EP2930942A1 - Audio headset with active noise control (anc) with electric hiss reduction - Google Patents

Abstract

The headset includes an active noise control system, with an ANC microphone delivering a signal having an acoustic noise component. A digital DSP signal processor (50) comprises a feedback ANC branch (54) applying a filter transfer function (H FB) to the signal delivered by the microphone ANC, and signal mixing means (46) of the feedback branch with an audio signal to be reproduced (M). The microphone ANC is an internal microphone (28) placed inside the acoustic cavity (22), and the feedback filter ANC (54) is one of a plurality of preconfigured, selectively switchable ANC feedback filters. The DSP (50) includes means (62) for checking whether or not current characteristics of the microphone signal verify a battery of predetermined criteria, and for selecting one of the pre-configured feedback ANC filters based on the result of that check. The filtering (H EQ) of an equalization branch (58) of the signal to be reproduced (M) is also modified according to the current feedback filter ANC selected.

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, smartphone, 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 earphones type called "in-ear" 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 techniques have been developed called "active noise control" or ANC (Active Noise Control), the principle of which is to capture the incident noise component and superpose, temporally and spatially, 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 , closed loop, and feedforward, open loop filtering. 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 feedforvvard filtering channel uses the signal picked up by the external microphone to collect the 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.

Existing ANC systems face a limitation due to the presence of an "electrical blast", perceived when the ANC system is activated: indeed, the feedback microphone provides an electrical signal which is the image of the acoustic signal and which is accompanied by a low electrical noise, and the amplification by the ANC filter, which is typically of the order of 20 to 30 dB, increases this electrical noise. In addition, the noise picked up by the microphone is increased by the so-called waterbed effect: beyond the main frequency band of noise suppression, the noise will be amplified in a relatively narrow frequency band, general around 1 kHz, perfectly perceptible and of course harmful. If it is too important, this phenomenon can even generate a feedback effect, a phenomenon that can be seen for many helmets when the pad is accidentally removed. These phenomena create on the transducer a noise that can sometimes be more audible and more annoying than the noise to be removed, especially when it is weak.

Specifically, the attenuation of noise in the low frequencies is even better than the gain of the filter ANC feedback is strong, but in return the breath increases. It is therefore desirable to adapt the gain of the ANC feedback filter according to the ambient noise: if this ambient noise is low, a smaller ANC gain and / or less waterbed effect is required. The filter will be less effective, but the breath will also be decreased. Conversely, in the case of strong ambient noise, a high ANC gain is preferable because the generated electrical wind becomes negligible compared to the ambient noise.

The WO 2010/129219 A1 ( EP 2 425421 A0 ) describes 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 headphone earphones collects ambient noise, the level of which is analyzed to adjust the transfer function of the feedback filter .

The disadvantage of this method is that the ANC feedback does not adapt to the noise actually perceived by the user, but noise prevailing in the external environment of the helmet. However, the noise actually perceived can be modified by acoustic leakage, different from one individual to another depending on the positioning of the helmet on the head, the shape of the ear of the user, the different clamps of the helmet on the head, the presence of hair where the circumaural pads come into support, etc. When acoustic leaks are present, the efficiency of the ANC is decreased, so that to suppress more noise it would be necessary to increase the ANC gain, with consequently a higher level of electrical blast.

The EP 1 923 864 A2 describes an ANC system that includes an internal microphone, placed inside the acoustic ear cavity of the earphone of the helmet and which controls the feedback branch of the noise reducer. The captured signal is analyzed in several frequency bands during periods of silence of the music, or during periods when the music is forced to pause by the ANC system. The coefficients of the filter are modified according to the result obtained, in order to better adapt the response curve of this filter as a function of the characteristics of the signal picked up. The proposed noise reduction technique provides only a limited result on the reduction of electrical blast and waterbed effects . Above all, the switching operations between the different transfer functions are particularly noticeable by the user, especially since they intervene during periods of silence of the audio signal to be reproduced.

