EP2945399B1 - Audio headset with active noise control anc with prevention of the effects of saturation of a microphone signal feedback - Google Patents

Description

The present invention relates to an audio headset comprising an "active noise control" system.

Such a headset can be used for listening to an audio source (music for example) from a device such as an MP3 player, radio, smart-phone, etc. to which it is connected by a wired connection or by a wireless link, in particular a connection of the Bluetooth type (trademark of the Bluetooth SIG).

If it is equipped with a microphone unit capable of picking up the voice of the wearer of the headset, it is also possible to use this headset for communication functions such as "hands-free" telephony functions, in addition to the listen to the audio source. The headset transducer then reproduces the voice of the remote speaker with which the wearer of the headset is in conversation.

The headset usually includes two headphones joined by a hoop.

Each earpiece comprises a closed shell housing a sound reproduction transducer (hereinafter simply referred to as a "transducer") and intended to be applied around the user's ear with the interposition of a circumaural pad isolating the ear of the ear. external sound environment.

There are also so-called "ear-type" earphones with an element to be placed in the ear canal, thus having no cushion surrounding or covering the ear. In the following, we will mainly refer to headphones type "headphones" with a transducer housed in a shell surrounding the ear (helmet "circum-aural") or in support thereof (helmet "supra-aural") , but this example should not be considered as limiting, the invention being able to be applied as well, as will be understood, to intra-ear headphones.

When the headset is used in a noisy environment (metro, street, train, airplane, etc.) the wearer is partially protected from noise by the headphones headphones, which isolate it with the closed shell and the circumaural pad.

However, this purely passive protection is only partial, part of the sounds, especially in the lower part of the frequency spectrum, can be transmitted to the ear through the shell of the headphones, or through the cranial box of the carrier.

It is for this reason that have been developed techniques called "active noise control" or RDI (A ctive Noise Control), whose principle consists in capturing the component of incident noise and superimpose temporally and spatially, to this noise component an acoustic wave which is ideally the inverted copy of the pressure wave of the noise component. This is to create in this manner a destructive interference with the noise component and reduce, ideally neutralize, the pressure variations of the parasitic acoustic wave.

The EP 2 597 889 A1 (Parrot) describes such a headset, equipped with an ANC system combining feedback type filtering , closed loop, and feed- forward, open loop. The feedback filtering channel is based on a signal collected by a microphone placed inside the acoustic cavity delimited by the shell of the earphone, the circumaural pad and the transducer. In other words, this microphone is disposed near the ear of the user, and mainly receives the signal produced by the transducer and the residual noise signal, unneutralized, still noticeable in the front cavity. The signal from this microphone, from which the audio signal of the music source to be reproduced by the transducer is subtracted, constitutes an error signal for the feedback loop of the ANC system. The feedforward filtering channel uses the signal picked up by the external microphone to collect spurious noise in the helmet wearer's immediate environment. Finally, a third filtering channel processes the audio signal from the music source to be reproduced. The output signals of the three filter channels are combined and applied to the transducer to reproduce the signal of the musical source associated with a surrounding noise canceling signal.

The EP 2 518 724 A1 (Parrot) describes a combined microphone / headset type device, usable in particular for hands-free telephony functions. The helmet is provided with a physiological sensor applied against the cheek or temple of the wearer of the helmet and receiving vocal vibrations which have the characteristic of being, by nature, very little corrupted by the surrounding noise. The physiological sensor may be in particular an accelerometer placed on the inside of the skin of the earpiece of the earphone of the helmet, so as to be pressed against the cheek or the temple of the user with as close a coupling as possible . The signal thus collected allows, after filtering and combination with signals picked up by conventional external microphones, to deliver to the communication system a speech signal from the nearby speaker (the helmet wearer) whose intelligibility has been greatly improved. Another advantage of this sensor is the ability to use the signal it delivers to calculate a cutoff frequency of a dynamic filter.

The WO 2010/129219 A1 ( EP 2 425 421 A0 ) describes another apparatus, comprising an ANC system of adaptive type, that is to say using filters whose transfer function is dynamically modified, continuously, by a real-time analysis algorithm of the signal. An external microphone placed on the shell of the earphones of the headset collects ambient noise, the level of which is analyzed to adjust the transfer function of the feedback filter to adapt to the noise prevailing in the external environment of the headphones.

Existing ANC systems are subject to a phenomenon that occurs when the internal acoustic cavity of the earpiece undergoes sudden, inaudible but excessive decompression and decompression, the amplitude of which is so great that the microphone diaphragm crashes sharply and produces an electrical signal exceeding its nominal limit.

This phenomenon occurs in particular when handling the helmet, or when the user is walking heavily or short. The movements of the helmet then create excessive overpressures and depressions in the front cavity, which results in a strong electrical peak in the low frequencies. The excessive signal picked up by the microphone creates in the ANC feedback filter a saturation leading to an audible signal or "plop" produced at the output by the transducer and unpleasant for the user.

