EP0692923A1 - Selective sound pick-up device for reflecting and noisy environment - Google Patents

Abstract

The system for selectively picking-up sound uses an acoustic aerial. This is formed from a part of a concave cylindrical surface (S) including a number of electro-acoustic transducers (M0-M10). These are distributed over the surface, and in the vicinity of the cylindrical surface (S). Partial summing circuits (2.1 - 2.4) act on a number of analogue sound signals forming elementary acoustic aerials. A further summing circuit (5) is able to provide a resultant analogue output signal which is representative of the sound being produced in a useful speaking zone (L). <IMAGE>

Description

The invention relates to a selective sound pickup system for a reverberant and noisy environment.

In the most recent telecommunications services, such as teleconferencing, hands-free telephony, as well as in multimedia workstation computer applications, the sound recording is systematically disturbed by the acoustic environment, because the distance between the speaker of the sound recording system is much more important than in conventional telephony. Thus, before being transmitted, the audible message, to which the ambient noise is added, is modified and disturbed by the room effect, or reverberation phenomenon.

This phenomenon, as well as the greater relative importance of the noise, compared to a close sound pickup, reduce the intelligibility of the message transmitted and give an impression of distance from listening.

In addition, when two conference rooms are interconnected, the signal from the remote room is fed back into the sound system. This acoustic coupling is the source of the echo phenomenon and the instabilities of the electro-acoustic loop, known as the Larsen effect.

Unlike studio sound recording, the aforementioned applications take place in acoustically uncontrolled and noisier sound environments, in particular in the case of multimedia workstations.

In order to reduce these very annoying effects, one can consider using directional microphones whose acoustic sensitivity diagram favors the direction of the useful source compared to the other directions, sources of uncontrolled noise. However, the spatial selectivity of this type of conventional sensor, whose acoustic sensitivity diagram is of the cardioid type for example, is insufficient to obtain a satisfactory reduction in noise and room effect.

A more effective technique for solving the aforementioned problems posed by taking remote sound, consists in producing an acoustic antenna, which is in fact constituted by a network of microphones whose output signals are summed. These microphones are advantageously arranged so that the useful sound signals reach each of them at the same time and are therefore summed in phase. Speech or useful signal is thus favored over parasitic signals due to reverberation and noise sources.

In general, the technique of acoustic antennas is similar to known techniques for forming channels of radioelectric antennas.

However, this technique poses specific problems due, on the one hand, to sound signals belonging to a broadband frequency spectrum, occupying several octaves for example from 100 Hz to 7500 Hz and, on the other hand, to the presence of near-field sound sources, for which the hypothesis of propagation of sound waves, by plane waves, is not verified.

In order to maintain constant performance on the antenna as a function of frequency, one solution consists in breaking it down into sub-antennas, each characterized by a specific spacing between sensors or microphones and dedicated to a part of the overall frequency band. . The output signals of the sub-antennas are obtained by summing the signals delivered by the sensors. The limitation of each sub-antenna to its frequency band is then carried out by a bank of band-pass filters.
Such a device has been described in particular in the article published by Y. MAHIEUX, G. LE TOURNEUR, A. GILLOIRE, A. SALIOU, JP. JULLIEN, CNET LAA / TSS / CMC - Route de Trégastel, BP 40 - LANNION CEDEX, France, and entitled " A microphone array for multimedia workstations ". Third international Workshop on Acoustic Control, Plestin-les-Grèves, France. September 1993.

The system described by the aforementioned article is satisfactory.

However, the practical implementation of such a system on an industrial scale poses many problems. In particular, the arrangement of microphones or acoustic sensors directly influences the performance of the acoustic antenna. Regarding the wavelength, the physical size of this acoustic antenna must be as high as possible, in order to allow good spatial directivity. Such a characteristic poses numerous problems, in particular in the field of low frequencies.

The above requirement is all the more difficult to meet in applications relating to multimedia workstations, the overall dimensions of the antenna not being able to reasonably exceed those of the video display monitor.

Finally, the acoustic antenna must, to allow easy industrialization and marketing, be easy to install on any type of terminal in the absence of significant modification thereof.

The selective sound pickup system for reverberant and noisy environment, object of the present invention, comprises a plurality of electro-acoustic transducers intended to receive useful sound signals in phase coming from the same useful speech area, these useful sound signals being summed in phase and the sound signals coming from zones other than the useful area being summed in phase inconsistency to select the sound signals coming from the useful speech area.

It is remarkable in that it comprises, in combination, an air formed by a part of concave cylindrical surface. The concavity of the cylindrical surface is oriented towards the useful speech area and the plurality of electro-acoustic transducers is distributed over and in the vicinity of the cylindrical surface. Each electro-acoustic transducer is oriented towards the useful speech area and delivers an analog sound signal, all of the aerial and electro-acoustic transducers forming an acoustic antenna.

Circuits for partial summation of a plurality of analog sound signals delivered by each of the electro-acoustic transducers are provided, in a determined arrangement, to form from the electro-acoustic transducers, a plurality of elementary acoustic antennas, each delivering a resulting analog elementary sound signal and summing circuits of the resulting elementary analog sound signals make it possible to generate a resulting sound signal representative of the sound signal coming from the useful speech area.

The selective sound recording system for a reverberant and noisy environment, object of the invention, finds application in new telecommunication services such as teleconferencing, hands-free telephony as well as multimedia computing and tele-computing.

