EP0710427B1 - Sound pick-up and reproduction apparatus and use thereof for conference calls - Google Patents

Abstract

A sound pick-up and reproduction apparatus (Aac) which is rotatable about an axis (D) and includes a mounting (1), a sound pick-up device (2) and a sound reproduction device (3) with a convex or planar surface (revolving cone) opposite the sound pick-up device. Said sound reproduction device (3) fits within a revolving cone (B' C A') of which the axis coincides with the axis of symmetry (D), and is directly attached to the mounting (1). The apparatus is useful for conference calls.

Description

The present invention relates to a sound pickup and playback device.

The present invention has a main application in the field of audio conference, in which a device for picking up and reproducing sound is included in a single set of relatively small dimensions. This assembly must be able to be placed easily on a table and operate in any room without the need for acoustic treatment of these premises. We would like it to be usable by a person with great freedom of movement within a radius of at least 4 m around the device while continuing the conversation with his interlocutor under normal listening comfort conditions for both interlocutors.

• 1. Le dispositif doit être associé à deux régulateurs automatiques de niveau qui assurent l'envoi en ligne d'un signal de niveau correct quelle que soit la puissance acoustique recueillie par le(s) microphone(s) du dispositif en fonction de la position du(des) locuteur(s) par rapport à ce(s) microphone(s) et l'envoi vers le(s) hauts-parleur(s) d'un signal de niveau correct quel que soit l'affaiblissement apporté par la ligne.
• 2. Le son restitué par le(s) hauts-parleur(s) doit être perçu avec suffisamment de confort d'écoute indépendamment de la position occupée par l'(les)auditeur(s) dans le local.
• 3. Le son recueilli par le(s) microphone(s) doit garder des qualités de clarté, de netteté et d'agrément d'écoute suffisamment stables quelle que soit la position du(des) locuteur(s) par rapport au dispositif et quelle que soit la configuration du local.
• 4. Le dispositif doit présenter un bon découplage acoustique entre le(s) hauts-parleur(s) et le(s) microphone(s) de manière à pouvoir assurer une écoute d'un niveau sonore suffisamment élevé sans provoquer d'effet LARSEN, mais aussi pour renvoyer le moins possible d'écho acoustique vers l'interlocuteur distant.

Preferably, it can also be used by any number of people gathered in the same room and distributed around the piece of furniture on which the device is placed. To obtain these results, four conditions are sought:

• 1. The device must be associated with two automatic level regulators which ensure that a correct level signal is sent online regardless of the acoustic power collected by the device's microphone (s) depending on the position of the speaker (s) in relation to this microphone (s) and the sending to the speaker (s) of a signal of correct level regardless of the attenuation brought about by the line.
• 2. The sound reproduced by the loudspeaker (s) must be perceived with sufficient listening comfort regardless of the position occupied by the listener (s) in the room.
• 3. The sound collected by the microphone (s) must keep qualities of clarity, sharpness and listening pleasure sufficiently stable whatever the position. of the speaker (s) in relation to the device and whatever the configuration of the room.
• 4. The device must have a good acoustic decoupling between the speaker (s) and the microphone (s) so as to be able to listen to a sufficiently high sound level without causing a LARSEN effect. , but also to send the least possible acoustic echo back to the remote party.

Currently operational devices are known which satisfy condition 1.

There are also devices which favor condition 4 by using a single microphone and four loudspeakers oriented in four directions spaced by an angle of π / 2 radians from each other, and attacked in phase opposition two by two. This process effectively makes it possible to obtain a weak coupling because the microphone is placed at a point which is a center of symmetry with respect to the loudspeakers. As these are attacked in phase opposition two by two, and provided that they have identical characteristics, the sound coming from the speakers collected by the microphone will be very weak and therefore the decoupling will be very good.

But this type of device does not fulfill conditions 2 (because of the phase shifts of π radians between speakers, the radiation pattern of the speaker assembly will not be circular in the horizontal plane and will depend significantly on the frequencies emitted) and 3 (because the microphone indifferently picks up direct sounds and indirect reflected sounds, which means that the quality of the sound picked up by the microphone depends too largely on the position of the speaker in the room and the configuration of this room).

The document WO-A-9 210 917 relates to a sound recording device of simple structure containing a single microphone.

In a French patent application No. FR-A-2 682 251 filed on October 2, 1991, published on April 9, 1993, and therefore not part of the state of the art under Rule 33 PCT, the Applicant proposed a method and an apparatus for sound recording the main purpose of which is to give rise to a low sensitivity to sounds arriving in a predetermined direction. This document relates to a device for taking and reproducing sound from at least one sound source, intended to be placed on a work surface, of the type comprising a support provided with a flat surface, a sound taking device arranged on this planar surface and a sound reproduction device, said sound pick-up and reproduction devices being arranged relative to each other so as to present a structure of revolution around an axis of symmetry orthogonal to said surface plane.

Another object of the invention which is the subject of this patent application is that, in a plane perpendicular to the predetermined direction, a sensitivity varying relatively little is obtained as a function of the direction from which the sounds originate and as a function of the frequency components of these sounds.

