The present invention relates to wave-field synthesis systems and particularly to an apparatus and a method for calculating filter coefficients for a predefined loudspeaker arrangement.
There is an increasing demand for new technologies and innovative products in the field of consumer electronics. Here, it is important prerequisite for the success of new multimedia systems to offer optimum functionalities or capabilities, respectively. This is achieved by the usage of digital technologies and particularly computer technology. Examples therefore are applications offering an improved realistic audiovisual impression. In known audio systems, a significant weak point is the quality of the spatial sound reproduction of real but also virtual environments.
Methods for multichannel loudspeaker reproduction of audio signals have been known and standardized for many years. All common techniques have the disadvantage that both the location of the loudspeakers and the position of the listener are already imprinted in the transmission format. If the loudspeakers are positioned in a wrong way with regard to the listener, the audio quality suffers significantly. An optimum sound is only possible in a very small area of the reproduction room, the so-called sweet spot.
An improved natural spatial impression as well as stronger enclosure during audio reproduction can be obtained with the help of new technology. The basics of this technology, the so called wave-field synthesis (WFS) have been researched at the TU Delft and have been presented for the first time in the late 80ies (Berkhout, A. J.; de Vries, D; Vogel, P.: Acoustic control by Wave-field Synthesis. JASA 93, 1993).
The basic idea of WFS is based on the application of the Huygens principle of the wave theory.
Every point captured by a wave is the starting point of an elementary wave, which propagates in a spherical or circular way.
Applied to acoustics, any form of an incoming wave front can be reproduced by a large number of loudspeakers arranged next to another (a so called loudspeaker array). In the simplest case, a single point source to be reproduced and a linear arrangement of the loudspeakers, the audio signals of every loudspeaker have to be fed with a time delay and amplitude scaling such that the emitted sound fields of the individual loudspeakers overlay properly. With several sound sources, the contribution to every loudspeaker is calculated separately for every source and the resulting signals are added. In a virtual space with reflecting walls, the reflections can also be reproduced via the loudspeaker array as additional sources. Thus, the calculation effort depends heavily on the number of sound sources, the reflection characteristics of the recording room and the number of loudspeakers.
The particular advantage of this technique is that a natural spatial sound impression is possible across a large area of the reproduction room. In contrary to the known techniques, direction and distance from the sound sources are reproduced very accurately. To a limited degree, virtual sound sources can even be positioned between the real loudspeaker array and the listener.
The technique of wave-field synthesis can also be used advantageously to add a corresponding spatial audio perception to a visual perception. So far, during production in virtual studios, the focus was on the production of an authentic visual impression of the virtual scene. The acoustic impression matching the image is normally imprinted on the audio signal afterwards by manual operating steps in the so-called post production or is considered to be too expensive and too time-consuming to realize and is thus neglected. This causes normally a discrepancy between individual sense impressions, which causes the designed space, i.e. the designed scene, to be considered as less authentic.
For reproduction of surround sound, corresponding reproduction systems with a series of loudspeakers, which are arranged around the listener, are used. Each loudspeaker receives its own audio signal in a way, so that a spatial scene is established by the super position of the loudspeaker signals. In this process a mapping of the source data (audio and meta data) to the loudspeaker signals is done, wherein the target loudspeaker arrangement is usually known.
If an ideal or optimal arrangement of the loudspeakers is available for the reproduction system, this arrangement should also be used for the real loudspeaker arrangement. However, this is not possible every time, so that an incorrect reproduction may be caused. If the actual arranged loudspeaker setup differs from the ideal arrangement, reproduction errors may appear, which may falsify, for example, a localization of the sound source reproduced by the system.
For the calculation of the audio signals for surround sound mixes, audio signals of virtual sources are mapped to the existing loudspeaker arrangement. In this process the audio signals of the sources are linked with meta data, which influence the calculation (rendering) of the audio signals. Depending on the method, this meta data comprises for example direction information, 2D- or 3D-position information, information about the emission behavior of the source, etc. The calculation algorithm uses information about the arrangement positions of the loudspeakers and meta data of the sources for generating coefficients, which describe the mapping of the source audio data to the resulting loudspeaker signals.