• la figure 1a représente un schéma de principe du système de prise de son sélective, objet de la présente invention,
• les figures 1b et 1c représentent un premier respectivement un deuxième mode d'agencement de transducteurs électro-acoustiques susceptibles d'être utilisés pour réaliser une antenne acoustique conforme à l'objet de la présente invention,
• la figure 2 représente un schéma de principe de circuits analogiques de sommation partielle permettant de constituer une pluralité d'antennes acoustiques élémentaires,
• la figure 3 représente un détail de réalisation d'un circuit de mixage,
• la figure 4a représente les éléments mécaniques en vue éclatée, permettant la réalisation de la structure mécanique de l'aérien,
• la figure 4b représente une vue en perspective de l'élément central au cadre de la structure mécanique de l'aérien,
• la figure 4c représente un mode de réalisation particulier avantageux du support mécanique de l'aérien,
• la figure 4d représente une vue de face de l'antenne acoustique selon l'invention, après montage,
• la figure 5a représente un schéma synoptique illustratif du processus de filtrage appliqué, en technique numérique, aux signaux délivrés par chaque antenne élémentaire,
• la figure 5b représente un chronogramme d'un circuit d'interfaçage série des circuits de sommation des signaux analogiques sonores résultants,
• les figures 6a et 6b représentent un diagramme gain/fréquence de réponse en fréquence de filtres correspondant à une découpe classique et à une découpe optimisée en bandes élémentaires respectivement,
• les figures 7a et 7b illustrent chacune une application particulière du système de prise de son sélective, objet de la présente invention, à une station de travail multimédia.

A more detailed description of the selective sound pickup system which is the subject of the present invention will be given below in conjunction with the drawings in which:

• FIG. 1a represents a block diagram of the selective sound pickup system, object of the present invention,
• FIGS. 1b and 1c represent a first respectively a second mode of arrangement of electro-acoustic transducers capable of being used to produce an acoustic antenna conforming to the object of the present invention,
• FIG. 2 represents a block diagram of analog summation circuits making it possible to constitute a plurality of elementary acoustic antennas,
• FIG. 3 shows a detail of an embodiment of a mixing circuit,
• FIG. 4a represents the mechanical elements in exploded view, enabling the mechanical structure of the aerial to be produced,
• FIG. 4b represents a perspective view of the central element within the framework of the mechanical structure of the aerial,
• FIG. 4c represents a particular advantageous embodiment of the mechanical support of the aerial,
• FIG. 4d represents a front view of the acoustic antenna according to the invention, after assembly,
• FIG. 5a represents a block diagram illustrating the filtering process applied, in digital technique, to the signals delivered by each elementary antenna,
• FIG. 5b represents a timing diagram of a series interfacing circuit of the summing circuits of the resulting analog sound signals,
• FIGS. 6a and 6b represent a gain / frequency response frequency diagram of filters corresponding to a conventional cutting and to an optimized cutting into elementary bands respectively,
• Figures 7a and 7b each illustrate a particular application of the selective sound pickup system, object of the present invention, to a multimedia workstation.

A more detailed description of the selective sound pickup system for reverberant and noisy environment, in accordance with the object of the present invention, will now be given in conjunction with FIG. 1a.

According to the above-mentioned figure, the system which is the subject of the present invention comprises a plurality of electro-acoustic transducers, denoted M₀ to M₁₀, intended to receive useful sound signals in phase originating from the same useful area of locution, noted L on the aforementioned figure. The electro-acoustic transducers M₀ to M₁₀ are constituted for example by microphones and are intended to receive the useful signals summed in phase, the sound signals coming from zones other than the useful zone L being summed in phase inconsistency to select the sound signals from the aforementioned useful phrase area.

Of course, the electro-acoustic transducers M₀ to M₁₀ are of the substantially unidirectional type, these microphones having a spatial sensitivity diagram of the cardioid type for example.

According to a particularly advantageous characteristic of the system which is the subject of the present invention, it comprises in combination an aerial, denoted 1, formed by a part of concave cylindrical surface, this surface being denoted S in FIG. 1a. The concavity of the cylindrical surface S is oriented towards the useful locution area L and the plurality of electro-acoustic transducers M₀ to M₁₀ is distributed over and in the vicinity of the above-mentioned cylindrical surface S.

Each transducer M₀ to M₁₀ is oriented towards the useful locution area L and delivers an analog sound signal, all of the aerial and electro-acoustic transducers M₀ to M₁₀ forming an acoustic antenna. The analog sound signals delivered by each electro-acoustic transducer are noted respectively s0 to s10. Advantageously, it is indicated that each electro-acoustic transducer is connected to a pre-amplifier, denoted A₀ to A₁₀, which delivers an amplified sound analog signal, denoted S₀ to S₁₀.

Circuits for partial summation of a plurality of the aforementioned analog sound signals, in particular the amplified analog sound signals S₀ to S₁₀, are provided, these circuits bearing the reference 2₁, 2₂, 2₃ and 2₄ in FIG. 1. The partial summation of the plurality of amplified analog sound signals S₀ to S₁₀ is carried out according to a determined arrangement of these signals, which makes it possible to form, from the aforementioned electro-acoustic transducers, a plurality of elementary acoustic antennas, more commonly referred to as sub-antennas, each sub-antenna delivering a resulting elementary analog sound signal. These signals being noted SA₁, SA₂, SA₃ and SA₄ in Figure 1a.

In a first simplified version of the selective sound pick-up system, object of the present invention, it is indicated that the resulting analog signals SA₁ to SA₄ delivered by the aforementioned summing circuits 2₁ to 2₄ in the form of analog signals can be kept under this form. In such a case, following an analog filtering 4₁ to 4₄, a circuit 5 of summation of these resulting elementary analog sound signals is provided to generate an analog sound signal resulting representative of the sound signal coming from the useful locution area L previously mentioned. This signal is noted SUL in FIG. 1a.

