EP2182744B1 - Replaying a sound field in a target sound area - Google Patents

Abstract

The method involves determining control signal for electro-acoustic transducer in an analytical manner, so that resulting sound field corresponds in an area around the center of the contour of the reproduced sound field. The control signal is changed so that the area, in which the resulting sound field reproduces sound field, is moved to a desired position within acoustic target area (10). The amended control signal is supplied to electro-acoustic transducer. An independent claim is included for a device for reproducing a sound field in a target area.

Description

The present invention relates to the reproduction or reconstruction of a sound field in a target sound area with the aid of a plurality of loudspeakers.

For the authentic physical reconstruction of a sound field over a wide range, the technique of wave field synthesis can be used. This technique uses a large number of loudspeakers and thereby avoids the problem that in an area within the loudspeaker arrangement, the so-called "sweet spot", the reconstruction of the desired sound field is significantly more accurate than in the rest of the listening area, as is the case for example with the stereo. Technique (in stereo, however, a soundfield is not physically reconstructed, but two speakers create an illusion that sounds very similar to the natural model, but has very different physical properties.) In stereo, there is a sweet spot in that the illusion sounds most authentic at this point). The speaker drive signals for each speaker are calculated analytically. In the practical implementation, however, there are large deviations from the desired sound field over the entire potential listening area. The technique of wave field synthesis is, for example, in the publication of S. Spors, R. Rabenstein and J. Ahrens, "The Theory of Wave Field Synthesis Revisited", in: Proceedings of the 124th Convention of the Audio Engineering Society, May 17-20, Amsterdam, The Netherlands, 2008 described.

Another known technique for reconstruction of a sound field is called Ambisonics and is for example in the WO 2005/015954 A2 described. This technique requires circular or spherical arrangements of loudspeakers, where the loudspeaker signals are generated using numerical algorithms. The limitation of the spatial bandwidth of the drive signals necessary in the calculation path causes the reconstruction of the desired sound field in the center of the loudspeaker arrangement to be most accurate. In the center of the loudspeaker arrangement is therefore a sweet spot. With increasing distance from the center of the loudspeaker arrangement, the deviations in the reconstructed sound field become larger. In the publication of J. Hannemann and KD Donohue, "Virtual Sound Source Rendering Using a Multipole Expansion and Method-of-Moments Approach, J. Audio Eng. Soc., Vol. 56, No. 6, June 2008 , a procedure related to the Ambisonics method is described, which makes it possible to choose relatively freely the arrangement of the loudspeakers and the location of the sweet spot. Again, the loudspeaker signals are calculated using numerical and thus very computationally intensive algorithms.

With enhancements by Ambisonics, such as the technique called Higher Order Ambisonics, a more efficient method can be provided in conjunction with analytic calculation of the speaker drive signals. Such a method is for example in the publication of J. Ahrens and S. Spors, "Analytical driving functions for higher order ambisonics", in: IEEE International Conference on Acoustics, Speech, and Signal Processing, Las Vegas, Nevada, March 30 to April 4, 2008 described. However, according to the method described there, the resulting reproduced sound field agrees even worse with the desired sound field, the further one moves away from the center of the arrangement. So again there is a sweet spot in the center of the loudspeaker arrangement.

It is therefore an object of the present invention to provide an efficient method and apparatus for reproducing a sound field wherein a sweet spot within a loudspeaker array can be freely selected. The arrangement of the loudspeakers should be subject to as few restrictions as possible.

This object is achieved with the features of the claims.

The present invention provides a method and a device for reproducing a sound field in a target sound area. For this purpose, electroacoustic transducers, ie loudspeakers, are arranged on a contour surrounding the target sound area. First, a drive signal for the electroacoustic transducer is determined analytically. In this analytical determination results in a resulting sound field, which corresponds exactly in an area around the center of the contour, the sweet spot, the sound field to be reproduced. The greater the distance from the center, the greater the deviations from the sound field to be reproduced, which is inherent in the analytical determination. The analytical determination can be achieved according to a known method by analytically solving an integral equation describing the sound field to be reproduced. To the solution In this integral equation, which is a surface or line integral via the drive signal of the electroacoustic transducers and the transfer function of the electroacoustic converters, the components of the integral equation can be decomposed into orthogonal basis functions, for example by Fourier decomposition.