• qui procure une réduction importante du phénomène de souffle électrique ;
• 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 ;
• de la façon la plus imperceptible possible par le porteur du casque lorsque des changements de filtrage sont opérés ;
• 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.

In view of the foregoing, the object of the invention is to propose a new ANC noise reduction technique:

• which provides a significant reduction in the phenomenon of electrical blast;
• 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;
• in the most imperceptible way possible by the helmet wearer when filtering changes are made;
• 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.

• un microphone ANC interne, placé à l'intérieur de la cavité acoustique d'oreille de l'écouteur du casque, apte à délivrer un signal capté dans cette cavité ; et
• un processeur numérique de signal, DSP, comprenant :
  • une branche feedback en boucle fermée, comprenant un filtre ANC feedback apte à appliquer une fonction de transfert de filtrage au signal capté par le microphone ANC, le filtre ANC feedback étant l'un d'entre une pluralité de filtres ANC feedback préconfigurés sélectivement commutables ;
  • des moyens d'analyse en temps réel du signal capté par le microphone ANC, aptes à vérifier si des caractéristiques courantes de ce signal, comprenant des valeurs d'énergie du signal dans une pluralité de bandes de fréquences, vérifient une batterie de critères prédéterminés ;
  • · des moyens de sélection, aptes à sélectionner l'un des filtres ANC feedback préconfigurés en fonction du résultat de la vérification de la batterie de critères effectuée par les moyens d'analyse ; et
  • des moyens de mixage, recevant en entrée le signal délivré par la branche feedback en sortie du filtre ANC feedback ainsi qu'un signal audio à reproduire, et délivrant en sortie un signal apte à piloter le transducteur.

To achieve these aims, the invention proposes an audio headset with an ANC active noise control system including, in a manner known in itself from the EP 1 923 864 A2 aforementioned:

• an internal microphone ANC, placed inside the acoustic ear cavity of the earphone of the headphones, adapted to deliver a signal captured in this cavity; and
• a digital signal processor, DSP, comprising:
  • a closed-loop feedback branch, comprising a feedback ANC filter adapted to apply a filter transfer function to the signal picked up by the microphone ANC, the feedback filter ANC being one of a plurality of selectively switchable preconfigured feedback ANC filters;
  • real-time analysis means of the signal picked up by the microphone ANC, able to verify whether current characteristics of this signal, including signal energy values in a plurality of frequency bands, verify a set of predetermined criteria;
  • Selection means, able to select one of the preconfigured feedback ANC filters according to the result of the verification of the set of criteria performed by the analysis means; and
  • mixing means, receiving as input the signal delivered by the feedback branch at the output of the feedback filter ANC as well as an audio signal to be reproduced, and outputting a signal able to drive the transducer.

• · 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 étant l'un d'entre une pluralité de filtres d'égalisation préconfigurés sélectivement commutables.

In a characteristic manner of the invention, the DSP furthermore comprises:

• An equalization branch, comprising an equalization filter able to apply an equalization transfer function to the audio signal to be reproduced before it is applied to the mixing means, the equalization filter being one of between a plurality of selectively preconfigurable equalization filters switchable.

1. i) l'un des filtres ANC feedback préconfigurés en fonction du résultat de la vérification de ladite batterie de critères ; et
2. ii) l'un des filtres d'égalisation préconfigurés, en fonction du filtre ANC feedback courant sélectionné.

In addition, the selection means are means able to select simultaneously:

1. i) one of the feedback ANC filters preconfigured according to the result of the verification of said set of criteria; and
2. ii) one of the preconfigured equalization filters, depending on the current feedback ANC filter selected.