In a more elaborate version of the selective pickup system, object of the present invention, it is indicated that, for reasons of simplification and improvement of the processing flexibility, each of the resulting elementary signals SA₁ to SA₄ is subjected successively to a process of analogical digital conversion by means of analogical digital converters 3₁ to 3₄, then to a filtering by means of digital filters, noted 4₁ to 4₄. These analog to digital conversion and digital filtering processes can be carried out by conventional type processes but nevertheless make it possible to improve the overall performance of the antenna, as will be described later in the description. In this case, the summing circuit 5 can be produced in digital form.

In general, it is indicated that the system for selective picking, object of the present invention, is made so that the spacings between electro-acoustic transducers M₀ to M1₀ are chosen so that the total number of aforementioned electro-acoustic transducers is minimized, thereby creating a so-called logarithmic spacing antenna.

As shown in FIG. 1a, the electro-acoustic transducers can be distributed so as to present a central transducer, the transducer M₀, as well as transducers adjacent to this central transducer, arranged on the aerial 1 symmetrically with respect to the central transducer M₀.

By way of example, it is indicated that the transducer M₁ is distant from the central transducer M₀ by a distance of 2.5 cm, the transducer M₃ is distant from the transducer M₁ by the same distance by 2.5 cm, the transducer M5 is distant from the transducer M₃ by a distance of 5 cm and the transducer M₇ is distant from the transducer M₅ by a distance of 10 cm, these transducers being placed on a guideline of the surface S previously mentioned. Of course, the transducers of even index M₂ to M₈ are symmetrical with respect to the central transducer M₀ with respect to the corresponding transducers M₁ to M₇.

In addition, it is indicated that at least two of the electro-acoustic transducers, such as the transducers M₉ and M₁₀, are located in the vicinity of the surface S but are mechanically decoupled from the aerial 1.

Advantageously and in order to improve the directivity of the acoustic antenna, object of the present invention, thus formed, it is indicated that the most extreme electro-acoustic transducers, such as the transducers M₉ and M₁₀, are placed at a distance about 30 cm from the extreme transducers located on the aerial 1, that is to say the transducers M₇ and M₈. Advantageously, in order to satisfy the aforementioned distance condition, without however prohibitively increasing the size of the antenna thus formed, it is indicates that the most extreme transducers M₉ and M₁₀ can be placed on a generating line of the surface S, close to the generating lines of this same surface S supporting the extreme transducers M₇ and M₈ placed on the aerial 1. We understand, as well as represented in FIG. la, that the distance of the most extreme transducers M₉ and M₁₀ with respect to the extreme transducers of the aerial 1, the transducers M₇ and M₈, in the direction of a guideline of the surface S, can be very small , of the order of 4 to 5 cm, which allows the size of the acoustic antenna thus formed to be reduced accordingly while maintaining the condition of distance between the most extreme transducers M₉ and M₁₀ and the extreme transducers of the aerial 1, M₇ and M₈.

In general, it is indicated that the arrangement of the electro-acoustic transducers or microphones directly influences the performance of the acoustic antenna thus produced.

Reduced to the wavelength, the physical size of this antenna must be as large as possible to allow the best possible spatial selectivity.

Such a constraint poses problems in the field of low frequencies, the size of multimedia workstations for example not having to be excessively increased.

A more detailed description of an arrangement of the microphones, or electro-acoustic transducers previously cited, will be given in two particularly advantageous embodiments in the case of FIGS. 1b and 1c respectively.

In the two cases of the abovementioned embodiments, it is indicated that the electro-acoustic transducers are distributed in a first, M₀ to M₈, and in a second group, M₉ and M₁₀, of electro-acoustic transducers.

The electro-acoustic transducers M₀ to M₈ of the first group are in odd number and aligned on a line of the curvilinear surface S parallel to a guideline of the cylindrical part, the guideline Dir 1 in FIGS. 1b and 1c. It is of course understood that the electro-acoustic transducers of the first group are placed symmetrically with respect to a plane of symmetry containing a generating line, the line G₀ in FIGS. 1b and 1c, of the cylindrical surface S, this plane of symmetry also containing the central electro-acoustic transducer M₀.

As regards the electro-acoustic transducers of the second group, the transducers M₉ and M₁₀, it is indicated that these are distributed over one or more guidelines of the aforementioned concave cylindrical surface S.

Thus, in the case of the embodiment of FIG. 1b, case where the electro-acoustic transducers M₀ to M₈ are placed on the horizontal guideline Dir 1 of the surface S, the transducers of the second group M₉ and M₁₀ can be placed on a common guideline Dir 2 of the surface S, each of them being placed on a separate generating line G₉, respectively G₁₀. It is understood that in the embodiment of FIG. 1b, the condition of distance between the most extreme electro-acoustic transducers M₉, M₁₀ relative to the extreme transducers of the aerial 1, M₇ and M₈, is satisfied without however the overall size of the acoustic antenna thus formed is prohibitively increased.

In the case of the embodiment of FIG. 1c on the contrary, the transducers of the first group, M₀ to M₈, are of course always placed on a guideline, the line Dir 1 of the surface S, this guideline this time being vertical . In such a case, the transducers of the second group, M₉ and M₁₀, can advantageously be placed on the same guideline Dir 2, and on two separate generating lines, denoted G₉ and G₁₀, which of course makes it possible to satisfy the distance condition of the most extreme electro-acoustic transducers M₉ and M₁₀ with respect to the extreme transducers M₇ and M₈ of the aerial 1, without however excessively increasing the overall size of the antenna acoustics thus formed.

It is of course indicated that in the previous embodiment, the electro-acoustic transducer M₁₀ for example can be placed on the generating line G₁₀ symmetrically to the electro-acoustic transducer M₉ with respect to the generating line G₀.

The electro-acoustic transducers being arranged as shown in FIGS. 1a and in particular 1b and 1c, it is indicated that each is connected to an adjustable electronic gain preamplifier, noted A₀ to A₁₀, this type of preamplifier can be formed by an operational pre-amplifier for example.

