit for deriving a frame can be implemented as follows. The incoming voltage signal can first be filtered using an analog low-pass filter, such as a filter with a cutoff frequency of 50 Hz. Next, the signal is applied to an analog-to-digital converter, which samples the applied voltage signal at a given sampling rate (e.g., 100 Hz) and a given resolution (e.g., 8 bits per sample). The obtained samples are then arranged in a frame of given dimensions. Preferably, the dimensions of the frame correspond to the configuration of multiple sensors. For example, in the case of a set of N aligned sensors (N is an integer value) mentioned above, the dimensions of the frame are Nx1. Depending on the rate at which the frames are provided to the processing unit, the movement of the head can be tracked in an almost continuous manner or a quasi-continuous manner, i.e., with very short interruptions.

[0055] The acquired frames can be considered to represent the state observed by the acquisition circuit at a given moment. Each frame (or in other words, each state) collected over time is stored in a memory. In some embodiments, the memory can be a separate memory. In other embodiments, the memory can be part of the processing unit (8) or the acquisition circuit (6).

[0056] Frames stored in a memory are processed to determine displacement vectors. A beneficial way of doing this is by applying the frames to a digital correlation algorithm running in a processing unit to compute the displacement vectors. Examples of such digital correlation algorithms are well known in the art, as the concept of using cross - correlation to measure shifts in a dataset is known and has been applied for a long time. The calculation step is repeated at a specific rate, which in a preferred embodiment is the same as the rate at which frames of sampled values are obtained. Thus, in an example where the sampling rate is 100 Hz, 100 pairs of frames per second will be fed into the correlation algorithm, resulting in a stream of 100 displacement vectors per second.

[0057] Figure 2 An example is provided in Figure 2 A new frame (13) acquired and stored in a storage device is shown. The storage device already contains an earlier frame (12) that is compared with the new frame. Through this comparison, an indication of the translation that has occurred between the two frames is obtained, and thus a non - zero displacement vector (15) is generated.

[0058] In an embodiment having a two - dimensional sensor array (e.g., an MxN sensor matrix), likewise, each sensor outputs an analog electrical signal (e.g., a voltage signal). An acquisition circuit receives the electrical signals and derives a frame, which is now also two - dimensional and contains M rows and N columns. Then, the algorithms perform two - dimensional correlation to determine the displacement vectors. Using a two - dimensional sensor array provides the advantage of obtaining a cost - effective solution.

[0059] One major advantage of the system is that the amount of computation required to determine if there is any movement of the head is moderate. This limits the computational requirements on the processing unit. For example, in some implementations, depending on the required accuracy, a computational load of less than 2 MIPS can be feasible. However, note that for applications requiring higher accuracy, a greater computational load may be necessary. In a preferred embodiment, the processing unit is implemented as a “simple” digital signal processor.

[0060] To explain how an indication of the change in the position and / or orientation of the head is derived, some examples are provided.

[0061] First, consider an embodiment having only one optical circuit and a two - dimensional sensor array. The optical circuit is positioned so that it faces the back of the user's head. Figure 3 An illustration is provided. In Figure 3, the circuit (21) is located in the XZ plane. The head has six degrees of freedom to move in three-dimensional space. In fact, the head is free to move forward / backward, up / down, left / right (translation on three vertical axes), and rotate around three vertical axes, usually referred to as elevation (or pitch), azimuth (also called yaw) and roll. Obviously, in this embodiment using only one optical circuit, it is impossible to handle all possible types of movement and rotation. However, it is found that users usually only move their heads around the elevation axis (up / down) and the azimuth axis (left / right). Therefore, in practice, an embodiment with one optical circuit is an important use case. If, after analyzing frames obtained at different times (for example, two consecutive frames), a non-zero displacement vector is found in the processing unit, and it can therefore be concluded that movement has occurred, then the displacement can be directly related to the change in the head orientation. In the case of a displacement in the left-right direction, it can be considered that the head has rotated around the azimuth axis. In the case of a displacement in the up-down direction, a rotation around the pitch axis has occurred.

