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1. (WO2007004147) SYSTEME DE REPRODUCTION DIPOLE STEREO A COMPENSATION D'INCLINAISON
Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

STEREO DIPOLE REPRODUCTION SYSTEM WITH TILT COMPENSATION

FIELD OF THE INVENTION
The invention relates generally to a stereo sound reproduction system and, more particularly, to a stereo dipole reproduction system for use in portable electronic devices and the like.

BACKGROUND OF THE INVENTION
Conventional stereo sound reproduction involves the provision of two loudspeakers, placed in front of a listener who perceives the original source as being between the sound sources. Changing the relative amplitudes of the signals sent to the loudspeakers 'moves' the apparent position of the source. With conventional stereo, the sounds reaching the listener's ears are simply an addition of the signals radiated by the loudspeakers.
More recently, digital signal processing techniques have been developed which provide much more scope for generating accurate virtual images of the original sound source and a better understanding of the way in which the brain interprets and locates sounds, along with these advances in digital signal processing, have made it possible to recreate "surround sound" without a large array of loudspeakers.
Conventional stereo sound equipment uses two loudspeakers placed apart from each other to produce the stereo effect. On the other hand, the current trend in the field of mobile electronic devices, such as mobile telephones, personal digital assistants (PDAs), MP3 players, etc., is to provide on-board stereo sound reproduction and, since the loudspeaker spacing on such devices is too small to obtain a wide stereo image, a stereo-based widening scheme, for example using stereo dipole reproduction, is implemented. Stereo dipole reproduction enables the provision of a virtual loudspeaker span much wider than the actual loudspeaker spacing, and such a scheme is extremely effective in producing the illusion of a virtual source of sound. One known stereo dipole reproduction system uses two loudspeakers in a single housing. This causes mutual interference of the sound waves, and a falsified stereophonic sound is produced. If the listener faces the loudspeaker, the sound appears not to come directly from the loudspeaker but from the front right and front left outside the physical width of the device. The listener has the perception of being completely enveloped in sound.

Many sound reproduction systems have been proposed for providing a wider sound image than the physical width of the device. For example, US Patent Application Publication No. US 2004/0131202 Al describes a sound system assembly for use in a handheld device in which sound is directed towards output ports at respective side faces of the device such that the apparent audio sources are displaced further apart horizontally. A sensor may be provided to detect the orientation of the device, i.e. upright or rotated through 90 degrees
("landscape'V'portrait"), so as to determine the output ports to which the sound should be directed for optimum effect.
However, this and other known types of arrangement tend only to work effectively if the device is maintained at a straight angle relative to the listener. Under (smaller) tilt angles, performance of the stereo dipole reproduction scheme drops considerably. When the device is tilted, one loudspeaker is more distant from the listener than the other, which can cause the stereo image to collapse completely. Less significantly, but nevertheless relevant, the loudspeaker span is reduced as a result of a tilt angle which results in a decrease in the width of the stereo image.

SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to provide a portable electronic device comprising a sound reproduction unit, which allows a user to hold the device under a tilt angle without sacrificing the sound image output by the loudspeakers.
In accordance with the present invention, there is provided a portable electronic device including a sound reproduction unit comprising a plurality of output means and processing means for rendering a sound image defined by a plurality of respective sound signals for output by respective output means, and a tilt compensation unit comprising sensing means for sensing a tilt angle of said portable electronic device and control means for tracking changes in said tilt angle and for generating one or more control signals for controlling said processing means so as to adjust one or more of said plurality of sound signals in dependence on changes in said tilt angle to maintain said sound image output by said respective output means.
Thus the above-mentioned object is achieved in accordance with the invention, by adjusting one or more of the signals defining the sound image output by the output means in dependence on changes in the tilt angle of the mobile electronic device.
The sensing means may comprise a camera unit for sensing an absolute tilt angle. Alternatively, the sensing means may comprise an inertia tracker for sensing a relative tilt angle. The inertia tracker may be an accelerometer. Beneficially, the sensing means comprises both a camera unit for delivering a first tilt angle and an inertia tracker for delivering a second tilt angle, the control means being adapted to deliver a final tilt angle based on the second tilt angle which is updated at regular time intervals based on the first tilt angle.
The processing means is beneficially arranged and configured to widen the apparent width of the sound image for output by respective output means.
Where the sound reproduction unit comprises two output means (e.g. loudspeakers), the processing means preferably comprises stereo widening processing means, such as a stereo dipole system, for widening the apparent width of the stereo sound image for output by respective output means. Said stereo widening processing means preferably includes means for generating a sum signal and a difference signal in respect of input left and right stereo signals. The stereo widening processing means preferably includes amplifying means for amplifying the difference signal. Said amplifying means preferably has a variable gain, said gain being varied in dependence on changes in said tilt angle. This solves the above-mentioned problem of the loudspeaker span being reduced as a result of the tilt angle.
Preferably, the stereo widening processing means further comprises conversion means for converting said sum signal and said difference signal to output left and right stereo signals, which define a wider stereo image than that defined by said input left and right stereo signals. Beneficially, the processing means further comprises one or more time delay units, one or more sound signals defining said sound image being output to the output means via said one or more time delay units. Said one or more time delay units are preferably controlled in dependence upon changes in said tilt angle. This solves the above-mentioned problem whereby, when the portable electronic device is tilted, one loudspeaker is more distant from the listener than one or more of the others. The time delay is adjusted to ensure that the sound signals reach the listener's ears at the same time, even though one or more of the
loudspeakers is more distant from the listener than one or more of the others.
The portable electronic device may further comprise means for adjusting the sound image in accordance with a distance between the portable electronic device and a listener and/or in accordance with an elevation of the portable electronic device.
In one exemplary embodiment, the processing means may be arranged and configured to render two or more different sound images independently in accordance with the relative position of each of two or more respective listeners, and to recombine said sound images prior to output thereof by said output means, such that each sound image can be heard only by a respective listener.
The present invention extends to a tilt compensation method for a sound reproduction unit in such a portable electronic device.
These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiment described herein.

BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawing, in which:
Figure 1 is a schematic block diagram illustrating the principal components of a stereo dipole reproduction system according to an exemplary embodiment of the present invention, and
Figure 2 is a schematic overview of a tracking system in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure 1 of the drawings, a stereo dipole reproduction system according to an exemplary embodiment of the present invention, and for use in a mobile device, is depicted. The stereo dipole reproduction system comprises a left L and a right R channel stereo signal source which is input to a sum-difference processor 12. A stereo widening processing scheme generally works by introducing cross-talk from the left input to the right loudspeaker, and from the right input to the left loudspeaker. The audio signals transmitted along direct paths from the left input to the left loudspeaker and from the right input to the right loudspeaker are usually also modified before being output from the left and right loudspeakers. In other words, widening of the stereo image can be achieved by manipulating the relationship of a difference or side signal L-R and a sum or mid signal L+R. A positive part of the side signal is then fed to the left channel and a part with its phase inverted to the right channel.
Thus, the sum-difference processor 12 is used as a stereo widening processing scheme mainly by boosting a part of the difference signal L-R in order to make the extreme left and right part of the sound stage appear more prominent. Accordingly, the left and right stereo signals, L and R, are transformed by the processor 12 into mid M and side S signals, where M=L+R and S=L-R. The side signal S is filtered and amplified by a dipole filter 14 so as to widen the stereo image reproduction over closely spaced loudspeakers. In a conventional stereo dipole reproduction system the gain associated with the dipole filter 14 is fixed.
However, in this exemplary embodiment of the present invention, a variable gain function 16 is provided in respect of the side signal path, as shown.
The mid and amplified side signals are then fed to a second processor 18, which converts the modified side and mid signals in a conventional manner back to right and left stereo signals R' and L' which are fed to respective loudspeakers 22, 24 via respective variable time delay units 20a, 20b. These time delay units enable output of one or other of the stereo signals to be delayed by an amount determined by the tilt angle 26, so as to ensure that the signals reach the listener's ears at the same time, even though the distance of one of the loudspeakers 22 from the listener will be different to that of the other 24 as a result of the tilt angle 26.
In one exemplary embodiment, two sound tracks could be rendered over a single set of loudspeakers for two listeners by rendering them independently in accordance with the respective positions of the listener and adding them together prior to playback.
The object of the present invention is achieved in accordance with this exemplary embodiment by the provision of the variable gain function 16 and the variable time delay functions, both of which functions are controlled by the tilt angle 26 output from a tilt sensor 28 which is coupled to a tracking pre-processor module 30. The tilt sensor 28 and tracking pre-processor module 30 track the tilt angle of the mobile device and use this tracking information to adapt the stereo reproduction by adjusting the variable gain and/or time delay.

As explained above, when the mobile device is tilted, two problems occur in conventional stereo dipole reproduction schemes:
- due to the tilt angle, one loudspeaker is more distant from the listener than the other.