In addition, in order to constitute the elementary antennas or the sub-antennas previously mentioned, it is indicated that the output of each of the aforementioned pre-amplifiers A₀ to A₁₀ is connected to certain inputs of the summing circuits 2₁ to 2₄, as illustrated on Figure 1a.

• sous-antenne 1 :
     A₉ A₇ A₀ A₈ A₁₀ / Circuit sommateur 2₁.
  Dans ce cas, on indique que la sous-antenne 1 est formée à partir des transducteurs les plus extrêmes M₉ et M₁₀, des transducteurs extrêmes de l'aérien 1, M₇ et M₈, ainsi que du transducteur central M₀. La sous-antenne 1 ainsi formée par l'intermédiaire des transducteurs électro-acoustiques précités et du circuit sommateur 2₁, correspond à la sous-antenne sensible aux fréquences basses, ainsi qu'il sera décrit ultérieurement dans la description.
• sous-antenne 2 :
     A₇ A₅ A₀ A₆ A₈ / Circuit sommateur 2₂.
  La sous-antenne 2 ainsi formée à partir des transducteurs électro-acoustiques précités et du circuit sommateur 22 correspond à une sous-antenne sensible aux signaux à fréquence moyenne inférieure, ainsi qu'il sera décrit ultérieurement dans la description.
• sous-antenne 3 :
     A₅ A₃ A₀ A₄ A₆ / Circuit sommateur 2₃.
  La sous-antenne 3 ainsi formée est sensible aux signaux à fréquence moyenne supérieure, ainsi qu'il sera décrit ultérieurement dans la description.
• sous-antenne 4 :
     A₃ A₁ A₀ A₂ A₄ / Circuit sommateur 2₄.
  La sous-antenne 4 ainsi formée est sensible aux fréquences hautes, ainsi qu'il sera décrit ultérieurement dans la description.

More specifically, it is indicated that, in order to constitute the successive sub-antennas, the connections are made in the following manner:

• sub-antenna 1:
  A₉ A₇ A₀ A₈ A₁₀ / Summing circuit 2₁.
  In this case, it is indicated that the sub-antenna 1 is formed from the most extreme transducers M₉ and M₁₀, extreme transducers from the air 1, M₇ and M₈, as well as from the central transducer M₀. The sub-antenna 1 thus formed by means of the aforementioned electro-acoustic transducers and the summing circuit 2₁, corresponds to the sub-antenna sensitive to low frequencies, as will be described later in the description.
• sub-antenna 2:
  A₇ A₅ A₀ A₆ A₈ / Summing circuit 2₂.
  The sub-antenna 2 thus formed from the aforementioned electro-acoustic transducers and from the summing circuit 22 corresponds to a sub-antenna sensitive to the signals at lower average frequency, as will be described later in the description.
• sub-antenna 3:
  A₅ A₃ A₀ A₄ A₆ / Summing circuit 2₃.
  The sub-antenna 3 thus formed is sensitive to signals at a higher average frequency, as will be described later in the description.
• sub-antenna 4:
  A₃ A₁ A₀ A₂ A₄ / Summing circuit 2₄.
  The sub-antenna 4 thus formed is sensitive to high frequencies, as will be described later in the description.

The aforementioned sub-antenna 1, sub-antenna 2, sub-antenna 3 and sub-antenna 4 respectively deliver the resulting elementary analog sound signals, ie the signals SA₁, SA₂, SA₃, SA₄ previously mentioned.

In order to facilitate the interconnection of each of the amplifiers A₀ to A₁₀ to the respective inputs of the aforementioned summing circuits 2₁ to 2₄, it is advantageous, as shown in FIG. 2, to produce a connection matrix advantageously comprising a battery of twelve preamplifiers in parallel , denoted A₀ to A₁₁, each adjustable gain preamplifier being connected via a connection matrix as shown in FIG. 2, to the four summing circuits, denoted 2₁ to 2₄. In practice, it is indicated that the channel formed by the preamplifier A₁₁ can be a free channel, this for convenience being provided and which can then be connected to the reference voltage of the device. Each summing circuit 2₁ to 2₄ can be constituted, as shown in FIG. 3, by an operational amplifier 20 whose positive input is connected to the reference voltage and whose negative input is connected in feedback by a adjustable resistance Rk. The negative input of the operational amplifier 20 is conventionally connected to twelve resistors in parallel, denoted R₁ to R₁₂, by means of connection bridges, which make it possible to configure the corresponding summing circuit 2₁ to 2₄ according to a desired configuration. . It is indicated that, as regards the summing circuit represented in FIG. 3, this corresponds to the sub-antenna 2 for example, the inputs e₀, e₁, e₅, e₈ and e₉ being connected to the negative input of the operational amplifier 20 operating as a summator and, respectively, to the pre-amplifiers A₀, A₅, A₆, A₇, A₈.

Of course, and in a nonlimiting manner, it is indicated that, with each aforementioned elementary acoustic antenna or sub-antenna, there is also associated means for filtering low-pass, band-pass or high-pass frequencies as a function of the rank of the aforementioned sub-antenna, the frequency bands transmitted by each filter associated with an elementary acoustic antenna being shifted so as to cover a resulting pass band covering frequencies between, for example, 25 Hz and 7500 Hz. The operating mode of the filtering proper will be described later in the description.

A more detailed description of the implementation of the structure of the aerial 1 will now be given in conjunction with FIGS. 4a, 4b, 4c and 4d.

According to a particularly advantageous aspect of the acoustic antenna, object of the present invention, it is indicated that the latter, in order to obtain the lowest possible production costs, uses electro-acoustic transducers constituted by microphone capsules bare electret type instead of complete sensors usually comprising, in addition to the actual capsule or sensitive element of the microphone, a protective shield, a fixing device and connection elements as well as a box for the power supply .