A corresponding algorithm for generating corresponding coefficients is mostly easier to develop for ideal loudspeaker arrangements. However, it may not be possible for real existing loudspeaker arrangements to represent the ideal loudspeaker positions. For example, due to structural reasons, it may not be possible to locate loudspeakers at their ideal positions. Occasionally, it is not possible at all to place parts of the loudspeaker arrangement. So, the real loudspeaker arrangement may differ from its ideal example due to missing loudspeakers and/or loudspeakers shifted in space.
Examples for the calculation of filter coefficients for the reproduction of virtual sources by a loudspeaker arrangement, as used for example in the field of wave-field synthesis, are described in “Berkhout, A. J., de Vries, D., and Vogel, P. (1993). Acoustic control by wave field synthesis. Journal Acoustic Society of America, 93(5):2764-2778.” and “Röder, T., Sporer, T., and Brix, S. (2007). Wave field synthesis device and method for deriving an array of loudspeakers.” However, the corresponding published calculation methods assume that the actual existing loudspeaker arrangement is used for the execution of the algorithm, although this arrangement might not be suitable for calculation since these algorithms do not provide handling for non-ideal loudspeaker placements or gaps in the speaker arrays.
The problem, that for the reproduction actual loudspeaker arrangements differ from an ideal arrangement, is made subject of discussion at various points and solutions are proposed. For example, “Jokinen, R. and Mäkivirta, A. (1997). A method and device for correcting the auditory image in a multichannel audio system” shows a possibility for correcting an incorrect positioned surround loudspeaker arrangement by delaying audio signals or by taking care of a listening position deviating from the system center. In “Goodwin, M. M. and Jot, J.-M. (2008). Multichannel surround format conversion and generalized upmix” the directions for different frequency components within the signal are reconstructed from the output signals for determined loudspeaker positions and distributed to the actual positioned loudspeakers, so that the original direction impression of the sound is kept as good as possible. In “Bruno, R., Laborie, A., and Montoya, S. (2006). Method and device for controlling a reproduction unit using a multi-channel signal” existing audio signals, which should be reproduced from different directions for a reconstruction of a sound field, are distributed to loudspeakers, whose positions are not corresponding to the optimal reproduction conditions. Common to all these examples is that it is assumed that the complete sound mixture exists as starting material, whose signals should have fixed set directions (as for example as position setups for loudspeakers according to the norm “5.1 ITU-R BF 775-1”).
Another motivation for reducing the channel number of multi-channel mixtures may be found in the field of coding data formats for a digital transmission. A method for saving transmission bandwidth is described for example in “Herre, J. and Faller, C. (2008). Apparatus and method for constructing a multi-channel output signal or for generating a downmix signal.” The channel number is reduced and audio data and meta data are created, which may be used by a decoder to reconstruct the original signal as purely as possible. Such methods need also input data, which represent final mixed loudspeaker signals.
In other publications, non-existing loudspeakers are handled as virtual sources. For example, in “Kuhn, C., Pellegrini, R., Rosenthal, M., and Corteel, E. (2008). Method and system for producing a binaural impression using loudspeakers”, initially an audio source is calculated for an arrangement of virtual loudspeakers and its signals in combination with their positions are again assumed as virtual sources and are finally reproduced on an real loudspeaker arrangement. Also in “Strauss, M. and Hörnlein, T. (2008). Device and method for generating a number of loudspeaker signals for a loudspeaker array which defines a reproduction area” an arrangement of loudspeakers of a non-existing surround system is assumed as virtual sound sources of a real existing loudspeaker arrangement, wherein the number of loudspeakers of the virtual system to be simulated is smaller than the number of actual existing loudspeakers. For such simulations of virtual sources, the wave-field synthesis seems to be a suitable reproduction method to approximate the positions of the virtual loudspeakers in a sufficiently exact manner.
Disadvantages of known methods are the high computational efforts for calculating the filter coefficients of the loudspeaker arrangements and/or a poor audio quality of reproduced audio signals.