This phenomenon can even occur under normal walking conditions, where pitch noise resonances in low frequencies below 100 Hz are heard and sometimes annoying. The feedback ANC filter can attenuate these pitch noise resonances by amplifying the internal microphone signal but, when the steps become stronger, the electrical level of the microphone signal can exceed the limits of its normal operation and cause, here again, a saturation ANC filter and transducer.

This saturation can occur at several points in the signal processing chain: electrical overshoot of the input dynamics of the analog / digital converter, exceeding of the maximum digital value in the digital signal processor DSP, or else output saturation if the signal reproduced by the transducer exceeds the maximum value that can be produced by the digital / analog converter, each of these phenomena can cause a "plop" unpleasant.

• en compensant les phénomènes pneumatiques de surpression/dépression dans la cavité acoustique de l'écouteur, notamment du fait des mouvements de pas de l'utilisateur du casque ;
• sans dégradation des performances antibruit du système ANC, c'est-à-dire que le bruit résiduel perçu par l'utilisateur sera toujours réduit au mieux, avec notamment i) une forte atténuation des basses fréquences et ii) une grande largeur de bande de fréquence de suppression ;
• le tout, sans que le signal audio issu de la source musicale (ou la voix du locuteur distant, dans une application de téléphonie) ne soit distordu, et sans que le spectre de ce signal ne soit amputé par le traitement ANC - bien que le signal de neutralisation du bruit et le signal audio à reproduire soient amplifiés par le même canal et reproduits par le même transducteur.

The aim of the invention is to propose a new ANC noise reduction technique making it possible to overcome these phenomena:

• compensating pneumatic overpressure / depression phenomena in the acoustic cavity of the earpiece, in particular due to the movements of the helmet user's steps;
• without degrading the ANC's noise-canceling performance, ie the residual noise perceived by the user will always be reduced at best, notably with i) a strong attenuation of the low frequencies and ii) a large bandwidth of suppression frequency;
• all without the audio signal from the music source (or the voice of the remote speaker, in a telephony application) being distorted, and without the spectrum of this signal is reduced by the ANC processing - although the noise canceling signal and the audio signal to be reproduced are amplified by the same channel and reproduced by the same transducer.

Another object of the invention is to implement a digital (and not an analogue) technology as in the EP 2 597 889 A1 mentioned above) for such an ANC system, which can be implemented in particular within a digital signal processor (DSP).

• deux écouteurs comportant chacun un transducteur de reproduction sonore d'un signal audio à reproduire, ce transducteur étant logé dans une cavité acoustique d'oreille ;
• au moins un microphone apte à délivrer un signal capté comportant une composante de bruit acoustique ;
• un capteur de mouvement monté sur au moins l'un des écouteurs et apte à délivrer un signal accélérométrique ; et
• un processeur numérique de signal, DSP, comprenant :
  • des moyens de mixage, recevant en entrée un signal provenant du microphone ainsi que ledit signal audio à reproduire, et délivrant en sortie un signal apte à piloter le transducteur ; et
  • des moyens de réduction de bruit, comprenant des moyens aptes à analyser concurremment i) le signal microphonique délivré par le microphone et ii) le signal accélérométrique délivré par le capteur de mouvement, et vérifier si des caractéristiques courantes de ces signaux microphonique et accélérométrique vérifient ou non une première batterie de critères prédéterminés.

To achieve these goals, the invention proposes a headset as disclosed by the EP 2 518 724 A1 supra. Such a helmet includes:

• two earphones each comprising a sound reproduction transducer of an audio signal to be reproduced, this transducer being housed in an acoustic ear cavity;
• at least one microphone capable of delivering a captured signal comprising an acoustic noise component;
• a motion sensor mounted on at least one of the earphones and adapted to deliver an accelerometric signal; and
• a digital signal processor, DSP, comprising:
  • mixing means, receiving as input a signal from the microphone as well as said audio signal to be reproduced, and outputting a signal adapted to drive the transducer; and
  • noise reduction means, comprising means capable of concurrently analyzing i) the microphone signal delivered by the microphone and ii) the accelerometer signal delivered by the motion sensor, and checking whether current characteristics of these microphone and accelerometer signals verify or not a first set of predetermined criteria.

• le casque comprend un système de contrôle actif de bruit, ANC ;
• le microphone est un microphone ANC interne placé à l'intérieur de la cavité acoustique ;
• le DSP comprend :
  • une branche feedback en boucle fermée comprenant un filtre ANC feedback apte à appliquer une fonction de transfert de filtrage au signal délivré par le microphone ANC interne ; et
  • lesdits moyens de mixage, qui reçoivent en entrée le signal délivré par la branche feedback en sortie du filtre ANC feedback ainsi que ledit signal audio à reproduire, et délivrent en sortie ledit signal apte à piloter le transducteur ; et
• le DSP comprend en outre des moyens de prévention des effets sur la branche feedback d'une saturation du signal délivré par le microphone interne, comprenant :
  • lesdits moyens aptes à analyser concurremment i) le signal microphonique délivré par le microphone et ii) le signal accélérométrique délivré par le capteur de mouvement, et vérifier si des caractéristiques courantes de ces signaux microphonique et accélérométrique vérifient ou non une première batterie de critères prédéterminés ; et
  • dans la branche feedback en amont du filtre ANC feedback, un filtre anti-saturation feedback sélectivement commutable en fonction du résultat de la vérification de la première batterie de critères.