The desired sound field is thus decomposed into basic functions which are orthogonal on the contour on which the loudspeakers are located. The desired sound field is then tape-bound with respect to the decomposition around the desired sweet spot. Furthermore, the desired sound field is band-limited with respect to the decomposition around the center of the loudspeaker arrangement. The connection between the two decompositions represents mathematical translation theorems. The starting point is in any case an integral equation. In this case, it is possible according to the invention to obtain the loudspeaker activation signals by way of the explicit solution of the integral equation (as described in detail below). Alternatively, the loudspeaker drive signals can be obtained via the relationships of the Kirchhoff-Helmholtz integral (as in wave field synthesis).

The detected drive signal is changed such that the area where the resulting sound field corresponds to the sound field to be reproduced is shifted from the center of the contour to a desired position within the target sound area. This can be done for example by means of mathematical translation theorems. The desired position can be predetermined at a fixed position, but it can also be shifted depending on the position of a receiver. In the latter case, the change of the drive signal then preferably takes place in real time. For example, this method can then be used to determine a position of a receiver within the target sounding area and then to change the control signal in such a way that the desired position is adjusted accordingly.

The changed drive signal is supplied to the electroacoustic transducers. The converters convert the input electrical signals into sound signals, and emit them into the target PA range.

Furthermore, the present invention also provides an apparatus for reproducing a sound field in a target sound area. The device has a plurality of electroacoustic transducers arranged on a contour surrounding the target sound area, and a processing module. The processing module is able to analytically determine from signals about the desired position and audio input signals a drive signal for the electroacoustic transducers and to change the drive signal such that the sweet spot, ie the area in which the resulting sound field corresponds to the sound field to be reproduced, is moved to the desired position. The drive signals can be amplified by means of an amplifier unit and supplied to the electroacoustic transducers.

With the present invention, the area where the reconstruction of the desired sound field is most accurate can be shifted to a desired position. Thus, the playback can be optimized to any location within the Zielbeschallungsbereichs. This shift or translation can be performed in real time, so that the playback can be tracked, for example, to a moving receiver. The determination of the location of the receiver can be carried out here by means of sensors, for example a camera.

The invention can be used to reproduce audio signals, in particular multi-channel audio signals. The audio signals can be provided by a variety of devices, such as CD, DVD or Blu-ray Disc devices, MP3 devices, computers or the like. The reproduced audio signals may be MPEG2 or MPEG4 signals, for example, or be in a Dolby format.

• Fig. 1 schematisch eine Vorrichtung gemäß der vorliegenden Erfindung zeigt und
• Fig. 2 ein erzeugtes Schallfeld zeigt, wobei in Fig. (a) der Sweet Spot im Zentrum der Lautsprecheranordnung ist und in Fig. (b) zu einem anderen Punkt innerhalb der Lautsprecheranordnung verschoben wurde.

The invention will be described in more detail below with reference to the accompanying figures, wherein

• Fig. 1 schematically shows an apparatus according to the present invention and
• Fig. 2 shows a generated sound field, wherein in Fig. (a) the sweet spot is in the center of the loudspeaker arrangement and has been shifted to another point within the loudspeaker arrangement in Fig. (b).

In Fig. 1 schematically a device according to an embodiment of the present invention is shown. On a contour 11 to a target PA range 10 more speakers 20 are arranged. The drive signals for the speakers 20 are in a processing module 30 determined and supplied via an amplifier unit 31 to the speakers 20. For this purpose, the processing module 30 is fed with information 1 about the current location for which the sound reconstruction is to be optimized, ie the so-called sweet spot or the sweet area, and audio input signals 2. The sweet spot should be located within the target PA area 10. In addition, the processing module 30 receives a third input signal 3 which carries information about the desired sound field (eg position of the virtual sound source).