In a preferred embodiment, the set of predetermined criteria further comprises detecting the presence or absence of said audio signal to be reproduced, and the predetermined criteria comprise two different sets of respective thresholds to which said signals are compared. energy values, one or the other of these two series being selected according to whether or not an audio signal to be reproduced is present. Advantageously, the helmet may furthermore comprise an external microphone placed outside the acoustic cavity and capable of picking up acoustic noise prevailing in the helmet environment, the DSP then comprising an open loop feedforward branch, comprising a filter ANC feedforward able to apply a feedforward filter transfer function to the signal delivered by the external microphone. The feedforward ANC filter is one of a plurality of preconfigured, selectively switchable, feedforward ANC filters, and the selection means is also able to select one of the preconfigured feedforward ANC filters based on the selected current feedback ANC filter.

• 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 de façon schématique, sous forme de blocs fonctionnels, la manière dont est réalisé le traitement de débruitage selon l'invention.
• La Figure 5 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 6 est un organigramme décrivant le fonctionnement de la machine d'états de la fonction d'analyse et de sélection de la Figure 5.
• La Figure 7 représente, en amplitude et en phase, le diagramme de Bode de la fonction de transfert de deux filtres ANC alternativement sélectionnés de façon automatique en fonction des conditions de bruit extérieures.
• La Figure 8 illustre des exemples d'atténuations obtenues avec les deux filtres exemplifiés Figure 7.
• La Figure 9 est homologue de la Figure 4, pour un système comportant une branche feedforward en sus de la branche feedback.

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 schematically, in the form of functional blocks, the manner in which the denoising treatment according to the invention is carried out.
• The Figure 5 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 6 is a flowchart describing the operation of the state machine of the function of analysis and selection of the Figure 5 .
• The Figure 7 represents, in amplitude and in phase, the Bode diagram of the transfer function of two ANC filters, alternately selected automatically according to the external noise conditions.
• The Figure 8 illustrates examples of attenuation obtained with the two exemplified filters Figure 7 .
• The Figure 9 is counterpart of the Figure 4 , for a system with a feedforward branch in addition to the feedback branch .

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 ear region. 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 making it possible to obtain a reinforcement 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. La neutralisation du bruit par l'onde sonore 34 n'étant jamais parfaite, le microphone interne 28 recueille un signal résiduel qui est utilisé comme signal d'erreur e appliqué à une branche de filtrage feedback 36 en boucle fermée.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
• of a sound wave 34 generated by the transducer 18, which is, ideally according to the principle of destructive interference, the inverted copy

residual noise 32, that is to say the noise to be removed 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 pick up the voice of the wearer of the helmet, it does not intervene in the active control of the noise and, in the following, it will be considered as an external microphone possibly used by the ANC system that the microphone 38 dedicated 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 36 carried by a grid 48 maintaining it nearby. of the ear canal of the user.

The Figure 4 schematically illustrates, in the form of functional blocks, the ANC active noise control system according to the invention.

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

We find the feedback branch 36 whose principle was described above with reference to the Figure 2 , with 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 filter 54, then converted into analog by the DAC 56 in order to be restored 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 canceling signal for conversion by the DAC 56 and reproduction by the transducer 18.

The block 54 defining the transfer function of the feedback branch comprises a plurality of predetermined filter configurations, selectively switchable, each of these X filters making it possible to obtain a greater or lesser attenuation of the ambient noise, to the detriment of a breath. also more or less electric, with an intelligent rocking mechanism between the X filters according to the signal picked up by the internal microphone 28.

In this regard, it is important that the switching between the different selectable filters is made from the signal picked up by the internal microphone 28, because it is this (and not the external microphone 38), close to the ear of the user, who provides the ANC system with an image of the residual noise actually perceived by the user, taking into account possible acoustic leaks between the inside and the outside of the shell of the earphone.

The switching between the different filters of the feedback branch , making it possible to optimize the attenuation / breath compromise, will thus depend on the level and the spectral content inside the front cavity 22 of the headphone earpiece.

Note also that the choice of a digital system makes it easy to program a large number of filters (unlike an analog system, where a large number of electronic components would be required to have this equivalence), and especially to be able to integrate an algorithmic intelligence to analyze in real time the signal and to switch with a very low response time that of the filters which will provide the best compromise attenuation / breath.

