EP1502475B1 - Method and system of representing a sound field - Google Patents

Abstract

A method of representing a sound field includes the steps of acquiring measurement signals (c<SUB>n</SUB>) which are delivered by simple sensors (2<SUB>n</SUB>) that are exposed to sound field (P) determining encoding filters which are representative of at least the structural characteristics of the sensors and processing the measurement signals (c<SUB>n</SUB>) by applying the encoding filters to the signals (c<SUB>n</SUB>), in order to determine a finite number of representative coefficients over time and in the three-dimensional space of the sound field (P).

Description

The present invention relates to a method and a representation device an acoustic field from signals delivered by means acquisition.

Processes and systems of acquisition and representation Existing sound environments use modelizations based on means of acquisition that are physically unfeasible, particularly as regards the electro-acoustic and / or structural characteristics of these means acquisition.

The acquisition means are, for example, constituted of a set measuring elements or elementary sensors arranged in specific places of space and exhibiting electro-acoustic characteristics intrinsic acquisition.

Existing systems are limited by structural features acquisition means, such as the physical arrangement of the elementary sensors as well as by their electro-acoustic characteristics, and deliver degraded representations of the sound environment to acquire.

For example, systems grouped under the term "Ambisonic" do not consider that the directions of provenance of the sounds in relation to the center of acquisition means formed of a plurality of elementary sensors, which leads to assimilate the acquisition means to a punctual microphone.

However, the impossibility of positioning all the elementary sensors at the same point limit the performance of these systems.

In addition these systems represent the sound environment by a modeling of virtual sources whose angular distribution around the center theoretically allows to obtain such a sound environment.

However, the lack of availability of elementary sensor characteristics of high directivity restricts these systems to a level of accuracy of representation commonly called order one on a mathematical basis called base of spherical harmonics.

In other systems, such as the one implementing the method and the acquisition device described in the patent application WO-01-58209, the acquisition is based on the measurement, in a plan, of representative information of the sound environment to acquire.

However, these systems use modelizations based on perfect elementary sensors necessarily arranged on a circle and result to a significant amplification of the background noise of the sensors.

These systems therefore require sensors whose intrinsic background noise is extremely weak, and therefore unrealistic in practice.

In addition, in these systems, the sound environment is described only by two-dimensional modeling, which corresponds to an approximation important and reducing real sound characteristics.

It therefore appears that representations made of sound environments existing systems are incomplete and degraded and no system does not allow to obtain a faithful representation.

The object of the invention is to solve this problem by providing a method and a device delivering a representation of the acoustic field substantially independent of the characteristics of the means of acquisition.

• une étape de détermination de filtres d'encodage représentatifs de caractéristiques au moins structurelles desdits moyens d'acquisition ; et
• une étape de traitement desdits signaux de mesure par l'application desdits filtres d'encodage à ces signaux pour déterminer un nombre fini de coefficients représentatifs dans le temps et dans les trois dimensions de l'espace dudit champ acoustique, lesdits coefficients permettant d'obtenir une représentation dudit champ acoustique sensiblement indépendante des caractéristiques desdits moyens d'acquisition.

The subject of the present invention is a method for representing an acoustic field comprising a step of acquiring measurement signals delivered by acquisition means formed from one or more elementary sensors exposed to said acoustic field, characterized in that 'it comprises :

• a step of determining encoding filters representative of at least structural characteristics of said acquisition means; and
• a step of processing said measurement signals by applying said encoding filters to these signals to determine a finite number of representative coefficients in time and in the three dimensions of the space of said acoustic field, said coefficients making it possible to obtain a representation of said acoustic field substantially independent of the characteristics of said acquisition means.

• lesdites caractéristiques structurelles comportent au moins des caractéristiques de position desdits capteurs élémentaires par rapport à un point de référence prédéterminé desdits moyens d'acquisition ;
• lesdits filtres d'encodage sont en outre représentatifs de caractéristiques électro-acoustiques desdits moyens d'acquisition ;
• lesdites caractéristiques électro-acoustiques comportent au moins des caractéristiques liées aux capacités électro-acoustiques d'acquisition intrinsèques desdits capteurs élémentaires ;
• lesdits coefficients permettant d'obtenir une représentation du champ acoustique sont des coefficients dits de Fourier-Bessel et/ou des combinaisons linéaires de coefficients de Fourier-Bessel ;
• ladite étape de détermination des filtres d'encodage comprend :
  • une sous-étape de détermination d'une matrice d'échantillonnage représentative des capacités d'acquisition desdits moyens d'acquisition ;
  • une sous-étape de détermination d'une matrice d'intercorrélation représentative de la ressemblance entre lesdits signaux de mesure délivrés par les capteurs élémentaires formant lesdits moyens d'acquisition ; et
  • une sous-étape de détermination d'une matrice d'encodage à partir de ladite matrice d'échantillonnage, de ladite matrice d'intercorrélation, et d'un paramètre représentatif d'un compromis souhaité entre la fidélité de représentation du champ acoustique et la minimisation du bruit de fond induit par les moyens d'acquisition, laquelle matrice est représentative desdits filtres d'encodage ;
• lesdites sous-étapes de détermination des matrices sont réalisées pour un nombre fini de fréquences de fonctionnement ;
• ladite étape de détermination de la matrice d'échantillonnage est réalisée, pour chacun desdits capteurs élémentaires formant lesdits moyens d'acquisition, à partir :
  • de paramètres représentatifs de la position dudit capteur par rapport au centre desdits moyens d'acquisition ; et/ou
  • d'un nombre fini de coefficients représentatifs des capacités d'acquisition dudit capteur ;
• ladite étape de détermination de la matrice d'échantillonnage est réalisée en outre à partir au moins d'un des paramètres parmi :
  • des paramètres représentatifs des réponses en fréquence de tout ou partie des capteurs ;
  • des paramètres représentatifs des diagrammes de directivité de tout ou partie des capteurs ;
  • des paramètres représentatifs des orientations de tout ou partie des capteurs, à savoir de leur direction de sensibilité maximale ;
  • des paramètres représentatifs des densités spectrales de puissance du bruit de fond de tout ou partie des capteurs ;
  • d'un paramètre spécifiant l'ordre auquel est conduite la représentation ; et
  • d'un paramètre représentatif d'une liste de coefficients dont on exige que la puissance soit égale à la puissance du coefficient correspondant dans le champ acoustique à représenter ;
• il comporte une étape de calibrage permettant de délivrer tout ou partie des paramètres utilisés dans ladite étape de détermination des filtres d'encodage ;
• ladite étape de calibrage comporte, pour au moins l'un desdits capteurs élémentaires formant lesdits moyens d'acquisition :
  • une sous-étape d'acquisition de signaux représentatifs des capacités d'acquisition dudit au moins un capteur ; et
  • une sous-étape de détermination de paramètres représentatifs de caractéristiques électro-acoustiques et/ou structurelles dudit au moins un capteur ;
• ladite étape de calibrage comporte en outre :
  • une sous-étape d'émission d'un champ acoustique spécifique vers ledit au moins un capteur, ladite sous-étape d'acquisition correspondant à l'acquisition des signaux délivrés par ce capteur lorsque exposé audit champ acoustique spécifique ; et
  • une sous-étape de modélisation dudit champ acoustique spécifique en un nombre fini de coefficients afin de permettre la réalisation de ladite sous-étape de détermination de paramètres représentatifs de caractéristiques électro-acoustiques et/ou structurelles du capteur ;
• ladite étape de calibrage comporte une sous-étape de réception d'un nombre fini de signaux représentatifs des caractéristiques électro-acoustiques et structurelles desdits capteurs formant lesdits moyens d'acquisition, lesquels signaux sont directement utilisés lors de ladite sous-étape de détermination des caractéristiques électro-acoustiques et/ou structurelles desdits moyens d'acquisition ; et
• il comporte une étape de saisie permettant de déterminer tout ou partie des paramètres utilisés lors de ladite étape de détermination des filtres d'encodage.

