EP3979663A1 - Audio headset with noise-reduction - Google Patents

Abstract

The helmet (10) comprises:
- a transducer (16) (12);
- an anti-noise processing chain (30, 40) comprising:
- a microphone (31, 41);
- an anti-noise processing filter (34, 44) which comprises in series:
- a stabilization filter (34A, 44A) whose transfer function is substantially equal to the inverse of the transfer function of the secondary path processed, and
- a noise cancellation filter (34B, 44B) whose transfer function is a noise cancellation transfer function.

The secondary path is formed between the transducer (16) and the eardrum, and the transfer function of the processed secondary path is the transfer function of the secondary path affected by the transfer functions of the components providing the processing, with the exception of the filter of treatment (34, 44).

Description

• un transducteur électro-acoustique placé dans une cavité de restitution sonore ;
• au moins une chaine de traitement anti-bruit comportant :
  • un microphone de capture du son ambiant ;
• un filtre de traitement anti-bruit du signal issu du microphone pour produire un signal anti-bruit ;
• des moyens d'application du signal anti-bruit pour l'excitation du transducteur électro-acoustique.

The present invention relates to noise reduction headphones, of the type comprising:

• an electro-acoustic transducer placed in a sound reproduction cavity;
• at least one anti-noise processing chain comprising:
  • an ambient sound pickup microphone;
• an anti-noise processing filter of the signal coming from the microphone to produce an anti-noise signal;
• means for applying the anti-noise signal for the excitation of the electro-acoustic transducer.

The noise reduction audio headset comprises at least one microphone placed either inside the cavity between the electro-acoustic transducer and the auditory canal, or outside this cavity. Ideally, such a headset has microphones placed in both positions.

To ensure the creation of an anti-noise signal reproduced by the transducer, the signals coming from the two microphones are processed by digital filters which may be the combination of one or more filters.

Headsets with an internal microphone generally have good performance in terms of noise reduction level, commonly of the order of 20 to 30 dB, but in a limited frequency range, typically 50 to 1000 Hz, due to the instability of the feedback loop formed by the internal microphone and its filter directly receiving the signals emitted by the electro-acoustic transducer. This instability can cause feedback and the filters therefore have a range of action deduced to avoid this phenomenon.

Headsets with an external microphone do not have this restriction in terms of instability in that there is only a very attenuated signal of the order of 50 dB coming from the electro-acoustic transducer picked up by the external microphone. , which does not create feedback. They generally provide attenuations of up to only 10 dB, since the microphone favors a direction for picking up external noise coming towards the ear.

Earmuffs with an external microphone can theoretically attenuate noises above 1 kHz, but the performance is highly dependent on the direction of the noise source, especially at higher frequencies.

The filters used at the output of the external or internal microphones are designed to seek to overcome the problems previously mentioned, namely feedback for the internal microphones and reduced performance due to the strong directivity of the measurements in the case of the external microphone.

These filters are commonly set empirically to modulate gain as a function of frequency.

The object of the invention is to propose a solution to this problem of feedback for interior microphones, and of the difficulty of constructing a high-performance filter taking into account the directivity for external microphones while allowing the satisfactory frequency ranges of the reducer to be widened. noise.

• un filtre de stabilisation dont la fonction de transfert est égale sensiblement à l'inverse de la fonction de transfert du chemin secondaire traité, et
• un filtre d'annulation de bruit dont la fonction de transfert est une fonction de transfert d'annulation du bruit,

• le chemin secondaire étant formé entre le transducteur électro-acoustique et le tympan de l'utilisateur, et
• la fonction de transfert du chemin secondaire traité étant la fonction de transfert du chemin secondaire affecté des fonctions de transfert des différents composants assurant le traitement jusqu'au transducteur dans la chaine de traitement anti-bruit, à l'exception du filtre de traitement.