The analysis of the error signal picked up by the microphone 28 is performed in the DSP 50 by an "Auto-ANC" module 62, which analyzes the signal e and defines that of the X filters of the feedback branch 54 which should be select and, likewise, that of the Y filters of the equalization branch 58 of the musical signal that should be selected (Y may be equal to X, but not necessarily).

• 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_FB étant la fonction de transfert du filtre feedback 54,
• H_EQ 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 4 ) given by : $$e=H_{\mathit{ext}}/\left(1-H_{\mathrm{at}}*H_{\mathit{FB}}\right)*B+H_{\mathrm{at}}/\left(1-H_{\mathrm{at}}*H_{\mathit{FB}}\right)*H_{\mathit{EQ}}*M$$

• 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 _FB being the transfer function of the feedback filter 54,
• H _EQ being the transfer function of the equalization filter 58, and
• H _a being the transfer function between the transducer 18 and the internal microphone 28.

de sorte que si le filtre H_FB de la branche ANC feedback 54 est modifié, la perception de la musique s'en trouve également modifiée. Ainsi, pour que la perception de la musique reste la même pour l'utilisateur, l'algorithme de contrôle Auto-ANC 62 devra modifier également le filtre H_EQ de la branche 58 d'égalisation de la musique en même temps que celui de la branche ANC feedback 54.From this equation, we notice that a played music signal is subjected to a transfer function: $$H_{\mathrm{at}}/\left(1-H_{\mathrm{at}}*H_{\mathit{FB}}\right)*H_{\mathit{EQ}}$$

so that if the filter H _FB ANC branch feedback 54 is changed, the perception of music is also changed. Thus, so that the perception of the music remains the same for the user, the control algorithm Auto-ANC 62 will also have to modify the H _EQ filter of the branch 58 of EQ of the music at the same time as that of the ANC branch feedback 54.

In other words, the switching, by the auto-ANC algorithm 62, of one of the X filters of the ANC feedback branch 54 will be accompanied simultaneously by the switching of one of the Y branch branch filters. equalizing the music 58 to rebalance the effects of the filtering, of course if a music signal is present.

The Figure 5 illustrates more precisely the elements implemented by the auto-ANC block 62 for analyzing the signal and selecting the ANC feedback filters and equalization filters.

The digitized signal e collected by the internal microphone 28 is subjected to a frequency decomposition by a battery of filters 64 in order to calculate at 66 the energy Rms _i of this signal e in each of its N frequency components.

In the context of active noise control with headphones, the study of the "color" of the surrounding noise via its spectral analysis makes it possible to distinguish various significant situations: for example, for use of the headset in a noisy environment of transport type (plane, train), the ratio between low and high frequencies is much more important than in a quieter environment such as in an office. So, by for example, Rms ₁ may be the power of the microphone signal below 100 Hz, Rms ₂ the signal power around 800 Hz, etc.

The values obtained ₁ Rms, Rms Rms ₂ ... _N are applied to a state machine 68, which compares the energy values with respective thresholds and determines based on these comparisons the X filters in the feedback ANC branch 54 and if applicable (if music is present) that of the Y filters of the equalizer branch 58 must be selected.

The Figure 6 illustrates more precisely how this state machine operates 68.

The state machine 68 will decide, given the current energy levels Rms ₁ , Rms ₂ ... Rms _n , whether or not to modify the transfer functions H _FB and H _EQ such that they are in the initial state (block 70).