In the context of the preferred, although not limiting, use of the invention, the subject of this patent application, of an audio conference device with sound pickup and reproduction, the achievement of the above aim then allows, by orienting one or more loudspeakers in said predetermined direction, fully satisfying condition 3 above, while satisfying conditions 1, 2 and 4 at least as well as the devices of the prior art.

The invention which is the subject of said French patent application thus proposes a sound pickup method using several sound reception devices, according to which the sound reception devices are arranged substantially in the same plane and they are distributed symmetrically with respect to to a direction of symmetry perpendicular to this plane, a phase shift is applied between the signals originating respectively from different sound reception devices and the signals thus phase shifted are added, so as to substantially cancel the signals relating to any sound wave arriving in phase and with the same intensity on each of the sound receiving devices.

Thanks to the symmetrical arrangement of the sound reception devices, the incident sounds in the direction of symmetry reach them in phase and with the same intensity. Consequently, due to the applied phase shifts and the addition of the phase-shifted signals, these incident sounds in the direction of symmetry are substantially eliminated after the processing. On the other hand, the incident sounds perpendicular to the direction of symmetry reach the various reception devices with phase and / or amplitude differences between these devices. These sounds are therefore preserved and correctly taken into account.

According to a preferred version of the method of this invention, an even number of sound receiving devices are used which are associated in pairs, the sound receiving devices of each pair being arranged symmetrically with respect to the direction of symmetry, and subtract one from the other the signals coming respectively from the sound reception devices of each pair to add them with a phase shift of 2 π radians between them.

Thus, the incident sounds in the direction of symmetry, as well as various parasites can be effectively eliminated by a simple subtraction of the signals originating respectively from the reception devices of each pair. This subtraction can advantageously be carried out in conjunction with a preamplification by means of a differential preamplifier connected to the output of the reception devices of each pair.

Preferably, in the above method, 2n associated sound reception devices are used in pairs and arranged at regular intervals along a circumference centered on the direction of symmetry, n denoting an integer at least equal to two , and a phase shift of 2 π radians / 2n is applied between the signals originating respectively from any two adjacent sound reception devices. These features make it possible to obtain a regular radiation pattern in a plane perpendicular to the direction of symmetry. In principle, the higher the number n of pairs of sound receiving devices, the more homogeneous the radiation pattern in the plane perpendicular to the direction of symmetry. In practice, it can be seen that with two pairs of reception devices, an excellent compromise can be obtained between this homogeneity and the cost of the components used.

The aforementioned patent application also has the second object of a sound recording system conforming to said process.

According to a third subject, the invention which is the subject of this patent application proposes a sound pickup and restitution apparatus comprising at least one speaker oriented in a direction of symmetry and sound pickup means, in which the means for sound recording comprise a system according to the second object of the invention, with the direction of symmetry of the system identical to the direction of orientation to the loudspeaker.

This device can be used for audio conferences and satisfies very satisfactorily the criteria 1 to 4 listed at the beginning of this description and therefore satisfies the set goals. Figure 1, which will be detailed later, shows such an apparatus.

It comprises three main parts, a base 1 having a flat surface 20, a sound pickup device 2 (microphones) and a sound reproduction device 3 (loudspeaker).

Experience has shown, however, that it is difficult to master and reproducibly minimize the mechanical coupling between the sound pickup 2 and sound reproduction devices 3, in the configuration of the device according to the aforementioned request.

On the other hand, experience has also shown under certain conditions of use irregularities in the amplitude response curve as a function of the frequency of the sound recording.

The invention sets itself the aim, while retaining the advantages of the device of the above-mentioned French patent application, of overcoming these drawbacks. To do this, the invention provides to fix the sound reproduction device directly on the flat surface of said base and to provide the part of the reproduction device with his view of the sound recording device. of an appropriate form, that is to say a form such that its intersection with any horizontal plane passing through it is a circle.

In a preferred variant, it is a convex shape comprised between two limiting shapes: a cone and a hemisphere.

Still in a preferred variant, the sound reproduction device is fixed to the base by means of a spacer of small dimensions. Advantageously, there are four spacers, regularly spaced by π / 4 radians.

The subject of the invention is therefore a device for taking and reproducing sound from at least one sound source, intended to be placed on a work surface, of the type comprising a support provided with a flat surface, a taking device of its arranged on this flat surface and a sound reproduction device, said sound pick-up and reproduction devices being arranged relative to each other so as to have a structure of revolution around an axis of symmetry orthogonal to said flat surface, characterized in that said sound pickup device comprises several elementary sound reception devices in number n greater than two, and processing means for processing the signals coming from the sound reception devices, the devices sound reception elements (M1 to M4) being arranged in the same plane, perpendicular to said axis of symmetry, at regular intervals, on a circumference centered relative to audi t axis of symmetry, said processing means being arranged to apply a phase shift equal to 2 π / n radians between the signals respectively from two elementary sound reception devices of any adjacent type and to add the signals thus phase shifted so as to obtain reception substantially uniform and not weakened signals relating to the components parallel to the plane regardless of the direction of the waves and reception substantially sound recording of an appropriate form, that is to say a form such that its intersection with any horizontal plane passing through it is a circle.