[0062] Now consider another embodiment in which there are two optical circuits (see Figure 4 ), and in which a two-dimensional sensor array is applied. As in the previous example, again, one optical circuit (26) is placed in the XZ plane and faces the back of the person's head. The second optical circuit (27) is also placed in the same XZ plane and faces the left ear of the user in this particular example. The processing unit receives input signals (frames) from the two acquisition circuits. For each optical circuit, a displacement vector can be determined by comparing frames obtained at different times. Combining the displacement information related to the two circuits then provides an indication of head movement. For example, the displacement vectors can be combined using a weighted sum or a two-dimensional lookup table or a Kalman filter.

[0063] For example, both the optical circuit 1 at the back of the head and the optical circuit 2 at the front of the left ear did not detect any movement (indicated as 'No' in the following scheme) or did not detect movement in the up-down direction (indicated as ), the available options can be summarized in the following scheme:

[0064]

[0065] Obviously, if none of the optical circuits detects any displacement, the head has not moved. If only optical circuit 1 detects an up / down movement, and optical circuit 2 does not observe any movement, it can be concluded that the head has rotated around the pitch axis. If only optical circuit 2 detects an up / down displacement, and optical circuit 1 does not observe any displacement at all, it can be considered that the head has moved along the roll axis (i.e., the Z axis). If both optical circuit 1 and optical circuit 2 observe an up / down displacement, the head has translated along the Y axis.

[0066] Similarly, both the optical circuit 1 at the back of the head and the optical circuit 2 in front of the left ear detected no movement (indicated as 'No' in the following scheme) or no movement in the left-right direction (indicated as ), the available options can be summarized in the following scheme:

[0067]

[0068] If only optical circuit 1 observes lateral motion, then it can be concluded that the head has translated along the X axis. If only optical circuit 2 observes left / right displacement, then the head must have moved along the Z axis.

[0069] In one aspect, the present invention also relates to a headrest for a seat in a car, train, airplane or other means of transportation. A seat headrest typically has a front surface configured to face (the back of) the head of a user sitting in the seat and a rear surface configured to face away from the head of the user sitting in the seat. A head tracking system as described above can be advantageously applied in such a seat headrest.

[0070] The seat may also be a seat for use outside of a vehicle. For example, the seat may be a seat for a computer game player, a seat for studio monitoring, or a personal home theater seat, but is not limited thereto. Other application areas may be studio monitoring applications, etc., or even just personal listening to music.

[0071] In such headrests, at least one optical circuit is preferably integrated in the front surface of the headrest.Preferably, at least one optical circuit of the head tracking system faces the back of the user's head so that the head is easily visible during use and the position of the head can be determined.

[0072] In another embodiment having one optical circuit in the head tracking system, the single optical circuit may also be located in the headrest so that it faces one of the user's ears.

[0073] In embodiments having multiple optical circuits, advantageously one optical circuit faces the back of the user's head and one optical circuit faces one of the user's ears.

[0074] In one aspect, the invention relates to a system comprising a head tracking system as described above and at least two loudspeakers. In an advantageous embodiment, such a system is included in a headrest of a seat as described above. Thus, in such a case, not only is the head tracking system integrated in the headrest, but also the at least two loudspeakers. Since typical headrests are designed to be at a small distance (e.g. 30 cm or less) from the ears of a user sitting in the seat, this is a convenient way to configure the seat in order to create a comfortable listening position for the user sitting in the seat.

[0075] Figure 5 An advantageous embodiment of a headrest (9) equipped with a head tracking system and also comprising two loudspeakers (18) is shown in FIG. Figure 5 The embodiment of the system depicted in also comprises a light source (5). In order not to overload the drawing, a head tracking system with one optical circuit (2) is shown. Obviously, embodiments with more than one optical circuit are also conceivable.