Characteristically, the invention

• the helmet includes an active noise control system, ANC;
• the microphone is an internal ANC microphone placed inside the acoustic cavity;
• the DSP includes:
  • a closed loop feedback branch comprising a feedback ANC filter adapted to apply a filter transfer function to the signal delivered by the internal microphone ANC; and
  • said mixing means, which receives as input the signal delivered by the feedback branch at the output of the feedback filter ANC as well as said audio signal to be reproduced, and outputs said signal able to drive the transducer; and
• the DSP further comprises means for preventing effects on the feedback branch of a saturation of the signal delivered by the internal microphone, comprising:
  • said means capable of concurrently analyzing i) the microphone signal delivered by the microphone and ii) the accelerometer signal delivered by the motion sensor, and check whether current characteristics of these microphone and accelerometer signals verify or not a first set of predetermined criteria; and
  • in the feedback branch upstream of the feedback filter ANC , a feedback saturation filter selectively switchable according to the result of the verification of the first set of criteria.

• le filtre anti-saturation feedback est l'un d'entre une pluralité de filtres préconfigurés sélectivement commutables, et le DSP comprend en outre des moyens aptes à sélectionner l'un des filtres anti-saturation préconfigurés en fonction du résultat de la vérification de la première batterie de critères ;
• le DSP comprend en outre : une branche d'égalisation, comprenant un filtre d'égalisation apte à appliquer une fonction de transfert d'égalisation au signal audio à reproduire avant application de celui-ci aux moyens de mixage ; et, dans la branche d'égalisation en amont du filtre d'égalisation, un filtre anti-saturation d'égalisation sélectivement commutable en même temps que le filtre anti-saturation feedback ;
• le filtre anti-saturation d'égalisation est l'un d'entre une pluralité de filtres préconfigurés d'égalisation sélectivement commutables, et le DSP comprend en outre des moyens aptes à sélectionner l'un des filtres préconfigurés d'égalisation en fonction du résultat de la vérification de la première batterie de critères ;
• les caractéristiques courantes du signal accélérométrique comprennent une valeur d'énergie du signal accélérométrique, et les critères prédéterminés comprennent un seuil auquel est comparée ladite valeur d'énergie. Il peut notamment s'agir de valeurs d'énergie dans une pluralité de bandes de fréquences respectives, les critères prédéterminés comprenant une série de seuils respectifs auxquels sont comparées ces valeurs d'énergie si la valeur d'énergie du signal accélérométrique dépasse le seuil ;
• le filtre ANC feedback est l'un d'entre une pluralité de filtres ANC feedback préconfigurés, sélectivement commutables, et le DSP comprend en outre : des moyens d'analyse du signal délivré par le microphone interne, aptes à vérifier si des caractéristiques courantes du signal délivré par le microphone interne vérifient ou non une seconde batterie de critères prédéterminés ; et des moyens de sélection, aptes à sélectionner l'un des filtres ANC feedback préconfigurés en fonction du résultat de la vérification de la seconde batterie de critères ;
• le DSP comprend en outre une branche d'égalisation, comprenant un filtre d'égalisation apte à appliquer une fonction de transfert d'égalisation au signal audio à reproduire avant application de celui-ci aux moyens de mixage. Le filtre d'égalisation est l'un d'entre une pluralité de filtres d'égalisation préconfigurés, sélectivement commutables, et les moyens de sélection sont également aptes à sélectionner l'un des filtres d'égalisation préconfigurés, en fonction du filtre ANC feedback courant sélectionné.

According to various advantageous subsidiary features:

• the anti-saturation feedback filter is one of a plurality of selectively switchable preconfigurable filters, and the DSP further comprises means for selecting one of the pre-configured anti-saturation filters according to the result of the verification of the first set of criteria;
• the DSP further comprises: an equalization branch, comprising an equalization filter adapted to apply an equalization transfer function to the audio signal to be reproduced before application thereof to the mixing means; and, in the equalization branch upstream of the equalization filter, an equalization anti-saturation filter selectively switchable at the same time as the feedback saturation filter;
• the equalization anti-saturation filter is one of a plurality of selectively switchable preconfigurable equalization filters, and the DSP further comprises means for selecting one of the preconfigured equalization filters according to the result the verification of the first set of criteria;
• the current characteristics of the accelerometer signal include an accelerometer signal energy value, and the predetermined criteria include a threshold at which said energy value is compared. It may in particular be energy values in a plurality of respective frequency bands, the predetermined criteria comprising a series of respective thresholds to which these energy values are compared if the energy value of the accelerometric signal exceeds the threshold;
• the ANC feedback filter is one of a plurality of preconfigured, selectively switchable ANC feedback filters, and the DSP further comprises: means for analyzing the signal delivered by the internal microphone, able to check whether current characteristics of the signal delivered by the internal microphone or not verify a second set of predetermined criteria; and selection means, adapted to select one of the preconfigured feedback ANC filters according to the result of the verification of the second set of criteria;
• the DSP further comprises an equalization branch, comprising an equalization filter adapted to apply an equalization transfer function to the audio signal to be reproduced before application thereof to the mixing means. The equalization filter is one of a plurality of preconfigured, selectively switchable equalization filters, and the selection means are also able to select one of the preconfigured equalization filters, depending on the ANC filter feedback current selected.