In a preferred variant, it is a convex shape comprised between two limiting shapes: a cone and a hemisphere.

Still in a preferred variant, the sound reproduction device is fixed to the base by means of spacers of small dimensions. Advantageously, there are four spacers, regularly spaced by π / 4 radians.

The subject of the invention is therefore a device for taking and reproducing sound from at least one sound source, intended to be placed on a work surface, of the type comprising a support provided with a flat surface, a taking device sound arranged on this flat surface and a sound reproduction device, said sound pick-up and reproduction devices being arranged relative to each other so as to have a structure of symmetry about an axis of symmetry orthogonal to said flat surface, characterized in that the sound reproduction device is fixed directly to said flat surface; in that the restitution device fits inside a cone of revolution with an axis coincident with said axis of symmetry; this cone being obtained by drawing a straight line between a first point distant by a first determined length from said axis of symmetry and by a determined height from said work plane and a second point located on said flat surface and distant from said axis of symmetry by a second determined distance, the line segment intersecting said axis of symmetry at the top of the cone, and by rotating this line segment by an angle of revolution equal to 2 π radians; and in that the surface of said sound reproduction device opposite said sound recording device is provided with a shape comprised between a hemisphere and a cone.

• la figure 1 représente une vue en coupe axiale d'un appareil conforme à la demande de brevet français FR-A-2 682 251 ;
• la figure 2 représente une vue en coupe d'une partie de l'appareil représenté à la figure 1, prise selon le plan II-II indiqué sur la figure 1 ;
• la figure 3 représente un schéma d'ensemble des moyens de traitement des sons captés par les microphones de l'appareil des figures 1 et 2 ;
• la figure 4 est un schéma explicitant la recommandation P34 du CCITT ;
• la figure 5 est un exemple de courbe de réponse en amplitude en fonction de la fréquence pour des dispositifs de prise de son conformes à la demande de brevet précité et à l'invention, respectivement ;
• la figure 6 illustre schématiquement un exemple de réalisation d'un appareil selon l'invention ;
• les figures 7 et 8 illustrent schématiquement deux variantes de réalisation d'un dispositif de restitution de son incorporé dans un appareil conforme à l'invention ;
• la figure 9 illustre des dispositions spécifiques à l'invention.

The invention will be better understood and other characteristics and advantages will appear on reading the description which follows with reference to the appended figures and among which:

• Figure 1 shows an axial sectional view of an apparatus according to French patent application FR-A-2 682 251;
• 2 shows a sectional view of part of the apparatus shown in Figure 1, taken along the plane II-II shown in Figure 1;
• FIG. 3 represents an overall diagram of the means for processing the sounds picked up by the microphones of the apparatus of FIGS. 1 and 2;
• FIG. 4 is a diagram explaining the recommendation P34 of the CCITT;
• FIG. 5 is an example of an amplitude response versus frequency curve for sound pick-up devices in accordance with the aforementioned patent application and with the invention, respectively;
• Figure 6 schematically illustrates an embodiment of an apparatus according to the invention;
• Figures 7 and 8 schematically illustrate two alternative embodiments of a sound reproduction device incorporated in an apparatus according to the invention;
• FIG. 9 illustrates arrangements specific to the invention.

We will first of all recall the main characteristics of the device which is the subject of the above-mentioned application, to the extent that they have an impact on the present invention.

We will refer to Figures 1 and 2. The apparatus illustrated in these figures comprises a housing 1, a body or sound recording device 2 in which are housed several sound reception devices M1, M2, M3, M4, and a sound reproduction device or element 3 in which a loudspeaker 4 is mounted. The body 2 and the element 3 have a general shape of revolution around in a direction of symmetry D. The element 3 is mounted on the body 2 which is itself mounted on the housing 1. Phonically insulating materials, and / or mechanically damping materials such as 5, can be interposed between the element 3 and the body 2, or even between the body 2 and the upper part of the housing 1. In general, the device has a symmetrical structure around the direction D to minimize the effect of mechanical vibrations which can affect the signals produced. by microphones M1, M2, M3, M4.

The housing 1 is provided at its lower part with feet 6 of rubber or the like for placing the apparatus on a horizontal surface such as a table. The direction of symmetry D is then vertical, which is the general case. Electrical circuits 7, 8 are mounted inside the housing 1. These circuits can be connected as shown diagrammatically, at 9, 10 in FIG. 1, to an external audio conference system (not shown) with which the apparatus according to the invention. These circuits include an amplification circuit 7 which receives signals from the audio conference system and sends them in amplified form to the loudspeaker 4 so that the latter emits the corresponding sounds, and processing means 8 for processing the signals from the elementary sound reception devices M1, m2, M3, M4, and address them after processing to the audio conference system. In a known manner, the amplification circuit 7 can include, to increase listening comfort, an electronic cell for correcting the response curve of the loudspeaker 4, in particular to reinforce the low frequencies and eliminate any resonances or anti-resonances. In addition, conventional means of echo cancellation are generally mounted between circuits 7 and 8.