According to other characteristics:

• said structural features include at least positional characteristics of said elementary sensors with respect to a predetermined reference point of said acquisition means;
• said encoding filters are further representative of electro-acoustic characteristics of said acquisition means;
• said electro-acoustic characteristics comprise at least characteristics related to the intrinsic electro-acoustic acquisition capabilities of said elementary sensors;
• said coefficients making it possible to obtain a representation of the acoustic field are so-called Fourier-Bessel coefficients and / or linear combinations of Fourier-Bessel coefficients;
• said step of determining the encoding filters comprises:
  • a substep of determining a sampling matrix representative of the acquisition capabilities of said acquisition means;
  • a sub-step of determining an intercorrelation matrix representative of the similarity between said measurement signals delivered by the elementary sensors forming said acquisition means; and
  • a sub-step of determining an encoding matrix from said sampling matrix, said cross-correlation matrix, and a parameter representative of a desired compromise between fidelity of representation of the acoustic field and the minimizing the background noise induced by the acquisition means, which matrix is representative of said encoding filters;
• said substeps for determining matrices are performed for a finite number of operating frequencies;
• said step of determining the sampling matrix is performed, for each of said elementary sensors forming said acquisition means, from:
  • parameters representing the position of said sensor relative to the center of said acquisition means; and or
  • a finite number of coefficients representative of the acquisition capabilities of said sensor;
• said step of determining the sampling matrix is furthermore carried out from at least one of:
  • parameters representative of the frequency responses of all or some of the sensors;
  • parameters representative of the directivity diagrams of all or some of the sensors;
  • parameters representative of the orientations of all or some of the sensors, namely their direction of maximum sensitivity;
  • parameters representative of the spectral power densities of the background noise of all or some of the sensors;
  • a parameter specifying the order in which the representation is conducted; and
  • a parameter representative of a list of coefficients whose power is required to be equal to the power of the corresponding coefficient in the acoustic field to be represented;
• it comprises a calibration step for delivering all or part of the parameters used in said step of determining the encoding filters;
• said calibration step comprises, for at least one of said elementary sensors forming said acquisition means:
  • a substep of acquiring signals representative of the acquisition capabilities of said at least one sensor; and
  • a substep of determining parameters representative of electro-acoustic and / or structural characteristics of said at least one sensor;
• said calibration step further comprises:
  • a substep of transmitting a specific acoustic field to said at least one sensor, said substep of acquisition corresponding to the acquisition of the signals delivered by this sensor when exposed to said specific acoustic field; and
  • a modeling sub-step of said specific acoustic field in a finite number of coefficients to allow the realization of said substep of determining parameters representative of electro-acoustic and / or structural characteristics of the sensor;
• said calibration step comprises a substep of receiving a finite number of signals representative of the electro-acoustic and structural characteristics of said sensors forming said acquisition means, which signals are directly used during said substep of determining the characteristics electro-acoustic and / or structural of said acquisition means; and
• it comprises an input step for determining all or part of the parameters used during said step of determining the encoding filters.

The invention also relates to a computer program comprising program code instructions for executing the steps of the method, as previously described when said program is executed on a computer.

The invention also relates to a mobile support of the type comprising at least one processing processor and a non-volatile memory element, characterized in that said memory comprises a program comprising code instructions for performing the steps of the method as described above when said processor executes said program.

The invention also relates to a device for representing a acoustic field connectable to acquisition means formed of one or more elementary sensors delivering measurement signals when exposed acoustic field, characterized in that it comprises a module of measurement signal processing by applying representative encoding filters at least structural characteristics of said acquisition means to these measurement signals to deliver a signal that has a finite number of representative coefficients in time and in all three dimensions of space said acoustic field, said coefficients making it possible to obtain a representation of said acoustic field substantially independent of the characteristics said acquisition means,

• lesdits filtres d'encodage sont en outre représentatifs de caractéristiques électro-acoustiques desdits moyens d'acquisition ;
• il comporte en outre des moyens de détermination desdits filtres d'encodage représentatifs de caractéristiques structurelles et/ou électro-acoustiques desdits moyens d'acquisition ;
• lesdits moyens de détermination de filtres d'encodage reçoivent en entrée au moins l'un des paramètres parmi les paramètres suivants :
  • des paramètres représentatifs des positions par rapport au centre desdits moyens d'acquisition de tout ou partie des capteurs ;
  • un nombre fini de coefficients représentatifs des capacités d'acquisition de tout ou partie des capteurs ;
  • des paramètres représentatifs des réponses en fréquence de tout ou partie des capteurs ;
  • des paramètres représentatifs des diagrammes de directivité de tout ou partie des capteurs ;
  • des paramètres représentatifs des orientations de tout ou partie des capteurs, à savoir de leur direction de sensibilité maximale ;
  • des paramètres représentatifs des densités spectrales de puissance du bruit de fond de tout ou partie des capteurs ;
  • un paramètre représentatif du compromis souhaité entre la fidélité de représentation du champ acoustique et la minimisation du bruit de fond induit par les moyens d'acquisition ;
  • un paramètre spécifiant l'ordre auquel est conduit l'encodage ; et
  • un paramètre représentatif d'une liste de coefficients dont on exige que la puissance soit égale à la puissance du coefficient correspondant dans le champ acoustique à représenter ;
• il est associé à des moyens de détermination de tout ou partie des paramètres reçus par lesdits moyens de détermination des filtres d'encodage, lesdits moyens comportant au moins l'un des éléments suivants :
  • des moyens de saisie des paramètres ; et/ou
  • des moyens de calibrage ;
• il est associé à des moyens de mise en forme desdits signaux de mesure afin de délivrer un signal mis en forme correspondant.

According to other features of the device of the invention:

• said encoding filters are further representative of electro-acoustic characteristics of said acquisition means;
• it further comprises means for determining said encoding filters representative of structural and / or electro-acoustic characteristics of said acquisition means;
• said encoding filter determining means receives as input at least one of the following parameters:
  • parameters representative of the positions relative to the center of said acquisition means of all or part of the sensors;
  • a finite number of coefficients representative of the acquisition capacities of all or some of the sensors;
  • parameters representative of the frequency responses of all or some of the sensors;
  • parameters representative of the directivity diagrams of all or some of the sensors;
  • parameters representative of the orientations of all or some of the sensors, namely their direction of maximum sensitivity;
  • parameters representative of the spectral power densities of the background noise of all or some of the sensors;
  • a parameter representative of the desired compromise between the representation fidelity of the acoustic field and the minimization of the background noise induced by the acquisition means;
  • a parameter specifying the order in which the encoding is conducted; and
  • a parameter representative of a list of coefficients whose power is required to be equal to the power of the corresponding coefficient in the acoustic field to be represented;
• it is associated with means for determining all or part of the parameters received by said means for determining the encoding filters, said means comprising at least one of the following elements:
  • means for entering the parameters; and or
  • calibration means;
• it is associated with means for shaping said measurement signals in order to deliver a corresponding shaped signal.