This has the potential to cause the stereo image to collapse completely; and
the loudspeaker span is reduced by the tilt angle, which results in a decrease in the width of the stereo image.
The variable time delay solves the first problem cited above, and is applied for example to the variable time delay unit 20a or 20b according to the equation:
_ dsinθ
* delay ~ where d is the spacing between the loudspeakers 22, 24, θ is the tilt angle 26 and c is the sound velocity in air. The signal channel to which the delay is applied is dependent on the direction of tilt.
The variable gain function 16 solves the second problem cited above, and is selected according to the equation:

COS0
where G0 is the default gain used for the stereo widening algorithm. Note that, in the above expression, G goes to infinity when θ to 90°. This can be avoided by setting a maximum value for G for example, Gmax = 2G0.
It will be appreciated that additional compensation can be provided to take into account the distance of a listener from the device and/or their elevation. So-called head-related transfer functions which are more generic filters and which are known to persons skilled in the art could facilitate this.
For example, tilting the device around the axis perpendicular to the portable electronic device screen surface also reduces the loudspeaker span, and this can be compensated for by adjusting the gain in accordance with the following equation:

G =^- cosψ
where G0 is the default gain for the stereo widening algorithm and ψ denotes the tilt angle. In this case, no further delay compensation is required.

For the tracking, several approaches can be used, depending on the type of application. For example, a camera unit is provided in or on the mobile device. Alternatively or in addition, accelero meter trackers may be used. In fact, many different forms of tracking suitable for use in various exemplary embodiments of the present invention will be apparent to a person skilled in the art and the present invention is not necessarily intended to be limited in this regard.
According to an exemplary embodiment of the invention, the tilt sensor includes a camera unit for sensing an absolute tilt angle. Camera tracking is performed according to conventional tracking algorithms such as eye tracking, face tracking. Other algorithms can be used for example based on skin tone detection, where the centre of the skin tone region is considered to be the centre of the head. As an example, the paper entitled "Eye Tracking for Viewpoint Adaptive Video Systems in Living Room Situations" by Bart de Liefde, Andre Redert, Emile Hendriks, Proceeding WIAMIS, describes a robust eye-tracking algorithm designed for complex indoor situations. The algorithm is based on the use of a structured infrared lighting scheme which exploits the bright and dark pupil effect.
According to another embodiment of the invention, the tilt sensor includes an inertia (e.g. accelero meter) tracker for sensing a relative tilt angle. The inertia tracker gives the acceleration plus some error epsilon. This result is integrated twice to obtain the estimate of the tilt angle. In order to prevent this estimate from drifting because of the error epsilon, a leaky integrator is used. As a result, the estimated tilt angle becomes zero after the mobile device has not moved for a while (e.g. tilt to the left and hold). Therefore the absolute position is only correct for not too slow movements of the mobile device around an average of 0 degree.
According to another exemplary embodiment of the invention, the tilt sensor includes both a camera unit and an inertia tracker. The inertia tracker is used to track quick, short-term orientation changes, while the camera unit is used to estimate and compensate for the drift and other non- ideal properties of the inertia tracker and tracks user movements from time to time. Thanks to the camera unit, the exact tilt angle is known and the inertia tracker is set to this value at regular time intervals. In this way a tracking system can be achieved which is responsive, accurate and cost effective.
Figure 2 is a schematic overview of the tracking system in accordance with this embodiment of the invention. A camera unit 40 provides a coordinate estimation 42 of the mobile device. A built-in inertia tracker 50 (e.g. an accelerometer) provides an inertia tracker output signal 52. The tracking pre-processor module 30 then combines the camera coordinate estimation 42 and the inertia tracker output signal 52 and provides tracker coordinate estimation 32 so as to ensure that the sound signals reach the mobile device user's ears at the same time irrespective of the tilt angle. Through the inertia tracker and camera unit information, the drift and other non- ideal properties of the inertia-tracking mechanism are compensated.
Since an inertia tracker can have non-ideal and varying properties (production tolerances, drift etc.), its response is not accurate. By combining the inertia tracker output signal with the camera coordinate estimation, a number of advantages can be achieved:
the camera unit can compensate for long-term drift of the inertia tracker;
the camera unit can track movements of the user as well;
the camera unit can track the performance/properties of the inertia tracker. By comparing the camera coordinate estimation with the inertia tracker output signal, the used tracking model (i.e. the model implemented in the tracking pre-processor module that converts the output signal of the inertia tracker to orientation/translation coordinates) is able to adjust parameters such that the inertia tracker becomes more accurate.
The camera unit is very accurate and can work at a very low rate to save
computational costs. The camera unit is used for compensation of drift and other non-ideal properties of the inertia tracker by adapting the inertia tracker model parameters. High-speed movements of the stereo dipole reproduction system are taken care of by the inertia tracker.

The tracking system of the present invention can be applied to mobile, handheld devices that involve audio recording or audio reproduction, for which information on the accurate relative position/orientation of the user with respect to the device can be used to improve the experience of the user and possible of the person the user may be in
communication with through the mobile device. Examples of handheld devices include mobile phones, personal digital assistants PDAs, MP3 players with integrated loudspeakers, etc. An example of application is video conferencing using a mobile phone. Speech intelligibility can be improved in this case on both audio recording and audio reproduction, if the accurate relative user position is known.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice- versa.
The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.