In the example shown, there are four elementary sound reception devices, each consisting of a single microphone M1, M2, M3, M4. These four microphones M1, M2, M3, M4 are all arranged in the same horizontal plane P perpendicular to the direction of symmetry D.

As seen in Figure 2, the four microphones M1, M2, M3, M4 are distributed symmetrically with respect to the direction of symmetry D, which is perpendicular to the plane of Figure 2. These four microphones are located on a circumference 13 parallel to P and centered on the direction of symmetry D. These four microphones are associated in pairs, respectively M1, M3 and M2, M4, the microphones of each pair being arranged symmetrically with respect to the direction of symmetry D, and the two pairs microphones being arranged along two radial lines 14, 15 forming between them a right angle.

Each of the microphones M1, M2, M3, M4 is housed in a respective cavity 12 machined in the body 2. This body 2 is metallic, for example made of brass. It is traversed by an axial bore 16 in the direction of symmetry D, and it further comprises four radial bores 17, each extending between the axial bore 16 and one of the four cavities 12. The axial bore 16 serves the passage of the connecting wires (not shown) of the loudspeaker 4 to the amplification circuit 7, with a corresponding bore 18 provided at the base of the element 3. The axial bore 16 and the four radial bores 17 are used for passage of the connection wires (not shown) of the microphones M1, M2, M3, M4, to the processing means 8 located in the housing 1.

The four microphones M1, M2, M3, M4 are advantageously of the condenser type, and have a small dimension (for example a cylindrical shape with a diameter of 6 mm, and a height of 4.5 mm). It is known that, for a given production series, such microphones have substantially the same response curve, with an offset between them not exceeding 3 to 4 decibels. For the realization of the device, it is therefore easy to sort four microphones having response curves identical to a predetermined tolerance (for example 0.5 decibel).

The body 2 is mounted on a flat metal plate 20, parallel to the plane P of the microphones and constituting the upper face of the housing 1. The cylindrical body 2 has an axial cylindrical extension 21, of smaller diameter which rests on this flat plate 20 and which defines a spacing 22 between the flat plate 20 and the surface 23 of the body 2 which is parallel to the plane P, and on which the machined cavities open 12. The extension 21 of the body 2 provides a certain acoustic insulation between the microphones M1 , M2, M3, M4 vis-à-vis the sounds arriving in a plane perpendicular to the direction of symmetry D. As can be seen in FIG. 1, the cavities 12 have an axial height greater than the height of the cylinders of the microphones M1 , M2, M3, M4, and the latter are pressed into their respective cavities 12 so as to leave a gap 24 between the side of each microphone facing the plate 20 and the surface 23 defining the bor d of the cavities 12.

At the rear of the microphone M1, M2, M3, M4, each cavity 12 is extended into a part 25 of smaller diameter which defines a shoulder against which the rear face of the microphone rests, and into which the radial bore opens. 17, thus giving space for the connection wires not shown.

The element 3 mounted above the body 2 forms a resonance box for the loudspeaker 4. The loudspeaker 4 is mounted in the element 3 in the direction of symmetry D, and oriented in this direction of symmetry D , opposite the plane P where the microphones M1, M2, M3, M4 are located. This means that the membrane 29 of the speaker 4, which has a shape of revolution around an axis, is arranged in the element 3 so that this axis coincides with the direction of symmetry D of the device, the edge outside 30 of this membrane 29 being located in a plane perpendicular to the direction of symmetry D. For an application to audio conferencing, this outer edge 30 of the membrane 29 is typically located between 100 and 150 mm above the horizontal surface on which the device is placed. A protective grid 32 is mounted at the top of the element 3 to protect the membrane 29 of the loudspeaker 4.

The external peripheral surface 33 of the element 3 has a concave curvature and is tangentially connected to the external peripheral surface of the body 2, this external peripheral surface of the body 2 being a cylinder defined by generatrices substantially parallel to the direction of symmetry D.

Instead of four microphones, it is possible to use a number of microphones n greater than two, always arranged at regular intervals, on a circumference centered with respect to said direction of symmetry D.

In a particular variant embodiment (not shown), each elementary reception device is composed of several microphones and not of a single microphone as shown in FIG. 2 (the microphones M1 to M4 respectively). The output signals from these microphones are added in phase.

In a preferred embodiment, elementary sound reception devices are used in an even number, ie m = 2 n, with m greater than three.

Said French patent application also provides means for processing signals from sound reception devices arranged to apply a phase shift equal to 2 π radians / n (or 2 π radians / m in the last variant mentioned above) between the signals derived respectively from any two adjacent sound reception devices and for adding the signals thus phase-shifted so as to obtain a substantially uniform and not weakened reception of signals relating to the components parallel to the plane P, whatever the direction of the waves, and a reception substantially zero of the signals relating to the components parallel to the line of symmetry D of the captured acoustic waves.

In practical terms, differential amplifiers and cascade phase shifters are used to perform these functions.