• la Fig.1 est une représentation d'un repère sphérique ;
• la Fig.2 est un schéma représentatif de moyens d'acquisition utilisés ;
• la Fig.3 est un organigramme général du procédé de l'invention :
• la Fig.4 est un organigramme du détail d'un mode de réalisation de l'étape de calibrage du procédé de l'invention ;
• la Fig.5 est un organigramme du détail d'un mode de réalisation de l'étape de détermination des filtres d'encodage du procédé de l'invention ;
• la Fig.6 est un schéma du détail d'un mode de réalisation de l'étape d'application des filtres d'encodage ; et
• la Fig.7 est un schéma synoptique d'un dispositif adapté pour la mise en oeuvre du procédé de l'invention.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings, in which:

• Fig.1 is a representation of a spherical landmark;
• Fig.2 is a representative diagram of acquisition means used;
• Fig.3 is a general flowchart of the method of the invention:
• Fig.4 is a flowchart of the detail of an embodiment of the calibration step of the method of the invention;
• Fig.5 is a flowchart of the detail of an embodiment of the step of determining the encoding filters of the method of the invention;
• Fig.6 is a diagram of the detail of an embodiment of the step of applying the encoding filters; and
• Fig.7 is a block diagram of a device adapted for carrying out the method of the invention.

In FIG. 1, there is shown a conventional spherical landmark of to specify the coordinate system referred to in the text.

This reference is an orthonormal reference, of origin O and having three axes ( OX ), ( OY ) and ( OZ ).

In this reference, a noted position x is described by means of its spherical coordinates ( r , , ), where r denotes the distance from the origin O ,  the orientation in the vertical plane and  the orientation in the horizontal plane.

In such a reference, an acoustic field is known if one defines at every point at each instant t the acoustic pressure denoted by p (r, , , t ), whose Fourier transform is denoted P ( r,  , , f ) where f is the frequency.

The method of the invention is based on the use of spatio-temporal functions to describe any acoustic field in the time and in the three dimensions of space.

In the described embodiments, these functions are functions so-called Fourier-Bessel spherical of the first kind, later called Fourier-Bessel functions.

In a zone empty of sources and empty of obstacles, the functions of Fourier-Bessel correspond to the solutions of the wave equation and constitute a base that generates all the acoustic fields produced by sources located outside this area.

Any three-dimensional acoustic field can therefore be expressed by a linear combination of the Fourier-Bessel functions, according to the expression of the inverse Fourier-Bessel transform which expresses:

In this equation, the terms P _{l , m} ( f ) are defined as the Fourier-Bessel coefficients of the field p ( r , , , t ), k = 2π f / c , c is the speed of sound in the (340 ms ^-1 ), j _l ( kr ) is the spherical Bessel function of the first kind of order l defined by j l ( x ) = π 2 x J l +1/2 ( x ) where J _ν ( x ) is the Bessel function of the first kind of order v , and y m / l (, ) is the real spherical harmonic of order l and of term m, with m ranging from -l at l , defined by: there m l (, ) = P | m | l (Cos) trg m () with:

In this equation, the P m / l ( x ) are the associated Legendre functions defined by: P m l ( x ) = 2 l +1 2 ( l - m )! ( l + m )! (1- x 2 ) m / 2 d m d x m P l ( x ) with P _l ( x ) the Legendre polynomials, defined by: P l ( x ) = 1 d l 2 l l ! d x l ( x 2 - 1) l

The Fourier-Bessel coefficients are also expressed in the time domain by the coefficients p _{l , m} ( t ) corresponding to the inverse time Fourier transform of the coefficients P _{l , m} ( f ).

In other embodiments, the acoustic field is decomposed based on functions, where each of the functions is expressed by a possibly infinite linear combination of Fourier-Bessel functions.

FIG. 2 diagrammatically shows acquisition means 1 formed of N elementary sensors 2 ₁ to 2 _N.

These elementary sensors are arranged at specific points of the space around a predetermined point 4 designated as the center of the means of acquisition 1.

Thus, the position of each elementary sensor can be expressed in the space in a spherical coordinate system such as that described with reference to FIG. centered on the center 4 of the acquisition means 1.

When exposed to an acoustic field P, each sensor 2 _n of acquisition means 1 delivers a measurement signal c _n which corresponds to the measurement made by this sensor in the acoustic field P.

Thus, the acquisition means 1 deliver a plurality of signals c ₁ to c _N which are the measurement signals of the acoustic field P by the acquisition means 1.

These measurement signals c ₁ to c _N delivered by the acquisition means 1 are therefore directly related to the acquisition capabilities of the elementary sensors 2 ₁ to 2 _N.

FIG. 3 shows a general flow chart of the process of the invention.

The method starts with a step 10 of entering parameters and a step 20 of calibrating the acquisition means, which make it possible to define a set parameters representative of the structural characteristics and / or electro-acoustic acquisition means 1.

Some parameters and in particular representative parameters of Electroacoustic characteristics are dependent on frequency.

The input step 10 and the calibration step 20, which is described more in detail with reference to FIG. 4, can be carried out simultaneously or in any order.

Similarly, the method of the invention may comprise only step 10 input.

• des paramètres x _n représentatifs de la position du capteur 2 _n par rapport au centre 4 des moyens d'acquisition 1, qui s'écrivent en coordonnées sphériques (r _n , _n , _n );
• des paramètres d _n (f) représentatifs du diagramme de directivité du capteur 2 _n qui peut prendre toutes les valeurs entre 0 et 1 et permet de décrire la directivité du capteur 2 _n par une combinaison de diagrammes omnidirectionnels et de diagrammes bidirectionnels :
• si d _n (f) = 0, le capteur est omnidirectionnel
• si d _n (f) = ½, le capteur est cardioïde
• si d _n (f) = 1, le capteur est bidirectionnel ;
• des paramètres α _n (f) représentatifs de l'orientation du capteur 2 _n c'est-à-dire de sa direction de sensibilité maximale qui est donnée par le couple d'angles ( α / n, α / n)(f) ;
• des paramètres H _n (f) représentatifs de la réponse en fréquence du capteur 2 _n correspondant, pour chaque fréquence f, à la sensibilité du capteur 2 _n dans la direction α _n (f) ;
• des paramètres σ² _n (f) représentatifs de la densité spectrale de puissance du bruit de fond du capteur 2 _n ;
• des paramètres B _n,l,m (f) représentatifs des capacités d'acquisition du capteur 2 _n , c'est-à-dire de la façon dont le capteur 2 _n prélève des informations sur le champ acoustique P. Ainsi chaque B _n,l,m (f) est représentatif des capacités d'acquisition d'un capteur et notamment de sa position dans l'espace et l'ensemble des B _n,l,m (f) est représentatif de l'échantillonnage du champ acoustique P réalisé par les moyens d'acquisition 1 ;
• un paramètre µ(f) spécifiant un compromis entre la fidélité de représentation du champ acoustique P et la minimisation du bruit de fond apporté par les capteurs 2₁ à 2 _N et pouvant prendre toutes les valeurs entre 0 et 1 :
  • si µ(f) = 0, le bruit de fond est minimal ;
  • si µ(f) = 1, la qualité spatiale est maximale ;
• un paramètre L(f) spécifiant l'ordre auquel est conduite la représentation ; et
• un paramètre {(l _k ,m _k )}(f) représentatif d'une liste de coefficients dont on exige que la puissance soit égale à la puissance du coefficient correspondant dans le champ acoustique à représenter.