To calculate the drive signals for the loudspeakers 20, the drive signal is first of all calculated analytically, for example according to the method described in the above-mentioned publication by J. Ahrens and S. Spors.

In the method described there, which is described here by way of example only, from a description of the sound field P to be reproduced by an integral over the driving signal D of the loudspeakers arranged on a contour around the target sounding area, and the spatial transfer function G of the loudspeakers shown: $$P=\int D\cdot \mathit{Gd}\mathrm{\Omega }$$

The integral here is a surface or line integral, depending on whether the calculation is to take place in three dimensions or only in one plane.

wobei P̊ die entsprechenden Zerlegungskoeffizienten bezeichnet. Dies kann beispielsweise durch eine Fourierzerlegung erfolgen. Das Lautsprecheransteuerungssignal D kann somit folgendermaßen berechnet werden: $$D=\mathrm{\sum }\frac{\stackrel{\circ }{P}}{\stackrel{\circ }{G}}B$$

This integral equation is solved by decomposing the variables involved into orthogonal basis functions B and determining the respective decomposition coefficients. Accordingly, the sound field P can be represented by $$P=\mathrm{\Sigma }\stackrel{\circ }{P}B$$

where P̊ denotes the corresponding decomposition coefficients . This can be done for example by a Fourier decomposition. The loudspeaker drive signal D can thus be calculated as follows: $$D=\mathrm{\Sigma }\frac{\stackrel{\circ }{P}}{\stackrel{\circ }{G}}B$$

In the case of a Fourier decomposition, the quantities P and G thus denote the corresponding Fourier coefficients.

This sum theoretically has infinite summands. Since this is generally not implementable in practice, and also to avoid excessive spatial aliasing, the summation is limited. However, this has the consequence that the farther one moves away from the center of the decomposition, the worse the resulting reproduced sound field agrees with the desired sound field. The so-called sweet spot or the sweet area, ie the region of the optimal reproduction, is thus at the center of the loudspeaker arrangement in this analytical determination, since the position of the center of the decomposition coincides with the center of the loudspeaker arrangement.

P _v und G _v sind hierbei die Zerlegungskoeffizienten des Schallfeldes und der Übertragungsfunktion. Auch dies ist lediglich ein Beispiel für die Beschreibung der kontinuierlichen Ansteuerungsfunktion. Dieses Beispiels gilt für die zweidimensionale Betrachtung kreisförmiger Lautsprecheranordnungen.As a result of this calculation, the continuous drive function can be described as follows:

P _v and G _v the decomposition coefficients of the sound field and the transfer function are in this case. Again, this is just one example of the description of the continuous drive function. This example applies to the two-dimensional viewing of circular loudspeaker arrangements.

In order to shift the place where the reconstruction of the desired sound field is most accurate to a desired location, according to the invention, the center of the decomposition is shifted in the loudspeaker drive signal D equation shown above. This can be done for example by means of various mathematical translation theorems. Thus, the playback can be optimized to any location within the speaker assembly.

For example, based on the analytical calculation described above, the inherent sweet spot may be constrained by limiting the spatial bandwidth of the sound field to be reproduced P, which is a function of the location and the frequency, are shifted with respect to the center of the decomposition different from the center of the speaker arrangement. The sweet spot or the sweet area is then arranged around this new center of decomposition. The decomposition of the sound field around any center x _c with a restricted bandwidth 2M + 1 can be represented as follows:

In this equation, r 'and α ' denote the location coordinates with respect to a local coordinate system whose origin is at x _c , J _μ denotes the μ-th order Bessel function, P denotes the decomposition coefficients of the sound field. However, to calculate the drive signal, the decomposition coefficients are required with respect to the decomposition around the origin of the global coordinate system whose axes are parallel to those of the local coordinate system. For this purpose, the addition theorem for harmonic cylinder functions can be used in the above equation:

Here, r _c and α _{c describe} the position of the local coordinate system in the global coordinate system. To reproduce P _M , the decomposition coefficients become P _{v, M} (ω) is substituted into the above equation for calculating the speaker driving signal D. Thus, two bandwidth limitations become clear: on the one hand, P _{M is} bandwidth-limited with respect to the decomposition by x _c , this limitation being denoted by M. Nevertheless, P _M has an infinite spatial bandwidth with respect to the decomposition around the coordinate origin. Furthermore, the drive signal is bandwidth limited with respect to the decomposition around the origin of coordinates. This restriction is denoted by N. The desired component of the reproduced sound field is thus limited twice the bandwidth.