[0076] exist Figure 6 In FIG. 1 , a mechanism is shown in which a processing unit (8) of a head tracking system is connected to another digital signal processor (31) that receives stereo sound from a music source (30). The two music channels and information about head movement from the processing unit of the head tracking system are fed to the other digital signal processor to adjust parameters to facilitate an enhanced listening experience. The other digital signal processor feeds its output signal to audio amplifiers, each of which outputs an amplified signal to a loudspeaker.

[0077] In the case of an implementation of a head tracking system in which the speaker is built into the headrest of a vehicle seat, an attractive arrangement may be to integrate one or more optical circuits, an acquisition circuit and a processing unit in a first device, and a digital processor for audio signals as another device. The displacement vector is calculated in the first device and can be transmitted to the audio processor via a logic bus, such as a controller area network (CAN) bus. Another option may be to make one or more optical circuits and an acquisition circuit form a first device, and to make the processing unit and the digital processor for audio signals as another device. In this case, the frames output by the acquisition unit are transmitted via the bus. This may be based, for example, on a link technology that enables the transmission of frames over an unshielded twisted pair (UTP) cable.

[0078] Consider in more detail Figure 6, a processing unit (8) is shown which receives frames derived from sensor output signals from an acquisition circuit (6) and determines a set of displacement vectors in the manner described above. The displacement vectors are forwarded to another digital signal processor (31). The input signal (e.g. a music signal from a stereo source (30)) is fed to separate variable filters (32, 33) which are contained in the other digital signal processor for the left and right channels, respectively. The displacement vectors are used to adjust one or more processing parameters of the channels. The transfer function of the variable filter can be adjusted using a correction signal (e.g. in terms of amplitude, phase, delay) derived from the displacement vector. The adaptation can be performed, for example, at the same rate as the generation of the displacement vector. In this way, the filter transfer function can be adapted according to the latest value of the displacement vector. The variable filter output is then applied to an audio amplifier (39) and the amplified signal goes to the corresponding speaker (18) in the headrest. One speaker is facing the left ear of the listener and the other speaker is facing the right ear of the listener, such as Figure 5 shown.

[0079] exist Figure 5 In , a situation is sketched where the listener turns his head to the left (i.e. about the azimuth axis). This movement is detected by a sensor array in the optical circuit, such as a one-dimensional array as shown. The corresponding frame is derived. Comparison of this frame with the earlier frame produces a non-zero displacement vector. As mentioned above, this can be discovered even if the mechanism has only one optical circuit, such as Figure 5 Another digital signal processor (31) receives the displacement vector and determines updated correction signal values ​​for both the left and right channels. In this particular example, the amplitude of the correction signal for the left channel may be slightly reduced, while the amplitude of the correction signal for the right channel may be slightly increased, compared to the value before the shift.

[0080] In an advantageous embodiment, the processing unit (8) of the head tracking system and the further digital processor (31) responsible for acoustic virtualization are integrated in one digital processing component. In such an embodiment, the single digital processing component receives the frames derived from the sensor output signals and determines the displacement vectors. The digital processing component then calculates the adjusted correction signals and applies them to the corresponding variable filters. The resulting signals are then output to the loudspeaker via an amplifier after D / A conversion.

[0081] The headrest can obviously also be used for resting or sleeping. In some embodiments, the processing unit of the head tracking system is arranged to detect this situation. Figure 7In the illustrated embodiment, the optical circuit (2) is located in the headrest (9), and when the user is resting or sleeping, the optical circuit is completely or partially blocked, that is, the reflected light is no longer or no longer completely captured by the sensor of the optical circuit. Therefore, the total measured intensity of the reflected light received by the sensor drops below a given threshold level. This state change can be detected. For example, the optical circuit can be arranged to detect that the amount of reflected light drops below a certain level. Alternatively, the acquisition unit can be equipped to detect whether the incoming signal from the sensor is below a threshold level. The head tracking system can enable its processing unit to detect that the received frame is "too dark". The head tracking system can then enter a rest or sleep mode.