• La Figure 1 illustre de façon générale un casque audio reposant sur la tête d'un utilisateur.
• La Figure 2 est une représentation schématique montrant les différents signaux acoustiques et électriques ainsi que les divers blocs fonctionnels impliqués dans le fonctionnement d'un casque audio à contrôle actif de bruit.
• La Figure 3 est une coupe en élévation de l'un des écouteurs du casque selon l'invention, montrant la configuration des divers éléments mécaniques et organes électromécaniques de celui-ci.
• La Figure 4 illustre un exemple de forme d'onde typique du signal électrique délivré avant amplification par le microphone interne d'un casque ANC, lors de deux sauts par le porteur du casque.
• La Figure 5 illustre de façon schématique, sous forme de blocs fonctionnels, la manière dont est réalisé le traitement de débruitage selon l'invention.
• La Figure 6 illustre plus précisément les éléments mettant en oeuvre la fonction d'analyse du signal microphonique et de sélection des filtres à appliquer aux signaux à délivrer au transducteur du casque.
• La Figure 7 est un organigramme décrivant le fonctionnement de la machine d'états de la fonction d'analyse et de sélection de la Figure 6.
• La Figure 8 représente, en amplitude et en phase, les fonctions de transfert du filtre ANC avec et sans le filtrage anti-saturation selon l'invention, sélectionné de façon automatique en fonction des mouvements détectés.
• La Figure 9 illustre des exemples d'atténuation obtenus dans les deux cas exemplifiés Figure 8.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which the same references designate elements that are identical or functionally similar from one figure to another.

• The Figure 1 generally illustrates an audio headset resting on the head of a user.
• The Figure 2 is a schematic representation showing the different acoustic and electrical signals as well as the various functional blocks involved in the operation of an active noise control headphones.
• The Figure 3 is an elevational section of one of the earphones of the helmet according to the invention, showing the configuration of the various mechanical elements and electromechanical members thereof.
• The Figure 4 illustrates an example of a typical waveform of the electrical signal delivered before amplification by the internal microphone of an ANC helmet, during two jumps by the helmet wearer.
• The Figure 5 illustrates schematically, in the form of functional blocks, the manner in which the denoising treatment according to the invention is carried out.
• The Figure 6 illustrates more precisely the elements implementing the function of analyzing the microphone signal and filter selection to be applied to the signals to be delivered to the transducer of the helmet.
• The Figure 7 is a flowchart describing the operation of the state machine of the function of analysis and selection of the Figure 6 .
• The Figure 8 represents, in amplitude and in phase, the transfer functions of the ANC filter with and without the anti-saturation filtering according to the invention, selected automatically according to the movements detected.
• The Figure 9 illustrates examples of attenuation obtained in the two exemplified cases Figure 8 .

On the Figure 1 , there is shown a headphone placed on the head of his user. This headset comprises, in a conventional manner, two earphones 10, 10 'joined by a holding bar 12. Each of the earphones 10 comprises an outer shell 14 which is applied to the contour of the ear of the user. , with interposition between the shell 14 and the periphery of the ear of a flexible circumaural pad 16 for providing a sound seal, acoustically, between the region of the ear and the external sound environment.

As indicated in the introduction, this example configuration of "headphone" type with a transducer housed in a shell surrounding the ear or resting on it should not be considered as limiting, the invention can also be well applied to ear-phones including an element to place in the ear canal, so headphones devoid of shell and cushion surrounding or covering the ear.

The Figure 2 is a schematic representation showing the different acoustic and electrical signals as well as the various functional blocks involved in the operation of an active noise control headphones.

The earphone 10 encloses a sound reproduction transducer 18, hereinafter referred to simply as a "transducer", carried by a partition 20 defining two cavities, namely a front cavity 22 on the side of the ear and a rear cavity 24 on the opposite side. .

The front cavity 22 is defined by the inner partition 20, the wall 14 of the earpiece, the pad 16 and the outer face of the user's head in the region of the ear. This cavity is a closed cavity, with the exception of the inevitable acoustic leaks in the region of contact of the pad 16.

The rear cavity 24 is a closed cavity, with the exception of an acoustic vent 26 for obtaining a strengthening of low frequencies in the front cavity 22 of the earpiece.

Finally, for the active control of the noise, there is provided an internal microphone 28 disposed closer to the auditory canal of the ear, to capture the residual noise present in the internal cavity 22, which noise will be perceived by the user.

• du bruit résiduel 32 provenant de la transmission du bruit externe environnant 30 au travers de la coque 14 de l'écouteur, et
• d'une onde sonore 34 générée par le transducteur 18, qui est, idéalement selon le principe des interférences destructives, la copie inversée du bruit résiduel 32, c'est-à-dire du bruit à supprimer au point d'écoute.