• problèmes mécaniques de fixation de la capsule sur le support de l'antenne ou aérien 1,
• sensibilité de l'ensemble aux champs électromagnétiques en raison de l'absence de blindage des capsules utilisées, ce type de capsules étant bien entendu sensible aux radiations électro-magnétiques,
• incidences sur la directivité, et donc sur la prise de son sélective, le diagramme de directivité de chacune des capsules utilisées ne devant en aucun cas être dégradé par le montage sur l'aérien 1.

Taking into account the use of the aforementioned bare electret type microphone capsules, the practical implementation of the aerial 1 presented the following technical problems:

• mechanical problems with the attachment of the capsule to the antenna or aerial support 1,
• sensitivity of the assembly to electromagnetic fields due to the lack of shielding of the capsules used, this type of capsule being of course sensitive to electromagnetic radiation,
• effects on the directivity, and therefore on the taking of its selective, the directivity diagram of each of the capsules used must in no case be degraded by mounting on the aerial 1.

Given the aforementioned problems, the production of the mechanical structure constituting the aerial 1 as described in conjunction with the aforementioned figures, makes it possible to resolve all of these problems.

As shown in FIG. 4a, the aerial 1 or part of a cylindrical surface is formed by a frame, denoted 10, of substantially rectangular shape made of plastic material. This frame 10 is provided with spacers 100 and facing teeth 101, 102 formed in the plane of the frame 10 previously mentioned. Two opposite teeth form a housing intended to receive an electro-acoustic transducer, that is to say the microphone capsules M capsules to M₈ previously mentioned in the description. Of course, as it appears from the observation of FIG. 4a, the frame 10 is curved so as to form the part of the cylindrical surface previously mentioned.

In addition, the acoustic antenna, object of the present invention, comprises, at the level of the aerial 1, an electro-magnetic shielding making it possible to ensure immunity of the electro-acoustic transducers to parasites or electromagnetic disturbances. In FIG. 4a, the electromagnetic shielding consists of a metal grid, in fact by two metal grids bearing the references 103 and 104, these grids covering the concave part and the convex part of the cylindrical surface part, that is to say finally the rectangular frame 10 previously mentioned, and of course, the electro-acoustic transducers when these have been placed in the intended housings and intended to receive them. Of course, the metal grid 103, 104 is electrically connected to the reference electrical potential of the electro-acoustic transducers.

The assembly of the aerial 1 is carried out by means of four rods, also made of plastic, bearing the references 105, 106, 107 and 108, these plastic rods having a shape adapted to ensure the assembly of the frame. 10, provided with its electro-acoustic transducers and electro-magnetic shielding grids 103 and 104 overlapping, the rods then being fixed to the periphery of the frame 10 to form a compact assembly. The aforementioned rods are fixed by screws for example to the frame 10.

The main role of the frame 10 is the maintenance of the electro-acoustic transducers M₀ to M₈. The geometry of the latter therefore corresponds to that of the aerial 1 and of the antenna, the plastic mount constituting the frame therefore being curved in a semicircle as shown more significantly in FIG. 4b. The structure represented in FIG. 4b for the frame 10 has the advantage of good solidity in the longitudinal direction as well as an acoustic transparency, the mechanical structure thus formed not causing degradation of the acoustic characteristics of the electro-acoustic transducers inserted. in the housings formed between two teeth 101, 102.

The frame 10 can be made of PVC (polyvinyl chloride), an atactic polymer material resistant to impact and abrasion and whose machining is easy. The four previously mentioned rods 105, 106, 107, 108 thus surround the frame 10 by strengthening the mechanical strength of the latter.

The frame 10 is the part of the mechanical assembly which has the most effect on the acoustic transparency. This transparency can only be obtained if the frame 10 is as hollowed out as possible so that the capsules are released from the material structure of the latter. In fact, otherwise, the disturbances in the acoustic field modify the specific directional characteristics of the electret capsules. This is the reason why the frame 10 as shown in FIGS. 4a and 4b has the hollowed out structure between the teeth and the spacers previously mentioned. Tests carried out in the laboratory have shown that electrons presenting a sensitivity diagram of the cardioid type become omnidirectional in the case where, no precaution being taken, these are inserted in a frame of solid structure.

In a preferred nonlimiting embodiment, the frame 10 is formed by a single piece of dimensions L = 490 mm, I = 40 mm and H = 6 mm, machined as shown in FIG. 4b. The length L of the frame 10, that is to say the longitudinal dimension of the latter, corresponds to the opening of the acoustic antenna. Width 1 responds to mechanical considerations such as the maintenance of electro-acoustic, acoustic and aesthetic transducers. Finally, the thickness h of the frame 10 is equal to that of the electret capsule inserted in each housing formed between two teeth 101, 102.

Maintaining the microphone capsules can be ensured by truncated triangle shapes at the end of the teeth 101, 102, the truncation of each triangle can be rounded by drilling. The drilling diameter is then chosen so as to allow the forcible fixing of the electret capsules in the housings thus formed at the end of the teeth 101, 102. This machining and the method of fixing each capsule are identical for all the capsules.

The solidity of frame 10 is obtained by conservation of material constituting the spacers 100 of width equal for example to 5 mm. These spacers forming stiffeners are four in number and can advantageously be located on the abscissa ± 5 cm, ± 10 cm on the semicircle taking as origin the position of the central electro-acoustic transducer M₀. In the central part of the antenna and consequently of the frame 10, it is not necessary to provide a stiffener or spacers 100 because of the short distance between microphones and the allocation of these at high frequencies. Indeed, the directivity disturbances of the electro-acoustic transducers due to obstacles, are stronger at high frequencies than at low frequencies.