The input and 20 calibration steps make it possible to determine, for one or more sensors, all or part of the following parameters:

• parameters x _n representative of the position of the sensor 2 _n with respect to the center 4 of the acquisition means 1, which are written in spherical coordinates ( r _n ,  _n ,  _n );
• parameters d _n ( f ) representative of the directivity diagram of the sensor 2 _n which can take all the values between 0 and 1 and makes it possible to describe the directivity of the sensor 2 _n by a combination of omnidirectional diagrams and bidirectional diagrams:
• if d _n ( f ) = 0, the sensor is omnidirectional
• if d _n ( f ) = ½, the sensor is cardioid
• if d _n ( f ) = 1, the sensor is bidirectional;
• parameters α _n ( f ) representative of the orientation of the sensor 2 _n, that is to say of its direction of maximum sensitivity which is given by the pair of angles ( α / n ,  α / n ) ( f );
• parameters H _n ( f ) representative of the frequency response of the sensor 2 _n corresponding, for each frequency f, to the sensitivity of the sensor 2 _n in the direction α _n ( f );
• parameters σ ² _n ( f ) representative of the spectral power density of the background noise of the sensor 2 _n ;
• parameters B _{n , l , m} ( f ) representative of the acquisition capacitances of the sensor 2 _n , that is to say of the way in which the sensor 2 _n takes information on the acoustic field P. Thus, each B _{n , l , m} ( f ) is representative of the acquisition capacities of a sensor and in particular of its position in space and the set of B _{n , l , m} ( f ) is representative of the sampling of the acoustic field P realized by the acquisition means 1;
• a parameter μ ( f ) specifying a compromise between the fidelity of representation of the acoustic field P and the minimization of the background noise brought by the sensors 2 ₁ to 2 _N and which can take all the values between 0 and 1:
  • if μ ( f ) = 0, the background noise is minimal;
  • if μ ( f ) = 1, the spatial quality is maximal;
• a parameter L ( f ) specifying the order in which the representation is conducted; and
• a parameter {( l _k , m _k )} ( f ) representative of a list of coefficients whose power is required to be equal to the power of the corresponding coefficient in the acoustic field to be represented.

In simplified embodiments, all or part of the parameters described is considered independent of frequency.

The parameters μ ( f ), L ( f ) and {( l _k , m _k )} ( f ) are representative of the optimization strategies making it possible to control the extraction of spatio-temporal information from the acoustic field P from the measurement signals c ₁ to c _N and are input during the input step 10. The other parameters can be entered during the input step or determined during the calibration step.

In simplified embodiments, the method of the invention is realized only with the parameters μ ( f ), L ( f ) and all the parameters. x _n or all the parameters B _{n , l , m} ( f ) or a combination of parameters x _n and B _{n , l , m} ( f ), so as to have at least one parameter per elementary sensor 2 _n .

Of course, all or some of the parameters used can be delivered by memories or dedicated devices, these techniques being comparable to step 10 of direct entry by an operator as described.

At the end of the capture and / or calibration steps, the process comprises a step 30 of determining encoding filters representative of features at least structural and advantageously electro-acoustic acquisition means 1.

This step 30, described in more detail with reference to FIG. to take into account all the parameters determined during the input steps 10 and / or calibration.

These encoding filters are therefore representative of at least the positional characteristics of the elementary sensors 2 _n with respect to the reference point 4 of the acquisition means 1.

Advantageously, these filters are also representative of other structural characteristics of the acquisition means 1, such as the orientation of the elementary sensors 2 ₁ to 2 _N or their mutual influences, as well as their electroacoustic acquisition capabilities and especially their background noise, their directivity pattern, their frequency response, ...

The encoding filters obtained at the end of step 30 can be stored, so that steps 10, 20 and 30 are repeated only in case of modification of acquisition means 1 or optimization strategies.

These encoding filters are applied during a step 40 of processing signals c ₁ to c _N from the elementary sensors 2 ₁ to 2 _N.

This treatment corresponds to a filtering of the signals and to combinations filtered signals.

At the end of this step 40 of processing of the measurement signals by the application of the encoding filters to these signals, a finite number of coefficients representative in time and in three dimensions of the space of the field acoustic P is delivered.

These coefficients are so-called Fourier-Bessel coefficients, noted P _{l , m} ( f ) and correspond to a representation of the acoustic field P substantially independent of the characteristics of the acquisition means 1.

It therefore appears that thanks to the method of the invention, we obtain a faithful representation of the acoustic field whose characteristics are transcribed temporal and spatial, whatever the means of acquisition used.

FIG. 4 shows a flowchart of an embodiment of step 20 of calibration.

In this embodiment, the calibration step 20 makes it possible to directly determine the coefficients B _{n , l , m} ( f ) representative of the acquisition capacities of the acquisition means 1.

This step 20 begins with a sub-step 22 of issuing a specific acoustic field towards the acquisition means 1 and by a substep 24 acquisition of measurement signals by the acquisition means 1 exposed to the acoustic field emitted.

These substeps 22 and 24 are repeated for a plurality Q of different specific fields and require means for generating specific acoustic fields and means of displacement and / or rotation of the acquisition means 1.

For example, the calibration step 20 is implemented using means for generating an acoustic field that comprise only a loudspeaker fixed, assumed punctual and flat frequency response, the speaker and the acquisition means 1 being placed in an anechoic environment.

At each sub-step 22 of generation, the speaker emits the same acoustic field and the acquisition means 1 are placed in the same position but they are oriented in different and known directions.

Of course, it is also possible to move the speaker.

Thus, in the reference of the acquisition means 1, the speaker is in a position ( r hp / q ,  hp / q ,  hp / q ) different for each generated q field.

The acquisition means 1 are thus exposed to an acoustic field q whose Fourier-Bessel coefficients P _{l , m , q} ( f ), in the reference of the acquisition means 1, are known up to a given order, noted L ₃ .

In the embodiment described, the measurement signals delivered following the acquisition substep 24 are a finite number of coefficients representative of the acoustic field q generated, as well as acquisition capacities of the acquisition means 1. .

Parameters L ₃ and Q are chosen to respect the condition: Q ≥ ( The 3 +1) 2

Advantageously, subsequently, the method comprises a modeling sub-step 26 for determining a representation of the Q acoustic fields emitted during the sub-step 22.

Thus, during the sub-step 26, a modeling matrix P representative of all the known fields Q to which the acquisition means 1 are successively exposed is determined. This matrix P is a matrix of size ( L ₃ +1) ² on Q consisting of the elements P _{l , m , q} ( f ), the indices ( l , m ) denoting the line l ² + l + m and the index q designating the column q . The matrix P thus has the following form:

In the embodiment described, the acoustic field produced by the loudspeaker is modeled by spherical radiation, thus, in the reference of the acquisition means 1, the coefficients P _{l , m , q} ( f ) of each acoustic field. q thus generated are known, thanks to the relation: P l, m, q ( f ) = 1 r hp q e - j 2π r hp q f vs ξ l ( r hp q , f ) there m l ( hp q ,  hp q ) with

The coefficients obtained during the substep 26 are then used during a substep 28 to determine parameters representative of structural and / or acoustic characteristics of the acquisition means 1.

In the embodiment described, this substep 28 also uses the modeling matrix P determined during the substep 26.

This substep 28 begins with the determination of a matrix C representative of all the signals c _{n , q} ( t ) collected at the output of the N sensors in response to the Q known fields. This matrix C is a matrix of size N on Q consisting of the elements C _{n , q} ( f ), the index n designating the line n and the index q designating the column q . The elements C _{n , q} ( f ) are deduced from the signals c _{n , q} ( t ) by Fourier transform. The matrix C thus has the following form:

The matrix C is representative of the acquisition capabilities of the acquisition means 1 and the Q transmitted acoustic fields.