By ignoring the audio signal of the music source reproduced by the transducer (or the voice of the remote speaker, in a telephone application), the acoustic signal picked up by this internal microphone 28 is a combination:

• residual noise 32 from the transmission of the surrounding external noise 30 through the shell 14 of the earphone, and
• a sound wave 34 generated by the transducer 18, which is, ideally according to the principle of destructive interference, the inverted copy of the residual noise 32, that is to say the noise to be deleted at the point of listening.

The neutralization of the noise by the sound wave 34 is never perfect, the internal microphone 28 collects a residual signal which is used as an error signal e applied to a feedback filter branch 36 closed loop.

Optionally, an external microphone 38 may be placed on the shell of the earphones of the headset, to capture the surrounding noise outside the earpiece, schematized by the wave 30. The signal collected by this external microphone 38 is applied to a feedforward 40 filter stage of the active noise control system. The signals from the feedback branch 36 and, if present, from the feedforward branch 40 are combined at 42 to drive the transducer 18.

In addition, the transducer 18 receives an audio signal to be reproduced from a musical source (walkman, radio, etc.), or the voice of the remote speaker, in a telephony application. Since this signal experiences the effects of the closed loop distorting it, it will have to be preprocessed by an equalization so as to present the desired transfer function, determined by the gain of the open loop and the target response without active control.

The helmet may possibly wear, as shown Figure 1 , another external microphone 44 for communication functions, for example if the headset is provided with "hands-free" telephony functions.

This additional external microphone 44 is intended to capture the voice of the wearer of the helmet, it does not intervene in the active control of the noise and, in the Next, it will be considered as an external microphone possibly used by the ANC system that the microphone 38 dedicated to active noise control.

The Figure 3 illustrates, in section, an exemplary embodiment of the various mechanical and electroacoustic elements schematically represented on the Figure 2 for one of the headphones 10 (the other headphone 10 'being made identically). It contains the partition 20 dividing the inside of the shell 14 into a front cavity 22 and a rear cavity 24 with, mounted on this partition, the transducer 18 and the internal microphone 28 carried by a gate 48 maintaining it nearby. of the ear canal of the user.

The object of the invention is to overcome the phenomenon, explained in the introduction, resulting from the sudden surges / depressions in the front cavity 22 which are likely to produce, especially in the low frequencies below 100 Hz, extreme overruns in the value the signal delivered by the internal microphone 28.

So, the Figure 4 illustrates an example of a signal delivered by the internal microphone 28, in this case an electret microphone which delivers a signal not exceeding 100 mV for a sound pressure of 110 dB SPL ( Sound Pressure Leve /). However, as illustrated Figure 4 in the case of two small successive jumps, it is found that this value can be very much exceeded (in the example, it reaches and exceeds 600 mV), which can produce after amplification saturation effects in several places of the chain treatment.

The basic idea of the invention is to detect upstream of the feedback filter, with a very low latency time, the situations likely to produce such signal peaks, in order to avoid all saturation phenomena during sudden movements. helmet, especially while the user is walking or running.

The Figure 5 schematically illustrates, in the form of functional blocks, the active noise control system ANC incorporating, according to the invention, an anti-saturation function to compensate for this phenomenon.

It is a digital type ANC system implemented by a DSP 50 digital signal processor. It should be noted that although these diagrams are presented in the form of interconnected circuits, The work of the various functions is essentially software, this representation being only illustrative.

We find the feedback branch whose principle was described above with reference to the Figure 2 after scanning by means of an ADC converter 52 of the error signal e picked up by the internal microphone 28. The digitized error signal is processed by a feedback filter 54, then converted into analog by the DAC 56 to be returned by the transducer 18 in the cavity of the earpiece 10. The reproduced signal is optionally combined with a music signal M which, after equalization at 58, is combined at 60 with the noise cancellation signal for conversion by the DAC 56 and reproduction by the transducer 18.

The filtering operated by the blocks 54 (transfer function H feedback _FB2 on the microphone signal) and 58 (transfer function H equalizer _EQ2 music M) can be realized in particular as described in the application FR 14 53284 of 11.04.2014, in the name of the Applicant, entitled "Active noise control ANC noise headphones with electric breath reduction", which proposes to implement a plurality of predetermined filter configurations, selectively switchable according to the signal captured by the internal microphone 28, so as to optimize the compromise between the more or less attenuation of the ambient noise and that of an electric breath also more or less strong, depending on the level and the spectral content of the signal returned to the user as collected by the microphone 28 placed in the front cavity 22 of the earpiece.

This particular technique of anti-blast filtering is however in no way limiting, and the anti-saturation system according to the invention is equally applicable to feedback and equalization filtering operated by other techniques.

In the illustrated example, the active noise control ANC is controlled by an ANC module 62 which analyzes the signal e and adapts accordingly the transfer functions H _FB2 of the feedback branch 54 and H _EQ2 of the equalization branch 58 of the music signal.