During the machining of the part constituting the frame 100, the recesses can be made on a straight piece before bending, the radius of curvature of the frame 10 being then obtained by hot bending.

The rods 105, 106, 107, 108 can be formed from a copolymer material called impact polystyrene (SB). Of U-shaped section, the rods are constituted by the aforementioned four elements, which surround the frame 10 and are fixed on it. The main role of the rods is to strengthen the mechanical rigidity of the assembly. The rods 105, 106, 107 and 108 can be produced by thermoforming, a technique applicable to thermoplastic materials such as impact polystyrene, while allowing the formation of rods of small size but having good mechanical strength.

As regards the electromagnetic shielding grids 103 and 104, it is indicated that these are made necessary due to the absence of specific shielding of each electro-acoustic transducer used. The absence of an electromagnetic shielding has the effect of introducing harmful lines at 50 Hz and at the harmonics of this frequency in the spectrum of the output signal of the unshielded electro-acoustic antenna. The electro-magnetic shielding grids mentioned above make it possible to eliminate such a harmful phenomenon. However, the aforementioned grids must be acoustically transparent, easy to implement and correspond perfectly to the mechanical structure of the aerial 1.

The two electro-magnetic shielding grids 103 and 104 are formed by two tinned metal grids surrounding the frame 10 of the aerial 1. The two aforementioned grids overlap at the ends of the frame 10 and the electrical mass of each electret capsule forming electro transducer -acoustics is connected to the aforementioned grids by a ground wire.

The grids 103 and 104 are made of micromesh expanded metal of a material such as brass, 490 mm long and 40 mm wide, and have diamond-shaped openings of dimensions 1.45 x 0.25 mm. Their thickness is 0.2 mm and therefore has a finesse which makes it possible to closely match the shape of the mechanical structure of the aerial 1, the electrical contact being ensured at the end of the latter. The mesh dimensions of the aforementioned grids are entirely satisfactory to ensure effective electromagnetic shielding.

The metal grids forming the above-mentioned electromagnetic shielding also have a very good quality of acoustic transparency. This condition is verified due to the empty / full ratio of the grid, which is much higher than 50%.

Finally, it is indicated, as shown in FIG. 4c, that the aerial 1 can then be installed for example on the video monitor of a workstation by means of a support 110, as shown in the above-mentioned figure, which comprises a air attachment claw 10, denoted 111, the assembly being mounted on a ball joint 112 allowing the orientation of the air and in particular of the axis of symmetry thereof towards the locution zone L. The claw 111 is formed by a double clamp which encloses the aerial 1 at the rushes. The ball 112 is formed by a ball lockable by screws, allowing orientation on demand. The fineness of the frame ensures the acoustic transparency of the whole.

In Figure 4d, there is shown a view of the complete aerial 1, the capsules being put in place but the electromagnetic shielding grid 104 being removed in order to show the interior appearance of the assembly.

With regard to the two most extreme electroacoustic transducers, M₉ and M₁₀, these can be placed in a structure similar to the frame 10 shown in FIG. 4b but of greatly reduced dimensions since it suffices that the corresponding structure actually has a single pair of teeth 101, 102 for ensuring the placement of the electret capsule forming the electro-acoustic transducer. It is then indicated that the elementary aerials thus formed, for the electroacoustic transducers M₉ and M₁₀, can be placed on one or the sides of the video monitor of a multimedia workstation for example, as will be described later in the description, the only condition to be observed being the distance condition D of the order of 30 cm with respect to the aerial 1. It is in fact indicated that the electro-acoustic transducers M₉ and M₁₀ are intended for low frequencies for which positioning errors are less critical.

A more detailed description of the electronic elements necessary for the implementation of the acoustic antenna, object of the present invention, will now be given in conjunction with FIGS. 5a, 5b and 6a, 6b.

In general, it is indicated that the preamplifiers A₀ to A₁₀ can be produced by operational amplifiers as already mentioned previously in the description. However, and in order to ensure the polarization of the electret capsules constituting the electro-acoustic transducers used for putting implementation of the acoustic antenna, object of the present invention, and the amplification of the signals delivered by them, it is indicated that these preamplifiers can be the subject of a differential type assembly, each preamplifier having a gain adjustment to compensate for the differences in efficiency of the different electret capsules. This type of differential assembly will not be described in detail since it may correspond to a conventional type diagram normally used to supply and discriminate the analog sound signals delivered by an electret capsule normally used in the corresponding technique.

As regards the filtering of the signals delivered by each sub-antenna, signals SA₁, SA₂, SA₃ and SA₄ of FIG. 1a, a particular advantageous embodiment will be described in a digital technique allowing very great flexibility of implementation and the best filtering efficiency.

As shown in particular in the aforementioned figure, each summing circuit 2₁, 2₂, 2₃, 2₄ is followed for example in a block diagram by an analog digital converter 3₁, 3₂, 3₃, 3₄ and connected in cascade to each of these analog digital converters by a 4₁, 4₂, 4₃, 4₄ filter. The output of each filter is then connected to the summing circuit 5 making it possible to deliver the resulting analog sound signal SUL.

In a preferred mode shown in FIG. 5a, it is indicated that all of the four analog digital converters 3₁, 3₂, 3₃, 3₄ and the four filters 4₁, 4₂, 4₃, 4₄ can advantageously be replaced by a filtering processing chain such as as shown in Figure 5a above.