FIG. 3 schematically represents an exemplary embodiment of the processing means 8 for the signals coming from the microphones M1, M2, M3, M4. These processing means comprise on the one hand two differential preamplifiers A13, A24 of inputs E1 to E4, supplying the respective signals S13 and S14, and two phase shift channels D13, D24 for applying a phase shift between the signals coming respectively from different microphones of outputs SD13 and SD14, and on the other hand an adder circuit 40 intended to sum the phase-shifted signals coming from the phase-shifting channels D13, D24. At the output ST of the adder circuit 40 is mounted a circuit 41 which formats the signals with a view to their transmission to the external audio conference system (not shown) and provides an output signal VS. In accordance with the invention, the phase shifts applied and the addition carried out are such that the signals relating to any sound wave arriving in phase and with the same intensity on each of the microphones M1, M2, M3, M4 are substantially canceled at the output of the circuit. adder 40. In particular, when the device is placed horizontally on a table, the sounds emitted by the speaker 4 and reflected by the horizontal ceiling located above the device reach the four microphones in the direction of symmetry D and have, taking into account the symmetrical arrangement of the microphones, an identical phase and intensity on each of the microphones. Consequently, these reflected signals are advantageously eliminated from the output signal from the processing circuit 8. In addition, the symmetrical structure of the sound pickup system ensures that the mechanical vibrations of the device will reach each of the microphones in an identical manner. Therefore, the effect of these vibrations on the microphones is also eliminated from the output signal from the processing circuit 8.

In the example shown in FIG. 3, a differential preamplifier A13 (respectively A24) has two inputs E1, E3 (respectively E2, E4) each connected to one of the microphones M1, M3 (respectively M2, M4) of a pair of microphones arranged in diametrically opposite position relative to the direction of symmetry D. The differential preamplifiers A13, A24 carry out a preamplification of the output signals of the microphones, eliminate certain parasites present in these output signals. In this combination, therefore, the contributions of the sounds reaching the microphones in the direction of symmetry D and the effects of symmetrical mechanical vibrations are eliminated. In the preferred application of the device to audio conferencing, the sounds emitted by speakers are thus taken into account in a satisfactory manner regardless of the position of these speakers relative to the device, while the echoes of the speaker are noticeably eliminated. In addition, the arrangement of the microphones M1, M2, M3, M4 in the body 2 (FIG. 1) and the presence of the pressure zones between this body 2 (FIG. 1) and the metal plate 20 (FIG. 1) reflecting the sound waves. largely eliminate indirect echoes from microphones.

These provisions are entirely preserved in the context of the present invention.

Thanks to the structural arrangements as described in relation to FIGS. 1 and 2 and to the arrangements concerning the processing circuits 8 which have just been recalled in connection with FIG. 3, the apparatus for taking and restoring its sound fills in a manner very satisfactory criteria 1 to 4 of the preamble of this description.

However, as indicated, experience has shown that it is difficult to control and reproducibly minimize the mechanical coupling between the sound pickup device 2 (Figure 1) and the playback device 3. More specifically, if we refer again to the device of Figure 1, it is difficult to obtain that each microphone (M1 to M4 in the example illustrated) receives the same mechanical vibration in amplitude and phase, even if an insulating material is used 5.

On the other hand, under certain conditions of use, experience has also shown that the frequency response curve of the sound recording exhibits irregularities. In particular, these conditions generally exist if one falls within the framework of CCITT recommendation P34.

$${\text{l}}_{\text{1}}\text{= 500 mm}$$

$${\text{l}}_{\text{2}}\text{= 400 mm}$$

Figure 4 schematically illustrates some features of this recommendation. An artificial Bart mouth is used to test the reception of an Aac audio conference device, for example a device of the type described in relation to FIG. 1. This Aac device is placed on a plane, for example a table. Your used for audio conferencing. The Bart artificial mouth is placed vertically and at a height h from the edge of the table Ta. The angle a is therefore equal to π / 2 radians. The edge of the table is at a distance l ₂ from the Aac apparatus and the artificial mouth Bart at a distance l ₁ from the Aac apparatus. Under these conditions, the respective values of these parameters are: $$\text{h = 300 <figure-callout id="1" label="mm l" filenames="imgf0003.png" state="{{state}}">mm </figure-callout>}$$

$${\text{l}}_{}$$$${}_{\text{1}}\text{= 500 <figure-callout id="2" label="mm l" filenames="imgf0003.png,imgf0004.png" state="{{state}}">mm </figure-callout>}$$

$${\text{l}}_{}$$$${}_{\text{2}}\text{= 400 mm}$$

Under the aforementioned measurement conditions, with the device of FIG. 1, a curve C ₁ of typical frequency amplitude response illustrated by FIG. 5 is obtained. The vertical axis graduated in dB, represents the amplitude AS of the signal VS (Figure 3) as a function of the frequency F carried on the horizontal axis.

• courbe non lissée relevée dans une pièce non traitée acoustiquement;
• réalisation d'une transformée de Fourrier rapide sur une réponse impulsionnelle limitée à un intervalle de temps de 10 ms pour éliminer les accidents dus à des réflexions lointaines sur les murs et le plafond de la pièce.

More specifically, the conditions for obtaining the curve C1 is as follows:

• unsmoothed curve raised in a room not acoustically treated;
• realization of a fast Fourier transform on an impulse response limited to a time interval of 10 ms to eliminate accidents due to distant reflections on the walls and the ceiling of the room.