B

étant le signal de bruit extérieur 30,

M

étant le signal de musique entrant,

H_ext

étant la fonction de transfert entre une source de bruit extérieure et le microphone interne 28,

H_FB2

étant la fonction de transfert du filtre feedback 54,

H_EQ2

étant la fonction de transfert du filtre d'égalisation 58, et

H_a

étant la fonction de transfert entre le transducteur 18 et le microphone interne 28.

More precisely, the signal e picked up by the internal microphone 28 (which is assumed to be identical to the signal picked up by the ear of the headphone user) is (in the configuration of the Figure 5 ) given by : $$e=H_{\mathit{ext}}/\left(1-H_{\mathrm{at}}*{\mathrm{<figure-callout\; id="2"\; label="H\; FB"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">H\; </figure-callout>}}_{\mathit{FB}}\right)*B+H_{\mathrm{at}}/\left(1-H_{\mathrm{at}}*{\mathrm{<figure-callout\; id="2"\; label="H\; FB"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">H\; </figure-callout>}}_{\mathit{FB}}\right)*{\mathrm{<figure-callout\; id="2"\; label="H\; EQ"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">H\; </figure-callout>}}_{\mathit{EQ}}*M$$

B

being the external noise signal 30,

M

being the incoming music signal,

H _ext

being the transfer function between an external noise source and the internal microphone 28,

H _FB2

being the transfer function of the feedback filter 54,

H _EQ2

being the transfer function of the equalization filter 58, and

_H

being the transfer function between the transducer 18 and the internal microphone 28.

de sorte que si le filtre H_FB2 de la branche ANC feedback 54 est modifié, la perception de la musique par l'utilisateur s'en trouve également modifiée.From this equation, we notice that a played music signal is subjected to a transfer function: $$H_{\mathrm{at}}/\left(1-H_{\mathrm{at}}*{\mathrm{<figure-callout\; id="2"\; label="H\; FB"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">H\; </figure-callout>}}_{\mathit{FB}}\right)*{\mathrm{<figure-callout\; id="2"\; label="H\; EQ"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">H\; </figure-callout>}}_{\mathit{EQ}}$$

so that if the filter H _FB2 ANC branch feedback 54 is changed, the perception of music by the user is also changed.

In order for the perception of the music to remain the same, the control algorithm ANC 62 modifies the H _EQ2 filter of the music equalization branch 58 at the same time as that of the feedback ANC branch 54 in order to rebalance the effects of the filtering. , of course if a music signal is present.

In a manner that is characteristic of the invention, together with the signal of the internal microphone 28, the active noise control processing ANC uses an accelerometer 64 mounted on the helmet ( Figures 2 and 5 ), the role of which will be to detect with a very low latency the movements of the earphone likely to produce saturation effects of the signal picked up by the internal microphone 28, typically movements resulting from the movements of the user while the latter walk, short, jump ... or when it manipulates the headphones, for example to readjust their position on his ears.

The EP 2 518 724 A1 (Parrot) describes a headset with an accelerometer built into an earpiece, but in this document the accelerometer is used as a physiological sensor to collect vocal components non-acoustically transmitted by bone conduction, so not noisy, a speech signal emitted by the user, for example in the case where the headset is used as a "hands-free" device in combination with a mobile phone. In the case of the present invention, this same accelerometer can be used, but with a different role, namely the improvement of the ANC function of the headphones, in a configuration of listening (sound reproduction) and not of speech (voice of the user).

The signal of the accelerometer 64, after scanning by means of an ADC converter 66, is applied to an "anti-saturation" module 68 which also receives the signal e collected by the internal microphone 28, after scanning by the ADC converter 52 .

The two acceleration and microphonic signals are analyzed jointly by the anti-saturation module 68, which controls a filter 70 (transfer function H _FB1 ) placed in the feedback branch upstream of the actual feedback filter (block 54, function transfer H _FB2 ), and likewise an equalization filter 72 (transfer function H _EQ1 ) placed in the equalization branch upstream of the equalization filter (block 58, transfer function H _EQ2 ).

Very advantageously, but in a nonlimiting manner, it is possible to provide, for the blocks 70 and 72 respectively defining the transfer functions of the feedback and equalization branches, a plurality of predetermined, selectively switchable filtering configurations with a control mechanism. intelligent switching between these different filters based on the signals captured together by the accelerometer 64 and the internal microphone 28.

The anti-saturation module 68, from these signals, defines the one of the X filters of the block 70 of the feedback branch that should be selected and, likewise, that of the Y filters of the block 72 of the equalization branch of the a music signal that should be selected (Y can be equal to X, but not necessarily).