alors l'algorithme considère que le bruit extérieur est suffisamment fort pour nécessiter une adaptation du filtre H_FB et, dans le même temps, une adaptation éventuelle correspondante du filtre d'égalisation H_EQ pour le signal de musique (bloc 74).If these energies exceed predefined respective thresholds (test 72): $${\mathit{rms}}_{\mathit{1}}>\mathit{<figure-callout\; id="1"\; label="Threshold"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">Threshold</figure-callout>}\left(1,,1\right)\&\&{\mathit{<figure-callout\; id="2"\; label="rms"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">rms</figure-callout>}}_{\mathit{2}}>\mathit{<figure-callout\; id="2"\; label="Threshold"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">Threshold</figure-callout>}\left(2,,1\right)\&\&...\&\&{\mathit{rms}}_{\mathit{NOT}}>\mathit{Threshold}\left(\mathrm{NOT},,1\right),$$

then the algorithm considers that the external noise is strong enough to require an adaptation of the filter H _FB and, at the same time, a corresponding eventual adaptation of the equalization filter H _EQ for the music signal (block 74).

avec des seuils plus bas, c'est-à-dire que Seuil(1,2) < Seuil(1,1), Seuil(2,2) < Seuil(2,1)... Seuil(N,2) < Seuil(N,1).In the opposite case, that is to say if the previous condition is not verified, a new comparison is made (block 72 '): $${\mathit{<figure-callout\; id="1"\; label="rms"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">rms</figure-callout>}}_{\mathit{1}}>\mathit{<figure-callout\; id="1"\; label="Threshold"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">Threshold</figure-callout>}\left(1,,2\right)\&\&{\mathit{<figure-callout\; id="2"\; label="rms"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">rms</figure-callout>}}_{\mathit{2}}>\mathit{<figure-callout\; id="2"\; label="Threshold"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">Threshold</figure-callout>}\left(2,,2\right)\&\&...\&\&{\mathit{R}}_{\mathit{MSN}}>\mathit{Threshold}\left(\mathrm{NOT},,2\right)$$

with lower thresholds, that is, Threshold (1,2) < Threshold (1,1), Threshold (2,2) <Threshold (2,1) ... Threshold ( N , 2) < Threshold ( N , 1).

If this last test is positive, then the filters H _FB and H _EQ are modified (block 74 '), but with parameters different from the previous case.

In the negative, it is possible to continue iteratively in the same way (blocks 72 ", 74", etc.) with progressively lower thresholds, so as to choose, among the X selectable filters for the feedback branch 54, the one that produces the least noticeable electric breath possible, while providing the best possible attenuation of acoustic noise.

Very advantageously, upstream of this series of tests, it is intended to detect (block 76) the presence or absence of a music signal M in the restitution chain, for example by comparing the power of the signal on the branch intended to this music signal, with respect to a predetermined threshold.

The initial thresholds are then adjusted to different values (blocks 78 or 78 ') depending on whether one is in the presence of music, to take account of the fact that the music plays a masking role, in the same way as the external noise, on the perception of the electric breath introduced by the ANC control.

• un premier filtre F1 adapté aux environnements bruyants tels que ceux des transports en commun, qui atténue fortement les très basses fréquences avec un gain plus important dans le médium et une bande d'atténuation plus large ; et
• un deuxième filtre F2 adapté à un environnement calme, avec une atténuation ANC moindre, une réduction plus énergique du souffle électrique et une limitation de l'effet waterbed autour de 1000 Hz.

The Figures 7 and 8 illustrate examples of two ANC feedback filters, alternately selected automatically according to the external noise conditions: Figure 7 represents, in amplitude and in phase, the Bode diagram of the transfer function H _FB of these two filters, while the Figure 8 illustrates the corresponding attenuations obtained. We chose for these two filters:

• a first F1 filter adapted to noisy environments such as those of public transport, which strongly attenuates the very low frequencies with a greater gain in the medium and a wider attenuation band; and
• a second filter F2 adapted to a calm environment, with a less ANC attenuation, a more energetic reduction of the electric breath and a limitation of the waterbed effect around 1000 Hz.

The Figure 9 is a generalization of the invention to an ANC system comprising not only a feedback branch 54, but also a feedforward branch receiving at its input the signal n of an external microphone 38. This signal n, after scanning by the ADC 80, is processed to apply a transfer function H _FF (block 82) within the DSP 50.