Curve C1 has a "hole" with a depth of approximately 12 dB and a width of the order of 100 Hz for a frequency usually included in the frequency interval ranging from 2000 Hz to approximately 2500 Hz. In the present case, the central frequency of the irregularity is approximately 2.2 kHz. The curve typically has a growth of 3 dB per octave with undulations of the order of ± 2 dB depending on the specific responses of each microphone and the quality of their pairing. This growth of 3 dB per octave is recommended by recommendation P34.

We have drawn a straight line C3 with a slope of 3 dB per octave which approaches the shape that the perfectly smoothed curve C1 would have, in order to facilitate its reading and comparisons with the curve C2 which will be described later.

It should naturally be understood that the detail of this curve C1 depends on the nature and the characteristics of the microphones M1 to M4, on the configuration of the table Ta, on the angular position of the sound pickup device relative to the source Bart, on atmospheric pressure, etc.

This curve is given only as a better illustration of the parasitic phenomenon observed.

The invention while retaining the advantages of the device described in relation to Figure 1, aims to overcome the two faults which have just been recalled.

To do this, if we refer again to Figure 1, we will move the device 3 away from the device 2. But to keep the whole a compact appearance, this distance must remain as small as possible.

To more specifically remedy the first defect, the device 3 will be fixed directly no longer on the device 2, with or without the intermediary of an insulating material 5, but directly on the surface of the plane 20.

In a preferred variant, it is fixed using spacers, near the outer edge of this plane 20.

To more specifically remedy the second defect, we endow the part of the device 3 which is in "view" of the device 2, that is to say in the example illustrated in FIG. 1, the lower surface part 33, of an appropriate form which will be explained in the following.

FIG. 6 schematically illustrates an apparatus for taking and restoring its Aac according to the invention.

With the exception of the provisions specific to the invention, the other provisions which have been described in relation to FIGS. 1 to 3 are retained and will only be re-described as necessary.

In particular, the sound recording device 2 may be identical to that described in relation to FIGS. 1 and 2 or to its possible variants which have been indicated. It is the same for the housing 1, the plane 20, and the electrical processing and amplification circuits (respectively 7 and 8) contained in the housing 1. These circuits could naturally be arranged outside of this housing 1, but the arrangement inside the housing 1 has the advantage of reducing the length of the connections between the amplification circuits and the loudspeaker, on the one hand; and the processing circuits 7 and the microphones M1 to M4, on the other hand.

We naturally keep the symmetric of revolution around the axis D.

The elements of Figure 6 identical or similar to those of Figure 1 bear the same reference and will also be rewritten only when necessary.

According to one of the important characteristics of the invention, the sound reproduction device 3 is no longer mechanically coupled to the sound recording device 2. In the preferred embodiment illustrated in FIG. 6, the device 3 is fixed directly to the housing 1, preferably near the edges of the plane 20, using spacers or the like, four in number Et1 to Et4 (in the example illustrated) and regularly spaced. In FIG. 6, the spacer Et2 is not visible. In the present case, the spacing between spacers is π / 4 radians.

In horizontal section, the shape of the spacers Etl to Et4 can be any: round, oval, triangular, etc. However, the largest dimension must remain small, typically in a range of 10 to 17 mm. This dimension, as well as the material will be chosen according to the weight and dimensions of the device 3, on the one hand, and to avoid any parasitic vibrations on the other hand.

The number of these spacers is not critical either but it seems difficult to descend below three to ensure stability and minimum rigidity to the assembly. Conversely, their number cannot grow indefinitely because they must be "transparent" to sound waves. At least the screen effect must be negligible.

A number equal to three or four is an acceptable compromise.

The means of fixing these spacers Etl to Et4, on the one hand, to the device 3 and, on the other hand, to the housing 1, is chosen from known conventional means: screwing, etc.

According to another important characteristic of the invention, the part of the device 3 is "seen" from the device 2 in an appropriate shape, in this case the lower surface 33 of the device 3.

This is preferably convex but can be between a cone and a hemisphere, which constitute the two extreme forms. In the case of a cone, we must eliminate the borderline case for which the apex angle is equal to π radians, that is to say a degenerate shape which tends towards a plane. In fact, in this case, there is a risk that standing waves will develop between the plane 20 and the surface 33.

Figures 7 and 8 illustrate the two extreme cases: conical (Figure 7) and spherical (Figure 8). In addition to the intermediate shapes, one can imagine shapes combining these two aspects, for example a conical shape, in the lower part of the surface 33 is transformed into a convex shape in the upper part.

The precise choice of a form, among those possible, will be guided more by practical considerations (simplicity of realization for example) or esthetic than by purely technical reasons.

In addition, it is most often required to keep a compact appearance to the device as indicated. The lower part of the device 3 must therefore be placed as close as possible to the device 2.

However, bringing the sound reproduction device 3 too close to the sound recording device 2 is incompatible with the aim which the invention sets for itself, since experience has shown that the frequency amplitude response curve would again exhibit irregularities.