The selection between the X filters of the transfer function H _FB1 of the block 70 (or Y filters of the transfer function H _EQ1 of the block 72) is as follows.

les coefficients a₁, a₂, b₀, b₁ et b₂ de la fonction de transfert étant issus des paramètres f₀ , Q et G du filtre.For each of the filters, we interpolate its parameters (center frequency f ₀ , quality factor Q and gain G ) and during a transition the coefficients are calculated with respect to these interpolated parameters between the initial state and the final state. Typically, it is possible to use an infinite impulse response filter (IIR), that is to say a type of filter characterized by a response based on the values of the input signal as well as the previous values of the response. that this filter could produce. It is possible to use in particular an IIR filter of order 2, called "biquad" whose transfer function gives the output signal y at time n as a function of the input signal x at times n, n -1 and n- 2 is given by: $$\mathrm{there}\left(\mathrm{not}\right)={\mathrm{<figure-callout\; id="0"\; label="b"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">b</figure-callout>}}_{\mathit{0}}*x\left(\mathrm{not}\right)+{\mathrm{<figure-callout\; id="1"\; label="b"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">b</figure-callout>}}_{\mathit{1}}*x\left(\mathrm{not}-1\right)+{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">b</figure-callout>}}_{\mathit{2}}*x\left(\mathrm{not}-2\right)-{\mathrm{at}}_{\mathit{1}}*\mathrm{there}\left(\mathrm{not}-1\right)-{\mathrm{at}}_{\mathit{2}}*\mathrm{there}\left(\mathrm{not}-2\right),$$

the coefficients a ₁ , a ₂ , b ₀ , b ₁ and b ₂ of the transfer function being derived from the parameters f ₀ , Q and G of the filter.

The Figure 6 illustrates more precisely the elements implemented by the anti-saturation module 68 for the analysis of the signal and the selection of the filters of the blocks 70 and 72.

The digitized signal e collected by the internal microphone 28 is subjected to a frequency decomposition by a battery of filters 74 in order to calculate at 76 the energy Rms _i of this signal e in each of its N frequency components. For example, Rms ₁ may be the power of the microphone signal below 100 Hz, Rms ₂ the signal power around 800 Hz, etc. which allows via spectral analysis to distinguish various significant situations: for example, for use of the headset in a noisy environment of transportation type (plane, train), the ratio between low and high frequencies is much greater than in an environment quieter as in an office.

The obtained values Rms ₁ , Rms ₂ ... Rms _N are applied to a state machine 78, which compares these energy values with respective thresholds and determines, according to these comparisons, that of the X filters of the block 70 of the branch feedback and if appropriate (if music is present) that of Y filters block 72 of the equalization branch to be selected.

The Figure 7 illustrates more precisely how this state machine operates 78.

The power RMS _acc of the signal of the accelerometer present on the helmet is, possibly after prefiltering, analyzed constantly. If this power exceeds a predetermined threshold Threshold a (test 80) then the state machine considers that the headset undergoes a movement that may result in saturation of the ANC control and triggers an anti-saturation control process, corresponding to the left side of the algorithm of the Figure 7 .

On this algorithm, the parameters Activity and Attenuation are Boolean variables, while the parameters Timer ₁ and Trimer ₂ are count values of a timer which is reset by an action "Timer = 0", the notation " Timer ++ "indicating that the algorithm allows the timer to continue.

In the presence of an acceleration exceeding the prescribed threshold, the state machine analyzes the signal of the internal microphone 28. If the power RMS ₁ (power of the microphonic signal in a certain frequency range) exceeds a predetermined threshold Threshold_1 (test 82) , then the state machine modifies the transfer function H _FB1 of the feedback branch , for example by selecting one of the X filters which has the effect of reducing the attenuation ANC in the low frequencies, and also modifies the function of H _EQ1 transfer of the equalizer branch to keep the same perception of the music (block 84).

In the opposite case, the RMS power ₂ of the microphonic signal in another frequency band is tested in the same way (block 82 ') with respect to a second threshold Threshold_2 (with Threshold_2 <Threshold_1 ) . If RMS ₂ > Threshold_2, then a modification of the transfer functions H _FB1 and H _EQ1 (block 84 ') is also applied, typically with an attenuation of the feedback ANC present, but less important than in the previous case. It is thus possible to test a certain number of successive thresholds (test 82 ") iteratively with progressively lower thresholds, so as to choose, among the X selectable filters of the feedback branch H _FB1 , that which will optimize the compromise between the attenuation of the ANC control and protection against saturation of it (block 84 ").

If in all the bands the signal strength of the internal microphone 28 is lower than the lowest threshold, it is considered that there is no risk of saturation and, after expiry of a time delay of X2 seconds (Test 86) The state machine disables the anti-saturation modules 70 and 72 (block 88).

Assuming that, in test 80, the signal analysis of the accelerometer indicates that it does not exceed the prescribed threshold, if the anti-saturation treatment was active (test 90), then at the expiration of a timer of X1 seconds (test 92), this control is automatically deactivated by the state machine (block 94).

Turning off the saturation control and "waking it up" only at the right time offers the advantage of a significant saving on the power consumption of the DSP 50, thus increasing the autonomy of the headphones.

The Figures 8 and 9 illustrate two examples of transfer functions H _FB1 applied to the feedback branch of the control ANC, without (A) and with (B) modification by the anti-saturation module 68: the Figure 8 represents, in amplitude and in phase, the transfer function H _FB1 in these two cases, while the Figure 9 illustrates the corresponding attenuations obtained.