As shown in this figure, it is indicated that the signals delivered by each sub-antenna SA₁, SA₂, SA₃, SA₄ are each delivered to an analog low-pass filter with cutoff frequency 7 kHz, each of the filters analog delivering a corresponding filtered signal, noted SA _{1 *} , SA _{2 *} , SA _{3 *} , SA _{4 *} . Advantageously, the analog-to-digital conversion of the above-mentioned filtered signals is carried out by means of dual-channel analog digital converters, marketed for example under the brand CRYSTAL reference 5356, these two-channel converters, two in number and bearing, by similarity to FIG. 1 a, the reference 3 _1,2 , respectively 3 _3,4 , and the signals SA _{1 *} , SA _{2 *} , respectively SA _{3 *} , SA _{4 *} in digital form. A time multiplexing circuit is provided, which receives the aforementioned signals delivered by the two analog digital converters 3 _1,2 , respectively 3 _3,4 , this multiplexing circuit delivering the various aforementioned frames to a signal processor of the TMS 320C50 type marketed. by TEXAS INSTRUMENTS. An oscillator at 49.152 MHz makes it possible to control a time base making it possible to deliver signals of sampling frequency to the aforementioned analog digital converters. At the output of the signal processor, a digital analog converter circuit of the type marketed for example under the brand CRYSTAL reference 4328, makes it possible to restore the signal in analog form, the summation being carried out beforehand within the signal processor.

The operation of the entire device shown in FIG. 5a is as follows:

The analog-to-digital conversion carried out by the converters 3 _1,2 and 3 _3,4 is carried out at the same time from the same sampling clock signal CLK delivered by the time base in order to respect the perfect synchronism between the different sub- antennas. Each of the aforementioned analog digital converters 3 _1,2 respectively 3 _3,4 delivers the digitized data on an independent serial train.

The specificity of the implementation due to the large number of data to be processed, both in analog and digital, is emphasized here. Indeed, the circuits Up to date digital audio has only allowed the development of digital analog converters of stereo type with two channels, and not with four channels as represented in FIG. 5a.

It will of course be understood that the digital processing processor makes it possible to carry out the various digital filtering operations on multiplexing on the digital data streams each corresponding to the data and therefore to the signals delivered by each of the sub-antennas. Multiplexing is thus carried out without shifting the sampling clock signals in any way thanks to the creation of a specific time base with a system for tracking the start of the frame, in order to find the signals relating to each of the channels in the corresponding order for the same given sampling instant.

• L/R, le signal d'horloge d'échantillonnage des deux convertisseurs analogiques numériques d'entrée, soit les convertisseurs 3_1,2 respectivement 3_3,4,
• BIO, un signal de synchronisation temporelle de début de trame ou de train de données pour chaque instant d'échantillonnage adressé au processeur de signal DSP,
• FSR, un signal de commande de début d'enregistrement dans le processeur de signal DSP du train de valeurs numériques correspondant à une voie ou sous-antenne pour l'instant d'échantillonnage courant nT, par exemple,
• Drr, les valeurs numériques des signaux délivrés par chacune des voies ou sous-antennes pour les instants d'échantillonnage courant nT, respectivement (n+1)T,
• Iack, la réponse du processeur de signal au signal FSR de début d'enregistrement,
• Phase, le traitement correspondant de chacun des trains représentés par le signal Drr par le processeur de signal.

As shown in FIG. 5b, graduated on the abscissa in relative time values and on the ordinate in logical values 01 of the corresponding signals, the following signals represent:

• L / R, the sampling clock signal of the two analog input digital converters, ie the converters 3 _1.2 respectively 3 _3.4 ,
• BIO, a time synchronization signal at the start of the frame or data stream for each sampling instant addressed to the signal processor DSP,
• FSR, a command to start recording in the signal processor DSP of the stream of digital values corresponding to a channel or sub-antenna for the current sampling instant nT, for example,
• Drr, the digital values of the signals delivered by each of the channels or sub-antennas for the sampling moments current nT, respectively (n + 1) T,
• Iack, the signal processor’s response to the FSR start recording signal,
• Phase, the corresponding processing of each of the trains represented by the signal Drr by the processor signal.

The operation of the assembly is as follows: following the appearance of the signal FSR, the recording of each train corresponding to a sampling instant is carried out at the level of the signal processor, then, on interruption generated at the level of the processor signal, the filtering processing is carried out by the latter, the signal Iack representing a response to this interruption and therefore to the filtering carried out for each of the data streams represented by the signal Drr. The signals delivered by the signal processor, in which a summing was carried out, are then subjected to digital to analog conversion.

Specifically, it is indicated that the filtering performed by the signal processor is carried out in digital form in preference to an analog filtering for reasons of design flexibility. The sum of the filters applied to each of the corresponding signals delivered by the sub-antennas is equal to unity for all frequencies, these filters should not introduce any degradation of the signal. These filters are also of limited size in order to maintain a reasonable complexity of calculations.

Filters can be designed according to different criteria. In a first approach as shown in FIG. 6a, each filter thus produced by the filtering by the signal processor is a filter of the conventional bandpass type whose cut-off frequencies are chosen so that each sub-antenna is limited to its own frequency band. FIG. 6a presents the frequency responses of the filters thus retained. For this purpose, finite impulse response filters RIF having for example 31 coefficients are used. These filters are then synthesized using a standard RIF filter design procedure. An iterative algorithm ensures that the sum of the filters is equal to unity for all frequencies, the transfer function of each of the filters being well limited to its own frequency band. For a more complete description of this type of filtering, reference may be made to the article mentioned in the introduction to the description.

A second approach consists in implementing filters so as to minimize a determined criterion, which can for example correspond to minimizing the energy received outside the area where the speaker is located. One then calculates for each frequency the linear combination of the sub-antennas which minimizes the aforementioned criterion. This calculation can be performed using well known optimization procedures. At the end of this phase, the frequency response of the filters to be applied to each sub-antenna is available, and it is then sufficient, in order to know the impulse responses, to use conventional filter calculation procedures.