To this end, the subject of the invention is noise reduction headphones of the aforementioned type, characterized in that, for the or each anti-noise processing chain, the processing filter comprises in series:

• a stabilization filter whose transfer function is substantially equal to the inverse of the transfer function of the secondary path processed, and
• a noise cancellation filter whose transfer function is a noise cancellation transfer function,

• the secondary path being formed between the electro-acoustic transducer and the user's eardrum, and
• the transfer function of the processed secondary path being the transfer function of the secondary path affected by the transfer functions of the various components ensuring the processing up to the transducer in the anti-noise processing chain, with the exception of the processing filter.

• le filtre de stabilisation est construit de sorte que sa fonction de transfert est sensiblement égale à l'inverse de la fonction de transfert du chemin secondaire traité, de 5 Hz à 50 Hz et de 1 kHz à 10 kHz, à une erreur près sur le gain de 5 dB et avec un déphasage de +45 à -45° sur la phase corrigée de la phase linéaire due au retard pur résultant de la propagation dans l'air et du retard dû au processeur ;
• il comporte, une chaine de traitement anti-bruit interne ayant un microphone interne placé dans la cavité de restitution sonore ;
• dans la chaine de traitement anti-bruit interne, la fonction de transfert d'annulation du bruit a un gain supérieur à 20 dB sur toute la gamme audio ;
• dans la chaine de traitement anti-bruit interne, le filtre de stabilisation est un filtre proportionnel intégrale ou un filtre étagé ;
• il comporte une chaine de traitement anti-bruit externe ayant un microphone externe placé hors de la cavité de restitution sonore ;
• dans la chaine de traitement anti-bruit externe, la fonction de transfert d'annulation du bruit est sensiblement égale à l'opposé du quotient de la fonction de transfert correspondant à l'atténuation passive de la cavité par la fonction de transfert entre l'enveloppe externe de la cavité et le microphone externe.

According to particular embodiments, the noise reduction audio headset comprises one or more of the following characteristics:

• the stabilization filter is constructed so that its transfer function is substantially equal to the inverse of the transfer function of the processed secondary path, from 5 Hz to 50 Hz and from 1 kHz to 10 kHz, to within an error on the gain of 5 dB and with a phase shift of +45 to -45° on the corrected phase of the linear phase due to the pure delay resulting from the propagation in the air and the delay due to the processor;
• it comprises an internal anti-noise processing chain having an internal microphone placed in the sound reproduction cavity;
• in the internal anti-noise processing chain, the noise cancellation transfer function has a gain greater than 20 dB over the entire audio range;
• in the internal anti-noise processing chain, the stabilization filter is a proportional-integral filter or a stepped filter;
• it comprises an external anti-noise processing chain having an external microphone placed outside the sound reproduction cavity;
• in the external anti-noise processing chain, the noise cancellation transfer function is substantially equal to the opposite of the quotient of the transfer function corresponding to the passive attenuation of the cavity by the transfer function between the outer shell of the cavity and the external microphone.

• un transducteur électro-acoustique ;
• au moins une chaine de traitement anti-bruit comportant :
• un microphone de capture du son ambiant ;
• un filtre de traitement anti-bruit du signal issu du microphone pour produire un signal anti-bruit ;
• des moyens d'application du signal anti-bruit pour l'excitation du transducteur électro-acoustique

• comportant, pour la ou chaque chaine de traitement anti-bruit, les étapes consistant à :
  • 1/ mesurer la fonction de transfert d'un chemin secondaire traité ;
  • le chemin secondaire étant formé entre le transducteur électro-acoustique et le tympan de l'utilisateur, et
  • la fonction de transfert du chemin secondaire traité étant la fonction de transfert du chemin secondaire affecté des fonctions de transfert des différents composants assurant le traitement jusqu'au transducteur dans la chaine de traitement anti-bruit à l'exception du filtre de traitement
  • 2/ inverser la fonction de transfert du chemin secondaire traité ;
  • 3/ créer un filtre de traitement ayant une fonction de transfert formée du produit de:
    • l'inverse de la fonction de transfert du chemin secondaire traité, et
    • une fonction de transfert d'annulation du bruit; et
  • 4/ construire un casque dont le filtre de traitement anti-bruit est le filtre de traitement créé.