In the embodiment described, during sub-step 28, the coefficients B _{n , l , m} ( f ) are determined from matrices C and P using conventional generalized matrix inversion methods applied to the relationship between C to P. for example, the coefficients B _{n, l, m} (f) are arranged in a matrix B determined by the following relationship: B = CP T ( PP T ) -1 B is a matrix of size N over ( L ₃ +1) ² made up of the coefficients B _{n , l , m} ( f ), the index n denoting the line n and the indices ( l , m ) denoting the column l ² + l + m . The matrix B thus has the following form:

These substeps 26 and 28 are performed for each frequency of functioning and the coefficients thus determined constitute directly the parameters representative of the acquisition capacities of the means of acquisition 1.

The substeps 26 and 28 of the calibration step 20 can be performed in different ways, depending on the parameters to be determined.

For example, in the case where the calibration step 20 makes it possible to determine the position x _n of each sensor 2 _n , the substeps 26 and 28 exploit the delays of the waves emitted by the loudspeakers to reach the sensors 2 _n . The position of each sensor 2 _n is determined using at least three propagation time measurements according to triangulation methods.

In another case, the substeps 26 and 28 make it possible to determine, from the signals c _{n , q} ( t ), the impulse responses of each sensor 2 _n when the loudspeaker emits a given pulse.

For example, in this case the usual determination techniques are used. impulse responses, such as MLS (Maximum Lenght Sequence).

Advantageously, the calibration step 20 allows the determination of electro-acoustic characteristics of the sensors. It then begins by determining the directivity diagram of each sensor 2 _n for each frequency f considered, for example, by determining the frequency response of each sensor 2 _n for several directions.

• des paramètres α _n (f) représentatifs de l'orientation de chaque capteur 2 _n , c'est-à-dire de sa direction de sensibilité maximale donnée par les angles ( α / _n , α / _n )(f) pour lesquels le diagramme de directivité admet un maximum à la fréquence courante f;
• des paramètres H _n (f) représentatifs de la réponse en fréquence de chaque capteur 2 _n dans la direction de sensibilité maximale qui correspond donc à la valeur du diagramme de directivité pour la direction ( α / _n ,  α / _n )(f) ; et
• des paramètres d _n (f) représentatifs du diagramme de directivité de chaque capteur qui permet de décrire la directivité de chaque capteur par un modèle constitué d'une combinaison de diagrammes omnidirectionnels et de diagrammes bidirectionnels orientés selon la direction α _n (f), à l'aide du modèle de directivité suivant : 1 - d n (f) + d n (f) cos(α n (f).(,))

   où α _n (f).(,) désigne le produit scalaire entre les directions α _n (f) et (,).In a second step, all or some of the following parameters are determined:

• parameters α _n ( f ) representative of the orientation of each sensor 2 _n , that is to say of its direction of maximum sensitivity given by the angles ( α / _n ,  α / _n ) ( f ) for which the directivity diagram admits a maximum at the current frequency f ;
• parameters H _n ( f ) representative of the frequency response of each sensor 2 _n in the direction of maximum sensitivity, which corresponds to the value of the directivity diagram for the direction ( α / _n ,  α / _n ) ( f ); and
• parameters d _n ( f ) representative of the directivity diagram of each sensor which makes it possible to describe the directivity of each sensor by a model consisting of a combination of omnidirectional diagrams and bidirectional diagrams oriented along the direction α _n ( f ), to using the following pattern of directivity: 1 - d not ( f + d not ( f cos (α) not ( f ). (, ))

where α _n ( f ). (, ) denotes the dot product between the directions α _n ( f ) and (, ).

This parameter d _n ( f ) can be determined using the usual parameter estimation methods, for example by applying a least squares method providing the value of d _n ( f ) which minimizes the error between the real directivity and the modeled directivity diagram.

Advantageously, the calibration step 20 also makes it possible to determine the parameter σ ² _n ( f ) corresponding to the spectral power density of the background noise of the sensors. Thus, during this step 20, the signal delivered by the sensor 2 _n is collected in the absence of an acoustic field. The parameter σ ² _n ( f ) is determined by means of power spectral density estimation methods, for example the so-called periodogram method.

Depending on the embodiments, all or part of the sub-steps 22 to 28 is repeated, for example to allow the determination of several types parameters, some substeps that may be common to the determination different types of parameters.

The calibration step 20 can also be carried out by means other than those described such as direct measurements, for example by means of optical measurement means of the position of each elementary sensor 2 _n with respect to the center 4 of the sensors. means of acquisition 1.

In addition, the calibration step 20 can implement a simulation, for example using a computer, of signals representative of the acquisition capabilities of the elementary 2 _n sensors.

It therefore appears that this calibration step 20 makes it possible to determine all or part of the parameters representative of the structural characteristics and / or electro-acoustic acquisition means 1, which are used during step 30 of determining the encoding filters.

FIG. 5 shows a flowchart of an embodiment of step 30 of determining the encoding filters.

Step 30 comprises a substep 32 for determining a matrix B representative of the acquisition capacities of the acquisition means 1 or sampling matrix.

In the embodiment described, the matrix B is determined from the parameters x _n , H _n ( f ), d _n ( f ), α _n ( f ) and B _{n , l , m} ( f ) and is a matrix of size N on ( L ( f ) +1) ² consisting of elements B _{n , l , m} ( f ), the index n denoting the line n and the indices ( l , m ) denoting the column l ² + l + m . The matrix B thus has the following form:

Some elements of the matrix B can be directly determined during steps 10 or 20. The matrix B is then completed with elements determined from a modeling of the sensors.

In this embodiment, each sensor n is modeled by a point sensor placed at the position x _n , Having directivity composed of a combination of diagrams omnidirectional and bidirectional proportion d _n (f) oriented in the direction α _n (f) and having a frequency response H _n (f).

   où : j* l (kr n ) = l j l-1(kr n ) - (l+1) j l+1(kr n ) 2l+1

   et où : u r =sin n sinα n (f)cos( n -α n (f)) + cos n cosα n (f) u =cos n sinα n (f)cos( n -α n (f)) - sin n cosα n (f) u =sinα n (f)sin(α n (f)- n ) The complementary elements B _{n , l , m} ( f ) are then determined according to the relation:

or : j * l ( kr not ) = lj l -1 ( kr not ) - ( l +1) j l +1 ( kr not ) 2 l +1

and or : u r = sin not sin α not ( f ) Cos ( not - α not ( f )) + cos not cos α not ( f ) u  = cos not sin α not ( f ) Cos ( not -  α not ( f )) - sin not cos α not ( f ) u  = sin α not ( f ) Sin ( α not ( f ) - not )

In the case where the sensors are oriented radially, the relation admits a simpler expression:

Step 30 then comprises a sub-step 34 for determining an intercorrelation matrix A representative of the resemblance between the signals c ₁ to c _N delivered by the sensors 2 ₁ to 2 _N due to the fact that these sensors 2 ₁ at 2 _N make measurements on the same acoustic field P. The matrix A is determined from the sampling matrix B. A is a matrix of size N over N obtained by means of the relation: A = BB T

Advantageously, the matrix A is determined more precisely by using a matrix B completed to an order L ₂ according to the method of the preceding step.