It is assumed that the sound source, artificial or human, is at a height h from the plane of the table Ta or any other plane on which is placed the device for taking and restoring its Aac and at a distance 12 (projection onto the plane Ta) of the edge of the housing 1.

FIG. 9 explains what are the conditions to which the arrangement must subscribe in the space of the device 3. In this figure, the spacers Et1 to Et4 have not been shown.

On the plane 20, a line segment AB is defined perpendicular to the axis of symmetry D, centered on this axis and of determined length x. We consider the sound source SB placed at B ', on the left in FIG. 9 and as indicated above, ie at a height h of the plane Ta. We draw the line segment B'B. Similarly for a source SA, placed at A ′, on the right in FIG. 9, the line segment A ′ A is drawn.

If at first it is considered that the outer wall 100 (vertical in the example illustrated) of the housing 1 is a cylinder, it is easily realized that by moving the sources SA or SB in a plane P 'parallel to the plane Ta on a circle of radius 1, we define a cone whose vertex is the point C, located on the axis D. The length of the segment EC, E being the intersection of the plane P 'with the axis D, and the angle at the top naturally depend on the following parameters: h, l ₂ and x.

According to another important aspect of the invention, the surface 33 of the sound restoring device 3 must be inscribed inside the cone defined by A'A and B'B. The limit position towards the bottom (in the example illustrated) is such that all or part of the surface 33 merges with the surface of the cone thus defined. This is the case, in particular when the surface 33 is also conical (FIG. 7) and the angle at the top of this cone is equal to that of the cone B'CA '.

In the general case, if the outer edge 100 of the housing 1 is not cylindrical, an average value will be taken between the point furthest from the axis D and the nearest point, ie r. The length defining a _'2 = l ₂ + r and constructing a cylinder of height h and radius of the _two. This approximation is legitimate, because for the type of applications targeted by the invention, the Aac device is compact compared to the usual values of l ₂ .

In a preferred embodiment, there is the sound reproduction device 3 to get as close as possible to the aforementioned limiting case. Indeed, for a given shape of the surface 33, it is in this position that the device 3 will be closest to the sound pickup device 2 while meeting the conditions set out above.

Also in a preferred embodiment, the devices 3 and 2 will be arranged in such a way that the recommendation P34 defined by the CCITT is respected.

Under these conditions l ₂ = 400 mm and
h = 300 mm

The value of x, less than the circumference of the housing 1, is typically of the order of 140 mm, this value however not being limiting of the invention. We therefore define a circle on the surface 20, of radius equal to x / 2 and centered on the axis D.

With the arrangements adopted within the framework of the invention, the curve C2 of amplitude response as a function of frequency has been traced, under the same conditions as above for the device of FIG. 1.

We can compare, by referring again to FIG. 5, the new curve obtained C2 with the primitive curve C1.

It is easy to see if it still has an irregularity for a frequency centered on 2.2 KHz, approximately, this is much lower amplitude, of the order of -4 to -5 dB (in the example illustrated) by compared to the "smoothed" curve C3 and of very small width.

The sound pickup device 2 is also identical to that used in the device shown in FIG. 1. The same is true of the electronic circuits 7 and 8. In addition, in the sound reproduction device 3, the transducer ( speaker 4) is identical to that previously used.

Other measurements, carried out by the Applicant, repeatedly on several devices produced in accordance with the teachings of the invention, have shown that the mechanical vibrations due, essentially in the device of Figure 1, the mechanical coupling between the device 2 and 3, were now negligible. It follows that, even if each of the microphones picks up a non-homogeneous fraction, in amplitude and in phase, the differences due to this non-homogeneity no longer entail an appreciable dysfunction of the sound pickup device 2.

The invention is not limited to the embodiments precisely described in relation in particular to FIGS. 6 to 9. It should be understood, in particular that the numerical examples have been given only to better illustrate the invention and are not in no way limiting its scope.

Claims (17)