Figure 6b shows the frequency responses of the filters resulting from the above optimization. The filters thus produced make it possible to minimize the energy received from directions external to the useful opening area defined as an area of ± 60 ° in the plane containing the electro-acoustic transducers M₀ to M₈. These filters have a length of 128 coefficients. The second embodiment as shown in FIG. 6b makes it possible to increase the directivity of the antenna under the conditions previously indicated.

A particularly efficient acoustic antenna has thus been described insofar as it is capable of being used for multimedia workstations for example.

In such a case, as shown in FIG. 7a or 7b, it is understood, in a first application mode, that the aerial 1 can be placed on the video display monitor while the electro-acoustic transducers M₉ and M₁₀ are placed in the vicinity of the sides of this monitor symmetrically with respect to the main axis of the antenna, the plane P containing the electro-acoustic transducer M₀. We of course indicates that the distance separating each electro-acoustic transducer M₉, M₁₀ from the aerial 1 is taken substantially equal to D = 30 cm.

In another mode of use represented in FIG. 7b, the aerial 1 is placed laterally to the video display monitor on one of the sides thereof, the aerial 1 then being fixed on the side of the latter at by means of a suction cup system for example or the like. In this case, the electro-acoustic transducers M₉ and M₁₀ can on the contrary be placed symmetrically with respect to the plane of symmetry P of the aerial 1 on two generating lines G₁₀, G₉ of the convex surface S. In addition, an electroacoustic transducer M₁₁ belonging to the first sub-antenna, band 1 in FIG. 6a, can be placed at the top of the video display monitor, halfway between the aerial 1 and the electro-acoustic transducers M₉, M₁₀ in order to improve the overall performance of the antenna.

It is of course understood that in the case of the use according to FIG. 7b, the directivity diagram of the acoustic antenna thus formed approximates that of the acoustic antenna as used in FIG. 7a with a rotation of 90 ° .

The acoustic antenna object of the present invention is particularly advantageous insofar as it allows a selective sound which favors the user by greatly reducing the degradations linked to ambient noise and to the room effect. Compared to a single microphone, a current conventional solution, the gain in spatial selectivity is significant as well as the reduction of noise internal to the sensor.

In addition, the space constraints of the aerial 1 are not excessive compared to the solution consisting in using a single microphone.

In addition, analog to digital conversion and digital processing can be easily integrated into an echo control and coding system.

Finally, the design of the acoustic antenna, object of the present invention, makes the mechanical assembly simple and industrialization presents no problem.

Given the improvements made by the acoustic antenna, object of the present invention, better listening comfort is provided to the remote listener while, on the other hand, an improvement in the performance of automatic speech recognition systems. is obtained.

Claims (8)

Selective sound pickup system for reverberant and noisy environment, comprising a plurality of electro-acoustic transducers intended to receive useful sound signals in phase originating from the same useful speech area, said useful sound signals being summed in phase and the signals sound coming from zones other than the useful zone being summed in phase inconsistency to select the sound signals coming from said useful speech zone, characterized in that said electro-acoustic transducers are of substantially unidirectional type, said system comprising in combination: - an aerial formed by a part of concave cylindrical surface, the concavity of said cylindrical surface being oriented towards the useful speech area, said plurality of electro-acoustic transducers being distributed over and in the vicinity of said cylindrical surface, each electro-acoustic transducer being oriented towards said useful speech area and delivering an analog sound signal, all of the aerial and electro-acoustic transducers forming an acoustic antenna, said electro-acoustic transducers being divided into a first and a second group, said transducers electro-acoustic of the first group being in odd number aligned on a line of the curvilinear surface parallel to a guideline of the cylindrical part and placed symmetrically with respect to a plane of symmetry containing a line generating said cylindrical surface and an electro-transducer central acoustics, said transducts electro-acoustic urs of the second group being distributed over one or more guidelines of the concave cylindrical surface, means of partial summation of a plurality of analog sound signals delivered by each of the electro-acoustic transducers, according to a determined arrangement, which makes it possible to form from the transducers electro-acoustic a plurality of elementary acoustic antennas each delivering a resulting elementary analog sound signal, means for filtering and summing said resultant basic analog sound signals to generate a resulting analog sound signal representative of the sound signal from the useful speech area. Selective sound pickup system according to claim 1, characterized in that said electro-acoustic transducers of the second group are distributed symmetrically with respect to the plane of symmetry. Selective sound pickup system according to claim 1 or 2, characterized in that each elementary acoustic antenna is further associated with a means of filtering in low-pass, band-pass or high-pass frequencies, the frequency bands transmitted by each filter associated with an elementary acoustic antenna being offset so as to cover a resulting passband covering frequencies between 25 Hz and 7500 Hz. Selective sound pickup system according to one of claims 1 to 3, characterized in that said cylindrical surface part is formed by a frame of substantially rectangular shape made of plastic material, said frame being provided with spacers and screw teeth vis-à-vis formed in the plane of the frame, two teeth facing each other forming a housing intended to receive an electro-acoustic transducer, said frame being curved so as to form said part of cylindrical surface. System according to claim 1, characterized in that said acoustic antenna further comprises an electromagnetic shielding making it possible to ensure immunity of said electro-acoustic transducers to parasites or electromagnetic disturbances. System according to claims 4 and 5, characterized in that said electromagnetic shielding is formed by a metal grid covering the concave part and the convex part of the cylindrical surface part, and the electro-acoustic transducers, said metal grid being electrically connected to the reference electrical potential of the electro-acoustic transducers. System according to claim 1, characterized in that said transducers of the second group are placed on a mechanical structure independent of said cylindrical surface. Use of a selective sound pickup system according to one of claims 1 to 7, with a multimedia workstation, the aerial and the electro-acoustic transducers of the first group being placed in the vicinity of one side of the monitor d video display of the workstation, the transducers of the second group being placed on at least one guideline of the concave cylindrical surface.

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