Since the matrix A can be expressed as a function only of the matrix B, the sub-step 34 for determining the intercorrelation matrix A can be considered as an intermediate calculation step and can, as such, be integrated into another sub-step. step of step 30.

Step 30 then comprises a substep 36 for determining an encoding matrix E ( f ) representative of the encoding filters for a given frequency. The matrix E ( f ) is determined from the matrices A and B and the parameters L ( f ), μ ( f ), {( l _k , m _k )} ( f ) and σ 2 / n ( f ). The matrix E ( f ) is a matrix of size ( L ( f ) +1) ² on N consisting of elements E _{l , m , n} ( f ), the indices ( l , m ) denoting the line l ² + l + m and the index n designating the column n. The matrix E ( f ) thus has the following form:

The matrix E ( f ) is determined line by line. For each operating frequency f , each line E _{l , m} of index ( l , m ) of the matrix E ( f ) takes the following form: [ E l , m , 1 ( f ) E l , m 2 ( f ) ····· E L M n ( f )]

• si (l,m) appartient à la liste {(l _k ,m _k )}(f) alors :
  
     où λ vérifie la relation :
  
     et où la valeur de λ est déterminée au moyen de méthodes analytiques ou numériques de recherche de racines d'équations, en utilisant éventuellement des méthodes de diagonalisation de matrices ; et
• si (l,m) n'appartient pas à la liste {(l _k ,m _k )}(f), alors :
  

The elements E _{l , m , n} ( f ) of the line E _{l , m are} obtained by the following expressions:

• if ( l , m ) belongs to the list {( l _k , m _k )} ( f ) then:
  
  where λ verifies the relation:
  
  and where the value of λ is determined using analytical or numerical root search methods of equations, possibly using die diagonalization methods; and
• if ( l , m ) does not belong to the list {( l _k , m _k )} ( f ), then:
  

In these expressions, B _{l , m} is the column ( l , m ) of the matrix B and Σ _N is a diagonal matrix of size N on N representative of the background noise of the sensors where the element n of the diagonal is σ 2 / n ( f ).

The substeps 32, 34 and 36 for determining the matrices A, B and E ( f ) are repeated for each operating frequency f .

Of course, in simplified embodiments, the parameters are independent of the frequency and the substeps 32, 34 and 36 are performed once. Sub-step 36 then directly allows the determination of a matrix E independent of the frequency.

In a subsequent substep 38, FD parameters representative of the encoding filters are determined from the matrix E ( f ). Each element E _{l , m , n} ( f ) of the matrix E ( f ) represents the frequency response of an encoding filter. Each encoding filter can be described by the FD parameters in different forms.

• des réponses en fréquence, les paramètres FD sont alors directement les E _l,m,n (f) calculés pour certaines fréquences f ;
• des réponses impulsionnelles finies e _l,m,n (t) calculées par transformée de Fourier inverse de E _l,m,n (f), chaque réponse impulsionnelle e _l,m,n (t) est échantillonnée puis tronquée à une longueur propre à chaque réponse ; et
• des coefficients de filtres récursifs à réponses impulsionnelles infinies calculées à partir des E _l,m,n (f) avec des méthodes d'adaptation.

For example, the representative FD parameters of the filters E _{l , m , n} ( f ) are:

• frequency responses, the parameters FD are then directly the E _{l , m , n} ( f ) calculated for certain frequencies f ;
• finite impulse responses e _{l , m , n} ( t ) calculated by inverse Fourier transform of E _{l , m , n} ( f ), each impulse response e _{l , m , n} ( t ) is sampled and then truncated to a specific length each answer; and
• recursive filter coefficients with infinite impulse responses calculated from the E _{l , m , n} ( f ) with adaptation methods.

Thus, the step 30 for determining the encoding filters delivers FD parameters describing encoding filters representative of the at least structural and / or electroacoustic capabilities of the acquisition means 1.

• position des capteurs 2₁ à 2 _N ;
• caractéristiques électro-acoustiques intrinsèques des capteurs 2₁ à 2 _N , notamment densité spectrale de puissance du bruit de fond et capacités d'acquisition du champ acoustique ; et
• stratégies d'optimisation, notamment le compromis entre la fidélité spatiale d'acquisition du champ acoustique et la minimisation du bruit de fond apporté par les capteurs.

In particular, these filters are representative of the following characteristics:

• position of the sensors 2 ₁ to 2 _N ;
• intrinsic electro-acoustic characteristics of the sensors 2 ₁ to 2 _N , in particular spectral power density of the background noise and acquisition capabilities of the acoustic field; and
• optimization strategies, in particular the compromise between the spatial fidelity of acquisition of the acoustic field and the minimization of the background noise brought by the sensors.

FIG. 6 shows the detail of an embodiment of step 40 of processing the measurement signals delivered by the means acquisition 1 by applying the encoding filters to these signals and by summation filtered signals.

   où P _l,m (f) est la transformée de Fourier de p and _l,m (t) et C _n (f) est la transformée de Fourier de c _n (t).During step 40, the coefficients p and _{l , m} ( t ) representative of the acoustic field P are deduced from the signals c ₁ to c _N originating from the elementary sensors 2 ₁ to 2 _N , by the application of the filters d frequency response encoding E _{l , m , n} ( f ) as follows:

where P _{l , m} ( f ) is the Fourier transform of p and _{l, m} ( t ) and C _n ( f ) is the Fourier transform of c _n ( t ).

In the example, the case of finite impulse response filtering has been described. This filtering requires the initial determination of a parameter T _{n, l, m} , corresponding to the number of samples specific to each response e _{n, l, m} ( t ), which leads to the following convolution expression:

These coefficients p and _{l , m} are a finite number of coefficients representative in time and in the three dimensions of the space of the acoustic field and constitute a faithful representation of this acoustic field.

• si les paramètres FD fournissent directement les réponses en fréquence E _l,m,n (f), le filtrage est effectué au moyen de méthodes de filtrage dans le domaine fréquentiel comme par exemple, des techniques de convolution par blocs ;
• si les paramètres FD fournissent la réponse impulsionnelle finie e _l,m,n (t), le filtrage est effectué dans le domaine temporel par convolution ; et
• si les paramètres FD fournissent les coefficients d'un filtre récursif à réponse impulsionnelle infinie, le filtrage est effectué dans le domaine temporel au moyen de la relation de récurrence.

According to the nature of the parameters FD, other filtering by E _{l, m, n} ( f ) can be carried out according to different filtering methods, such as for example:

• if the parameters FD directly supply the frequency responses E _{l, m, n} ( f ), the filtering is performed by means of filtering methods in the frequency domain such as, for example, block convolution techniques;
• if the parameters FD provide the finite impulse response e _{l, m, n} ( t ), the filtering is done in the time domain by convolution; and
• if the FD parameters provide the coefficients of an infinite impulse response recursive filter, the filtering is performed in the time domain by means of the recurrence relation.

It therefore appears that the invention makes it possible to faithfully represent a acoustic field by a representation substantially independent of the characteristics acquisition means in the form of Fourier-Bessel coefficients.

Moreover, as has been said previously, the method of the invention can be implemented in simplified embodiments.

For example, if all the sensors 2 ₁ to 2 _N are substantially omnidirectional and substantially identical in sensitivity and in background noise level, the method of the invention can be implemented using only the knowledge of the parameters. x _n representative of the position of the sensors 2 _n with respect to the center 4 of the acquisition means 1 and the parameters μ and L relating to the optimization strategy.

In addition, in this simplified embodiment, it is considered that the parameters are independent of the frequency.