Such a translation of the decomposition center can be carried out in real time, so that the reproduction can be tracked, for example, to a moving receiver. In such a system, for example, a camera can be used to determine the position of the receiver and then optimize the playback to that position. The determination of the position of the receiver can also be done by a sensor that the listener carries, for example, in his pocket with it.

In Fig. 2 is shown the sound field generated by the speakers. The loudspeakers are located on the circular contour indicated by dashed lines, which surrounds the target PA range. In Fig. 2a the sound field is shown to result from the analytical determination by the method described above. Here, the sweet spot shown by the marked point is at the center of the contour.

However, if a recipient moves to the in Fig. 2b marked point within the target PA range, the present invention allows the sweet spot inherent in the system to track so that the sound field reproduction is optimal for the selected receiver. The corresponding sound field is in Fig. 2b shown, the only shifted sweet spot in turn marked with a cross.

Claims (10)

1. Method for reproducing a sound field in a target region for acoustic irradiation by means of multiple electroacoustic transducers arranged on a contour surrounding the target region for acoustic irradiation, having the steps:

   - analytical determination of a drive signal for the electroacoustic transducers through orthogonal decomposition of the sound field to be reproduced about the center of the contour and spatial band limiting of the sound field to be reproduced with respect to the decomposition in such a manner that the resulting sound field corresponds to the sound field to be reproduced in a region about the center of the contour;

   - alteration of the drive signal by displacing the center of the decomposition to a desired position and spatial band limiting of the sound field to be reproduced with respect to the decomposition displaced to the desired position by means of a mathematical translation theorem so that the region where the resulting sound field corresponds to the sound field to be reproduced is displaced to the desired position within the target region for acoustic irradiation; and

   - delivery of the altered control signal to the electroacoustic transducers.
2. Method according to claim 1, wherein the drive signal is determined by analytical solution of an integral equation describing the sound field to be reproduced.
3. Method according to claim 2, wherein the integral equation is a surface or line integral over the control signal for the electroacoustic transducers and the spatial transfer function.
4. Method according to claim 3, wherein the solution of the integral equation is accomplished through a decomposition of the sound field, the drive signal, and the transfer function into orthogonal basis functions.
5. Method according to claim 1, wherein the drive signal is determined through the relationships of the Kirchhoff-Helmholtz integral.
6. Method according to any one of the preceding claims, wherein the displacement to a desired position takes place in real time.
7. Method according to claim 6, wherein the desired position corresponds to the position of a moving receiver, wherein the desired position follows the position of the receiver.
8. Device for reproducing a sound field in a target region for acoustic irradiation (10), in particular by means of a method according to any one of the preceding claims, having multiple electroacoustic transducers (20) and a processing module (30) for delivering a drive signal to the electroacoustic transducers (20), wherein the processing module (30) is adapted to determine the drive signal analytically through orthogonal decomposition of the sound field to be reproduced about the center of the contour and through spatial band limiting of the sound field to be reproduced with regard to the decomposition in such a manner that the resulting sound field in a region about the center of the contour (11) corresponds to the sound field to be reproduced and to alter the drive signal by displacing the center of the decomposition to a desired position and spatial band limiting of the sound field to be reproduced with respect to the decomposition displaced to the desired position by means of a mathematical translation theorem in such a manner that the region where the resulting sound field corresponds to the sound field to be reproduced is displaced to the desired position within the target region for acoustic irradiation (10).
9. Device according to claim 8, further having an amplifier unit (31) for amplifying the altered drive signal determined by the processing module (30) and for delivering the amplified signals to the electroacoustic transducers (20).
10. Device according to claim 8 or 9, further having an audio reproduction device for providing an audio signal describing the sound field.

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