It is found that the detection of an acceleration triggers an attenuation of the gain of the feedback ANC branch of the order of 12 to 15 dB at 40 Hz between the curve A (without anti-saturation control) and the curve B (with anti-control -saturation). The change is mainly made in the low frequencies, below 150 Hz, because it is in this range of frequencies that the resonances of footsteps, etc. are located. usually encountered in practice. The anti-saturation control decreases, certainly, the attenuation performance of the ANC control but, in return, avoids the production of a very unpleasant "plop" output by the transducer due to the saturation of the control branch ANC feed - back.

Claims (8)

1. An audio headset, comprising:

   - two earphones (10), each including a transducer (18) for the sound reproduction of an audio signal to be reproduced, this transducer being housed in an ear acoustic cavity (22);

   - at least one microphone adapted to deliver a picked-up signal including an acoustic noise component;

   - a movement sensor (64) mounted on at least one of the earphones and adapted to deliver an accelerometric signal; and

   - a digital signal processor, DSP, (50) comprising:

   . mixing means (46), receiving as an input a signal coming from the microphone as well as said audio signal to be reproduced (S), and delivering as a output a signal adapted to pilot the transducer (18); and

   . noise reduction means, comprising means (68) adapted to analyse concurrently i) the microphonic signal delivered by the microphone and ii) the accelerometric signal delivered by the movement sensor (64), and to verify whether or not current characteristics of these microphonic and accelerometric signals verify a first set of predetermined criteria,

   characterized in that:

   - the headset comprises an active noise control, ANC, system;

   - the microphone is an inner ANC microphone (28) placed inside the acoustic cavity (22);

   - the DSP (50) comprises:

   . a closed-loop feedback branch (36) comprising a feedback ANC filter (54) adapted to apply a filtering transfer function (H_FB ) to the signal delivered by the internal ANC microphone (28); and

   . said mixing means (46), which receive as an input the signal delivered by the feedback branch at the output of the feedback ANC filter (54) as well as said audio signal to be reproduced (S), and deliver as an output said signal adapted to pilot the transducer (18); and

   - the DSP further comprises means for preventing the effects on the feedback branch of a saturation of the signal delivered by the internal microphone (18), comprising:

   . said means (68) adapted to analyse concurrently i) the microphonic signal delivered by the microphone (28) and ii) the accelerometric signal delivered by the movement sensor (64), and to verify whether or not current characteristics of these microphonic and accelerometric signals verify a first set of predetermined criteria; and

   . in the upstream feedback branch of the feedback ANC filter (54), a feedback anti-saturation filter (70) selectively switchable as a function of the result of the verification of the first set of criteria.
2. The audio headset of claim 1, wherein :

   - the feedback anti-saturation filter (70) is one from a plurality of selectively switchable preconfigured filters; and

   - the DSP (50) further comprises:

   . means (68) adapted to select one of the preconfigured anti-saturation filters as a function of the result of the verification of the first set of criteria.
3. The audio headset of claim 1, wherein :

   - the DSP (50) further comprises:

   . an equalization branch, comprising an equalization filter (58) adapted to apply an equalization transfer function (H_EQ ) to the audio signal to be reproduced (M) before application of the latter to the mixing means (60); and

   . in the equalization branch, upstream from the equalization filter (58), an equalization anti-saturation filter (72) that is selectively switchable at the same time as the feedback anti-saturation filter (70).
4. The audio headset of claim 1, wherein :

   - the equalization anti-saturation filter (72) is one from a plurality of selectively switchable equalization preconfigured filters; and

   - the DSP (50) further comprises:

   . means (68) adapted to select one of the preconfigured equalization filters as a function of the result of the verification of the first set of criteria.
5. The audio headset of claim 1, wherein the current characteristics of the accelerometric signal comprise a value of energy (Rms_acc ) of the accelerometric signal, and the predetermined criteria comprise a threshold (Seuil_a) to which is compared said value of energy.
6. The audio headset of claim 5, wherein the current characteristics of the microphonic signal comprise values of energy (Rms1, Rms2...) of the microphonic signal in a plurality of respective frequency bands (Filter1, Filter2...), and the predetermined criteria comprise a series of respective thresholds (Seuil1, Seuil2 ... SeuilN) to which are compared said values of energy of the microphonic signal if the value of energy (Rms_acc ) of the accelerometric signal exceeds said threshold (Seuil_a).
7. The audio headset of claim 1, wherein :

   - the feedback ANC filter (54) is one from a plurality of selectively switchable, preconfigured feedback ANC filters; and

   - the DSP (50) further comprises:

   . means (62) for analysing the signal delivered by the internal microphone, adapted to verify whether or not current characteristics of the signal delivered by the internal microphone verify a second set of predetermined criteria; and

   . selection means (62), adapted to select one of the preconfigured feedback ANC filters as a function of the result of the verification of the second set of criteria.
8. The audio headset of claim 7, wherein :

   - the DSP (50) further comprises:

   . an equalization branch, comprising an equalization filter (58) adapted to apply an equalization transfer function (H_EQ ) to the audio signal to be reproduced (M) before application of the latter to the mixing means (60);

   - the equalization filter (58) is one between a plurality of selectively switchable, preconfigured equalization filters; and

   - the selection means (62) are also adapted to select one of the preconfigured equalization filters, as a function of the current selected feedback ANC filter.

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