So, using these parameters; the matrices A and B are calculated simultaneously or sequentially in any order during the substeps 32 and 34.

   avec : B n,l,m (f)=4πj l j l (kr n )y m l ( n , n ) The elements B _{n , l , m} ( f ) of the matrix B are then organized as follows:

with: B not , l , m ( f ) = 4π j l j l ( kr not ) there m l ( not ,  not )

Similarly, the elements A _{n 1 , n 2} ( f ) of the matrix A are then organized as follows:

In this embodiment, the matrix A is obtained from the matrix B by means of the relation: AT = BB T

   où L ₂ est l'ordre auquel est conduite la détermination de la matrice A et est un entier supérieur à L. Plus L ₂ sera choisi grand, plus le calcul des A _{n 1,n 2} (f) sera précis mais long.Advantageously, the elements A _{n 1} , _{n 2} ( f ) of the matrix A are determined with a better precision by the relation:

where L ₂ is the order in which the determination of the matrix A is conducted and is an integer greater than L. Plus L ₂ will be chosen large, plus the calculation of the A _{n 1 , n 2} ( f ) will be precise but long.

In the sub-step 36, the encoding matrix E representative of the encoding filters is determined from the matrices A and B and the parameter μ according to the expression: E = μ B T (μ AT + (1-μ) I NOT ) -1

The elements E _{l , m , n} ( f ) of the matrix E are organized as follows:

The substeps 32, 34 and 36 for determining the matrices A and B then E are repeated for all the operating frequencies f .

Each element E _{l , m , n} ( f ) corresponds to an encoding filter that integrates the spatial distribution of the sensors 2 _{n as} well as the optimization strategy.

   où P and _l,m (f) est la transformée de Fourier de p and _l,m (t) et C _n (f) est la transformée de Fourier de c _n (t).During phase 40, the signals c ₁ to c _N coming from the sensors 2 ₁ to 2 _N are filtered using the encoding filters described by the parameters FD. Each coefficient p and _{l , m} ( t ) delivered is deduced from the signals c ₁ to c _N by the application of the filters as follows:

where P and _{l , m} ( f ) is the Fourier transform of p and _{l , m} ( t ) and C _n ( f ) is the Fourier transform of c _n ( t ).

In this embodiment, the coefficients p and _{l , m} ( t ) are determined by means of filtering methods in the frequency domain, such as block convolution techniques.

The representation of the acoustic field therefore takes into account the position selected sensors and optimization parameters and constitutes an estimate faithful of the acoustic field.

FIG. 7 shows a block diagram of a device adapted for the implementation of the method of the invention.

In this figure, a device 50 for representing the acoustic field P is connected to the acquisition means 1 as described with reference to FIG. 2.

The device 50 or encoding device is also connected as input to means 60 for determining the parameters representative of the characteristics structural and / or electro-acoustic acquisition means 1.

These means 60 comprise, in particular, means 62 for inputting parameters and calibration means 64 which are adapted to implement respectively the steps 10 and 20 of the method of the invention as described previously.

The encoding device 50 receives, determination means 60 parameters, a plurality of parameters representative of the characteristics acquisition means 1 distributed between a signal CL defining the characteristics structures and a signal CP of parameterization of the structural characteristics and / or electro-acoustic.

The device also receives parameters relating to the strategies of representation in an OS optimization signal.

• dans le signal CL de définition :
  • des paramètres x _n représentatifs de la position du capteur 2 _n ;
• dans le signal CP de paramétrage :
  • des paramètres H _n (f) représentatifs de la réponse en fréquence du capteur 2 _n ;
  • des paramètres d _n (f) représentatifs du diagramme de directivité du capteur 2 _n ;
  • des paramètres α _n (f) représentatifs de l'orientation du capteur 2 _n ;
  • des paramètres σ² _n (f) représentatifs de la densité spectrale de puissance du bruit de fond du capteur 2 _n ; et
  • des paramètres B _n,l,m (f) représentatifs des capacités d'acquisition du capteur 2 _n ; et
• dans le signal OS d'optimisation :
  • un paramètre µ(f) spécifiant le compromis entre la fidélité de représentation du champ acoustique et la minimisation du bruit de fond apporté par les capteurs ;
  • un paramètre L(f) spécifiant l'ordre auquel est conduite la représentation ; et
  • un paramètre {(l _k ,m _k )}(f) représentatif de la liste des coefficients dont on exige que la puissance soit égale à la puissance du coefficient correspondant dans le champ acoustique à représenter P.

In these signals, the parameters are distributed as follows:

• in the definition CL signal:
  • parameters x _n representative of the position of the sensor 2 _n ;
• in the parameter CP signal:
  • parameters H _n ( f ) representative of the frequency response of the sensor 2 _n ;
  • parameters d _n ( f ) representative of the directivity diagram of the sensor 2 _n ;
  • parameters α _n ( f ) representative of the orientation of the sensor 2 _n ;
  • parameters σ ² _n ( f ) representative of the spectral power density of the background noise of the sensor 2 _n ; and
  • parameters B _{n , l , m} ( f ) representative of the acquisition capacitances of the sensor 2 _n ; and
• in the OS optimization signal:
  • a parameter μ ( f ) specifying the compromise between the fidelity of representation of the acoustic field and the minimization of the background noise brought by the sensors;
  • a parameter L ( f ) specifying the order in which the representation is conducted; and
  • a parameter {( l _k , m _k )} ( f ) representative of the list of coefficients whose power is required to be equal to the power of the corresponding coefficient in the acoustic field to be represented P.

Advantageously, this device 50 comprises means 51 for shaping the input signals adapted to deliver from the signals c ₁ to c _N , a correspondingly shaped signal SI.

For example, the means 51 comprise analog-digital converters, amplifiers or filtering systems.

The device 50 furthermore comprises means 52 for determining the encoding filters which comprise a module 55 for calculating the sampling matrix B , a module 56 for calculating the matrix A for intercorrelation, which are both connected to a module 57 for calculating the matrix E ( f ) encoding.

This encoding matrix E ( f ) is used by a coding filter determining module 58 which delivers a signal S _FD which contains the parameters FD representative of the encoding filters.

This signal S _FD is used by a processing module 59 which applies the encoding filters to the signal S1 in order to deliver an IF signal _FB which comprises the Fourier-Bessel coefficients representative of the acoustic field P.

Optionally, the device 50 comprises a non-volatile memory in which are stored the parameters which constitute the signal S _FD which have been determined beforehand.

For example, the acquisition means 1 are tested and calibrated by their manufacturer in order to directly supply a memory comprising all the parameters of the signal S _FD that should be integrated into an encoding device in order to realize the acquisition of the acoustic field P and to deliver a faithful representation of the latter.

Likewise, in a variant, this memory comprises only the matrices B and possibly A and the device 50 comprises means for inputting the parameters constituting the optimization signal OS in order to implement the determination of the matrix E ( f ) d. encoding and determination of FD parameters representative of the encoding filters.

Of course, other distributions between the various modules described can be considered as needed.