As in the case of the feedback branch , the algorithm Auto-ANC 62 will modify the coefficients of the filter H _FF , that is to say select one of Z preconfigured digital filters (Z may be equal to X, but not necessarily ), based on the signal delivered by the internal microphone 28 and not the signal delivered by the external microphone 38. The equation of the signal e delivered by the internal microphone 38 is: $$e=H_{\mathrm{at}}*H_{\mathit{FF}}*B+H_{\mathit{ext}}/\left(1-H_{\mathrm{at}}*H_{\mathit{FB}}\right)*B+H_{\mathrm{at}}/\left(1-H_{\mathrm{at}}*H_{\mathit{FB}}\right)*H_{\mathit{EQ}}*M\mathrm{.}$$

Note that in such a system the feedforward filter H _FF does not affect the equalization of the music, and therefore it is not necessary to modify the filter H _EQ of the equalization branch 58.

The state machine for signal analysis e and the selection of the most appropriate feedforward H _FF filter is the same as illustrated Figure 6 , the only difference being that after testing the different thresholds (which remain the same) in the blocks 74, 74 ', 74' ..., the transfer function H _FF is modified in addition to the transfer functions H _FB and H _EQ .

Claims (3)

An audio headset, comprising two earphones (10) each comprising a transducer (18) for reproducing sound of an audio signal to be reproduced, this transducer being housed in an acoustic ear cavity (22), this headset comprising a control system active noise, ANC, with: an internal ANC microphone (28) placed inside the acoustic cavity (22), able to deliver a signal picked up in this cavity; and a digital signal processor, DSP, (50) comprising: a closed-loop feedback branch (36), comprising a feedback ANC filter (54) adapted to apply a filter transfer function (H _FB ) to the signal picked up by the microphone ANC, the feedback filter ANC (54) being one one of a plurality of preconfigured ANC feedback filters (F1, F2) selectively switchable; means (62) for real-time analysis of the signal picked up by the microphone ANC, able to check if current characteristics of this signal, including signal energy values ( Rms1, Rms2 ...) in a plurality of frequency bands (Filter1, Filter2 ...), check a set of predetermined criteria; selection means (62) capable of selecting one of the preconfigured ANC feedback filters (F1, F2) as a function of the result of the verification of the set of criteria performed by the analysis means; and mixing means (46), receiving as input the signal delivered by the feedback branch at the output of the feedback filter ANC (54) and an audio signal to be reproduced (M), and outputting a signal able to drive the transducer (18) characterized in that the DSP (50) further comprises: an equalization branch, comprising an equalization filter (58) adapted to apply an equalization transfer function (H _EQ ) to said audio signal to be reproduced (M) before application thereof to the mixing means (60) the equalization filter (58) being one of a plurality of selectively switchable preconfigurable equalization filters, and in that the selection means (62) are means able to select simultaneously: i) one of the preconfigured feedback ANC filters (F1, F2) according to the result of the verification of said set of criteria; and ii) one of the preconfigured equalization filters, depending on the current feedback ANC filter selected. The headset of claim 1, wherein: - The predetermined criteria battery further comprises detecting (76) the presence or absence of said audio signal to be reproduced (M); and the predetermined criteria comprise two different series of respective thresholds (Threshold (1,1), Threshold (2,1) ...) to which said energy values are compared, one or the other of these two series being selected (78, 78 ') according to whether an audio signal to be reproduced is present or not. The headset of claim 1, wherein: - The helmet further comprises an external microphone (38), placed outside the acoustic cavity and adapted to capture an acoustic noise (30) prevailing in the environment of the helmet; the DSP (50) further comprises: an open loop feedforward branch (40), comprising a feedforward ANC filter (82) adapted to apply a feedforward filter transfer function (H _FF ) to the signal output from the external microphone (38); the feedforward ANC filter is one of a plurality of preconfigured, selectively switchable ANC feedforward filters; and the selection means are also able to select one of the preconfigured feedforward ANC filters according to the current feedback ANC filter selected.

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