[0082] In an advantageous embodiment, detection of being in rest or sleep mode can be associated with taking measures related to safety enhancement and / or comfort enhancement. For example, music streaming can be paused during rest or sleep mode. Alternatively, the volume of the music signal can simply be reduced or the sound modified. In other embodiments, detection of rest or sleep mode can be a trigger to adapt one or more filter settings of an algorithm used for noise cancellation.

[0083] In still other embodiments, detection of a rest or sleep mode may be an indication that the reading light may be dimmed or turned off.

[0084] Another interesting feature of the head tracking system in the headrest could be the issuance of a warning signal when a rest or sleeping pattern is detected in the headrest of the driver's seat.

[0085] Detection of a rest or sleep mode may also be used to place the entire head tracking system into a reduced power mode.

[0086] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary, rather than restrictive. The foregoing description describes certain embodiments of the present invention in detail. However, it will be appreciated that no matter how detailed the foregoing description appears in text, the present invention can be implemented in many ways. The present invention is not limited to the disclosed embodiments.

[0087] By studying the drawings, the present disclosure and the appended claims, those skilled in the art may understand and implement other variations of the disclosed embodiments when practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude pluralities. A single processor or other unit may perform the functions of several items recited in the claims. The mere fact that certain measures are stated in mutually different dependent claims does not mean that a combination of these measures cannot be used to advantage. The computer program may be stored / distributed on a suitable medium (such as an optical storage medium or solid-state medium supplied together with or as part of other hardware), but may also be distributed in other forms (such as via the Internet or other wired or wireless telecommunications systems). Any reference numerals in the claims should not be interpreted as limiting the scope.

Claims

1. A head tracking system, include: Headrest; an optical circuit comprising a plurality of sensors, each sensor being arranged to detect light reflected from at least a portion of an illuminated scene, the portion of the scene containing at least a portion of a head to be tracked, and to provide an output signal corresponding to an amount of reflected light detected by the sensor, wherein the plurality of sensors of the optical circuit form an array, the array being positioned in the headrest so as to face a back of the head to be tracked; an acquisition circuit arranged to receive a plurality of output signals from the array of sensors and to derive a frame from the output signals, A processing unit is arranged to receive the frame and to derive an indication of a change in position and / or orientation of the head by comparing the received frame with one or more frames obtained at different times.

2. The head tracking system according to claim 1, It is characterized in that The processing unit is arranged to derive the indication by calculating a translation between the frames, and wherein the indication comprises one or more displacement vectors.

3. The head tracking system according to claim 1, It is characterized in that The array of sensors is a two-dimensional array.

4. The head tracking system according to claim 1, It is characterized in that The head tracking system comprises a light source for illuminating at least the portion of the scene.

5. The head tracking system according to claim 4, It is characterized in that The intensity and / or one or more wavelengths of the light emitted by the light source can be adapted.

6. The head tracking system according to claim 4, It is characterized in that The one or more wavelengths are in the infrared light spectrum.

7. The head tracking system according to claim 1, It is characterized in that The sensors are arranged to detect when a total amount of reflected light captured by the plurality of sensors falls below a given threshold level.

8. The head tracking system according to claim 1, It is characterized in that A further optical circuit is included that includes a sensor array positioned in the headrest so that it faces the user's ears.

9. A system comprising a head tracking system according to claim 1 and at least two loudspeakers, the system being arranged to adjust at least one parameter setting of a signal applied to the loudspeakers based on the indication of the change in position and / or orientation of the head.

10. The system according to claim 9, It is characterized in that The system is arranged to adjust and / or delay the signal applied to the loudspeaker in amplitude and / or phase.

11. The system according to claim 9, It is characterized in that The at least two speakers are positioned in the headrest.

12. The system according to claim 9, It is characterized in that The processing unit is arranged for calculating the at least one parameter setting to be adjusted.

13. A chair incorporating the head tracking system according to claim 1.

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