1. Apparatus (Aac) for recording and reproducing sound from at least one sound source (SB, SA), to be stood on a working plane (Ta), of the type comprising a support (1) provided with a plane surface (20), a sound recording device (2) disposed on said plane surface (20) and a sound reproducing device (3), said sound recording and reproducing devices being arranged with respect to each other thereby having a structure axisymmetric about an axis of symmetry (D) perpendicular to said plane surface (20), characterized in that said sound recording device (2) comprises plural elementary sound receiving devices (M1 to M4) in a number n greater than two, and processing means (8) for processing signals originating from the sound receiving devices, the elementary sound receiving devices (M1 to M4) being disposed on a same plane (P) perpendicular to said axis of symmetry (D), at regular intervals on a circumference (13) centered with respect to an axis of symmetry (D), said processing means (8) being arranged to apply a phase shift equal to 2π/n radians between the signals originating respectively from any two adjacent elementary sound receiving devices and to add the signals thus phase shifted so as to obtain a substantially uniform and non-attenuated reception of signals relating to the components parallel to the plane (P), regardless of the direction of the waves, and a substantially zero reception of the signals relating to the components parallel to the direction of symmetry (D), of picked up acoustic waves,
      and in that the sound reproducing device (3) is directly secured on said plane surface (20) and has an outer surface of revolution with a convex shape lying between an half sphere and a cone, opposite said sound recording device (2).
2. Apparatus according to claim 1, characterized in that the reproducing sound device (3) is contained inside a cone of revolution with an axis coinciding with said axis (D) ; this cone being obtained by drawing a segment of a line (B'B) between a first point (B') which is a first determined distance (1) away from said axis of symmetry (D) and a determined height (h) away from said working plane (Ta) and a second point (B) which is located on said plane surface (20) and a second determined distance (x/2) away from said axis (D), the segment of a line (B') intercepting said axis (D) at the vertex of the cone (C), and by turning this segment of a line (B'B) with an angle of revolution equal to 2 π, the first point (B') corresponding to the artificial mouth (B_art) of the recommendation P34 of CCITT.
3. Apparatus according to claim 1 or 2, characterized in that said sound reproducing device (3) is secured to said plane surface (20) by means of equidistributed spacers (Et1 to Et4).
4. Apparatus according to claim 2, characterized in that the number of the spacers (Et1 to Et4) is equal to four.
5. Apparatus according to any one of claims 1 to 4, characterized in that the first determined distance 1 is defined as follows : 1 = 400 mm + r, the height (h) is equal to 300 mm and the second determined distance (x/2) substantially equal to 70 mm, r being the middle value between the farthest point belonging to the outer edge (100) of the box (1) with regard to said axis of symmetry (D) and the nearest point belonging to the outer edge (100) of the box (1) with respect to said axis of symmetry (D).
6. Apparatus according to any one of claims 1 to 5, characterized in that the surface (33) of said sound reproducing device (3) opposite said sound recording device (2) is a cone with a vertex angle lower than π radians.
7. Apparatus according to any one of claims 1 to 6, characterized in that the sound reproducing device (3) comprises at least a loudspeaker (4), in that it comprises an active face (31) intended to radiate acoustic waves and in that the middle radiation direction coincides with said axis (D), following an opposite direction to said surface (33) opposite the sound recording device (2).
8. Apparatus according to any one of claims 1 to 7, characterized in that each elementary sound receiving device comprises a single microphone (M1 to M4).
9. Apparatus according to any one of claims 1 to 3, characterized in that each elementary sound receiving device is comprised of several microphones, and in that the processing means (8) are arranged in such a way as to add in phase the signals originating respectively from the microphones comprising each elementary sound receiving device so as to establish the outgoing signal of this sound receiving device.
10. Apparatus according to one of claims 1 to 9, characterized in that said elementary sound receiving devices (M1 to M4) are of an even number m = 2n greater than three, in that they are associated two by two in n pairs, in that the elementary sound receiving devices of each of said pairs are disposed symmetrically about the axis (D), in that the processing means (8) are arranged so that the signals originating respectively from the receiving devices of each pair are subtracted from each other so as to add them with a phase shift of π radians between them, in that a phase shift is applied to each signal (S13, S14) originating from a pair so as to obtain a phase shift of 2π/m between the signals originating respectively from two pairs of any adjacent sound receiving devices and in that the shifted signals are added.
11. Apparatus according to claim 10, characterized in that the processing means (8) comprise for each pair of sound receiving devices (M1 to M4) a differential preamplifier (A13 and A24) comprising two inputs (E1, E3 and E2, E4) receiving respectively the signals originating from the two sound receiving devices (M1 to M4) of the pair, and an output supplying the amplified difference (S13 and S24) between the two signals received at the inputs (E1 to E4).
12. Apparatus according to one of claims 10 or 11 characterized in that, so as to apply the phase shift of 360°/m between the signals originating respectively from any two adjacent sound receiving devices, the processing means (8) comprise n phase-shifter channels (D13, D24) each comprising one input receiving a signal (S13, S24) originating from one of said pairs and an output (SD13, SD24), the n outputs of the phase-shifter channels being added so as to constitute the signal originating from the sound recording system.
13. Apparatus according to one of claims 1 to 12, characterized in that the sound receiving devices comprise microphones (M1, M2, M3, M4), each microphone being housed in a recess (12) open on one side (23) facing the plane plate (20) reflecting the sound waves and disposed parallel to the plane (P) in which the sound receiving devices are situated.
14. System according to claim 13, characterized in that the recesses (12) in which said microphones (M1 to M4) are housed, are provided in a body (2) having a shape symmetrical about said direction of symmetry (D) and comprising on the side of said plane plate (20) reflecting the sound waves an extension (21) for defining a determined spacing (22) between the recesses (12) and said plate (20).
15. Apparatus according to one of claims 13 or 14, characterized in that each microphone (M1 to M4) is driven into its respective recess (12) so as to leave a gap (24) between one side of this microphone facing the flat plate (20) reflecting the sound waves and one edge (23) of this recess (12) facing said plate (20).
16. Apparatus according to one of claims 1 to 15, characterized in that it has a general symmetrical structure about the direction of symmetry (D).
17. Application of a sound recording and reproducing apparatus (Aac) to audioconferencing according to any one of claims 1 to 16.

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