The invention also relates to a method of manufacturing a noise reduction audio headset comprising:

• an electro-acoustic transducer;
• at least one anti-noise processing chain comprising:
• an ambient sound pickup microphone;
• an anti-noise processing filter of the signal coming from the microphone to produce an anti-noise signal;
• means for applying the anti-noise signal for the excitation of the electro-acoustic transducer

• comprising, for the or each anti-noise processing chain, the steps consisting in:
  • 1/ measure the transfer function of a processed secondary path;
  • the secondary path being formed between the electro-acoustic transducer and the user's eardrum, and
  • the transfer function of the processed secondary path being the transfer function of the secondary path affected by the transfer functions of the various components ensuring the processing up to the transducer in the anti-noise processing chain with the exception of the processing filter
  • 2/ invert the transfer function of the processed secondary path;
  • 3/ create a processing filter having a transfer function formed from the product of:
    • the inverse of the transfer function of the processed secondary path, and
    • a noise cancellation transfer function; and
  • 4/ build a headset whose anti-noise processing filter is the created processing filter.

According to particular modes of implementation, the method comprises the measurement of the transfer function of the processed secondary path is carried out on a complete headset but devoid of the or each anti-noise processing filter by excitation of the transducer by a sinusoidal function of variable frequency over the entire audio range and measurement of the signal obtained on an artificial ear.

• [Fig. 1] La figure 1 est une vue schématique d'un casque audio à réduction de bruit selon l'invention ;
• [Fig. 2] La figure 2 est une courbe illustrant, en fonction de la fréquence, les gains de l'inverse de la fonction de transfert du chemin secondaire traité, du filtre de stabilisation et de la combinaison de la fonction de transfert du chemin secondaire traité et du filtre de stabilisation ;
• [Fig. 3] La figure 3 est une courbe illustrant les phases des mêmes quantités que la figure 2 en fonction de la fréquence.

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

• [ Fig. 1 ] The figure 1 is a schematic view of a noise reduction audio headset according to the invention;
• [ Fig. 2 ] The picture 2 is a curve illustrating, as a function of frequency, the gains of the inverse of the transfer function of the processed secondary path, of the stabilization filter and of the combination of the transfer function of the processed secondary path and of the stabilization filter;
• [ Fig. 3 ] The picture 3 is a curve illustrating the phases of the same quantities as the figure 2 depending on the frequency.

On the figure 1 schematically illustrated is a noise reduction headset 10.

It comprises a cavity 12 for sound reproduction inside which is schematized an ear 14 of the wearer of the helmet.

As known per se, this cavity comprises an electro-acoustic transducer 16 arranged facing the auditory canal of the ear. This cavity 12 is formed for example by a shell covering most of the ear in the case of an external audio headset or takes the form of an anatomical box that can be introduced into the entrance to the auditory canal in the case of an in-ear headphone.

The transducer 16 is connected, for its excitation, to an amplifier 18, assumed to have unity gain, receiving a digital signal to be reproduced through a digital/analog converter 20.

The headphones have an input 22 for a musical signal to be reproduced connected to the input of the digital/analog converter 20 through an equalization filter 24.

In order to ensure the anti-noise treatment, the helmet 10 comprises an internal anti-noise treatment chain 30 comprising an internal microphone 31 arranged inside the cavity 12 facing the electro-acoustic transducer 16.

The external microphone 31 is capable of picking up the sound produced by the transducer 16 and the external noise at the level of the outer envelope of the cavity 12, denoted bext, filtered by the cavity 12 whose transfer function is denoted HPA.

The path formed between the transducer 16 and the user's eardrum is called the “secondary path” and its transfer function is denoted Ha.

The transfer function between the measurement point of the internal microphone 31 and the eardrum is denoted Hmici-t. Thus the transfer function between the transducer 16 and the measurement point of the microphone 31 is equal to Ha/Hmici-t.