Claims (22)

1. Method for representing an acoustic field comprising a step involving the acquisition of measurement signals (c_n) issued by acquisition means (1) comprising one or more elementary sensors (2_n) that are exposed to said acoustic field (P), characterised in that it comprises:

   a step (30) involving the determination of encoding filters that are representative of at least the structural characteristics of said acquisition means (1); and

   a step (40) involving the processing of said measurement signals (c_n) by applying said encoding filters to these signals (c_n) in order to determine a finite number of coefficients representative over time and in the three-dimensional space of said acoustic field (P), said coefficients allowing a representation of said acoustic field (P) to be obtained that is substantially independent of the characteristics of said acquisition means (1).
2. Method according to claim 1, characterised in that said structural characteristics comprise at least position characteristics of said elementary sensors (2_n) relative to a predetermined reference point (4) of said acquisition means (1).
3. Method according to either claim 1 or claim 2, characterised in that said encoding filters are also representative of electro-acoustic characteristics of said acquisition means (1).
4. Method according to claim 3, characterised in that said electro-acoustic characteristics comprise at least characteristics related to the intrinsic electro-acoustic acquisition capacities of said elementary sensors (2_n).
5. Method according to any one of claims 1 to 4, characterised in that said coefficients allowing a representation of the acoustic field (P) to be obtained are what are known as Fourier-Bessel coefficients and/or linear combinations of Fourier-Bessel coefficients.
6. Method according to any one of claims 1 to 5, characterised in that said step (30) involving the determination of the encoding filters comprises:

   a sub-step (32) involving the determination of a sampling matrix (B) that is representative of the acquisition capacities of said acquisition means (1);

   a sub-step (34) involving the determination of an intercorrelation matrix (A) that is representative of the similarity between said measurement signals (c_n) issued by the elementary sensors (2_n) forming said acquisition means (1); and

   a sub-step (36) involving the determination of an encoding matrix (E(f); E) from said sampling matrix (B), said intercorrelation matrix (A) and a parameter (µ(f)) that is representative of a desired compromise between faithfulness of representation of the acoustic field and minimisation of the background noise caused by the acquisition means (1), which matrix is representative of said encoding filters.
7. Method according to claim 6, characterised in that said sub-steps involving the determination of the matrices are carried out for a finite number of operating frequencies.
8. Method according to either claim 6 or claim 7, characterised in that said step (32) involving the determination of the sampling matrix (B) is carried out, for each of said elementary sensors (2_n) forming said acquisition means (1), from:

   parameters ( x _n) that are representative of the position of said sensor (2_n) relative to the centre (4) of said acquisition means (1); and/or

   a finite number of coefficients (B_n,l,m(f)) that are representative of the acquisition capacities of said sensor (2_n).
9. Method according to claim 8, characterised in that said step involving the determination of the sampling matrix (B) is also carried out from at least one of the following parameters:

   parameters (H_n(f)) that are representative of the frequency responses of all or some of the sensors (2_n);

   parameters (d_n(f)) that are representative of the directivity diagrams of all or some of the sensors (2_n);

   parameters (α_n(f)) that are representative of the orientations of all or some of the sensors (2_n), i.e. of their maximum sensitivity direction;

   parameters (σ² _n(f)) that are representative of the power spectral densities of the background noise of all or some of the sensors (2_n);

   a parameter (L(f)) specifying the order in which the representation is conducted;

   a parameter ({(l_k,m_k)}(f)) that is representative of a list of coefficients, the power of which must be equal to the power of the corresponding coefficient in the acoustic field to be represented (P).
10. Method according to any one of claims 1 to 9, characterised in that it comprises a calibration step (20) allowing all or some of the parameters used in said step (30) involving the determination of the encoding filters, to be issued.
11. Method according to claim 10, characterised in that said calibration step (20) comprises, for at least one of said elementary sensors (2_n) forming said acquisition means (1):

    a sub-step (24) involving the acquisition of signals that are representative of the acquisition capacities of said at least one sensor (2_n); and

    a sub-step (28) involving the determination of parameters representative of electro-acoustic and/or structural characteristics of said at least one sensor (2_n).
12. Method according to claim 11, characterised in that said calibration step (20) further comprises:

    a sub-step (22) involving the emission of a specific acoustic field toward said at least one sensor (2_n), said acquisition sub-step (24) corresponding to the acquisition of the signals issued by this sensor (2_n) when it is exposed to said specific acoustic field; and

    a sub-step (26) involving the modelling of said specific acoustic field in a finite number of coefficients, in order to allow said sub-step (28) involving the determination of parameters that are representative of electro-acoustic and/or structural characteristics of the sensor (2_n) to be carried out.
13. Method according to any one of claims 10 to 12, characterised in that said calibration step (20) comprises a sub-step involving the reception of a finite number of signals that are representative of the electro-acoustic and structural characteristics of said sensors (2_n) forming said acquisition means (1), which signals are used directly during said sub-step involving the determination of the electro-acoustic and/or structural characteristics of said acquisition means (1).
14. Method according to any one of claims 1 to 13, characterised in that it comprises an input step (10) allowing all or some of the parameters used during said step (30) involving the determination of the encoding filters to be determined.
15. Computer program comprising program code instructions for implementing the steps of the method according to any one of claims 1 to 14, when said program is executed on a computer.
16. Movable support of the type comprising at least one operation processor and a nonvolatile memory element, characterised in that said memory comprises a program comprising code instructions for implementing the steps of the method according to any one of claims 1 to 14, when said processor executes said program.
17. Device for representing an acoustic field that is connectable to acquisition means (1) comprising one or more elementary sensors (2_n) issuing measurement signals (c_n) when they are exposed to said acoustic field (P), characterised in that it comprises a module (59) for processing the measurement signals (c_n) by applying encoding filters that are representative of at least the structural characteristics of said acquisition means (1) to these measurement signals (c_n), in order to issue a signal (SI_FB) that comprises a finite number of coefficients representative over time and in the three-dimensional space of said acoustic field (P), said coefficients allowing a representation of said acoustic field (P) to be obtained that is substantially independent of the characteristics of said acquisition means (1).
18. Device according to claim 17, characterised in that said encoding filters are also representative of electro-acoustic characteristics of said acquisition means (1).
19. Device according to either claim 17 or claim 18, characterised in that it further comprises means (52) for determining said encoding filters that are representative of structural and/or electro-acoustic characteristics of said acquisition means (1).
20. Device according to claim 19, characterised in that said means (52) for determining encoding filters receive at the input at least one of the following parameters:

    parameters ( x _n) that are representative of the positions, relative to centre of said acquisition means (1), of all or some of the sensors (2_n);

    a finite number of coefficients (B_n,l,m(f)) that are representative of the acquisition capacities of all or some of the sensors (2_n);

    parameters (H_n(f)) that are representative of the frequency responses of all or some of the sensors (2_n);

    parameters (d_n(f)) that are representative of the directivity patterns of all or some of the sensors (2_n);

    parameters (α_n(f)) that are representative of the orientations of all or some of the sensors (2_n), i.e. of their maximum sensitivity direction;

    parameters (σ² _n(f)) that are representative of the power spectral densities of the background noise of all or some of the sensors (2_n);

    a parameter µ(f) that is representative of the desired compromise between faithfulness of representation of the acoustic field and minimisation of the background noise caused by the acquisition means (1);

    a parameter (L(f)) specifying the order in which the encoding is conducted;

    a parameter ({(l_k,m_k)}(f)) that is representative of a list of coefficients, the power of which must be equal to the power of the corresponding coefficient in the acoustic field to be represented (P).
21. Device according to claim 20, characterised in that it is associated with means (60) for determining all or some of the parameters received by said means (52) for determining the encoding filters, said means (60) comprising at least one of the following elements:

    means (62) for inputting parameters; and/or

    calibration means (64).
22. Device according to any one of claims 17 to 21, characterised in that it is associated with means (51) for formatting said measurement signals (c₁ to c_N), in order to issue a corresponding formatted signal (SI).

Images