In practice, the distance between the internal microphone 31 and the eardrum being very small, Hmici-t is substantially equal to 1. Consequently, it is considered in practice and in the rest of the document that the transfer function of the secondary path and the transfer function between the transducer 16 and the measurement point of the internal microphone 31 are both equal to Ha.

The microphone 31 is connected, in the chain 30, to an internal signal processing filter 34 capable of supplying an anti-noise signal, with the interposition of an analog/digital converter 32.

The output of the internal processing filter 34 is connected to the amplifier 18 through an adder 38 arranged upstream of the digital/analog converter 20. This adder ensures the addition of the equalized signals coming from the input 22 and the signals anti-noise from the internal processing chain 30.

Similarly, the headset 10 includes an external noise-cancelling processing chain 40 including an external microphone 41 installed outside the cavity 12.

The external microphone 41 is capable of picking up the external noise bext affected by a transfer function Hbext. Hbext is the transfer function between the external surface of the cavity 12 where the external noise bext applies and the external microphone 41, as illustrated in the figure 1 .

In the external processing chain 40, the external microphone 41 is connected through an analog/digital converter 42 to an external processing filter 44 whose output is connected to the adder 38.

The adder 38 thus ensures a routing to the amplifier 18, through the analog/digital converter 20, of the anti-noise signals produced at the output of the filters 34 and 44 and of the equalized musical signal to be reproduced from the input 22.

The filters and equalizers described here are digital filters implemented in a digital signal processor (DSP).

According to the particular embodiments, the helmet 10 comprises the two internal 30 and external 40 anti-noise treatment chains or either the internal 30 anti-noise treatment chain or the external 40 anti-noise treatment chain is eliminated and only one of the two associated microphones and filters is retained.

• l'inverse de la fonction de transfert du chemin secondaire traité, et
• une fonction de transfert d'annulation du bruit.

Whatever the embodiment and according to the invention, the external 34 and internal 44 anti-noise processing filters when they exist have a transfer function each formed of the product of:

• the inverse of the transfer function of the processed secondary path, and
• a noise cancellation transfer function.

The transfer function of the processed secondary path is the transfer function of the secondary path affected by the transfer functions of the various components ensuring the processing up to the transducer 16, with the exception of the internal 34 or external 44 processing filter depending on the case. Here, it is in particular the transfer functions of the microphone 31 or 41 depending on the case, of the analog/digital converter 32 or 42 depending on the case and of the digital/analog converter 20. The amplifier 18 is assumed to be unitary and if such n is not the case, its transfer function is also integrated into the transfer function of the secondary path processed.

The inverse of the transfer function of the processed secondary path is applied by a stabilization filter denoted respectively 34A and 44A for the filters 34 and 44. These filters 34A, 34B have a stabilization transfer function denoted HFBcorr and HFFcorr respectively.

Each stabilization filter 34A, 44A is followed, at the output, respectively in the processing filter 34, 44, by a noise cancellation filter 34B, 44B whose transfer function is denoted respectively HFB2, HFF2.

The digital filters 34A, 44A and 34B, 44B used are for example infinite impulse response (IIR) filters or finite impulse response (FIR) filters.

The construction and nature of filters 34 and 44 will now be described.

We note s, the residual noise received by the eardrum of the helmet wearer, assumed to correspond to the sound picked up by the internal microphone 31.

We note HPA the transfer function s/bext in the absence of an active noise reduction device, in other words, it is the passive attenuation of the cavity, bext being the ambient noise on the external envelope of this cavity.

This HPA transfer function is generally close to a low-pass filter, which means that the structure forming the cavity mainly reduces high frequencies.

Où:

• p : une variable complexe
• HFB : fonction de transfert du filtre 34
• HFF: fonction de transfert du filtre 44
• HPA : fonction de transfert de l'atténuation passive de la structure du casque délimitant la cavité 12
• Hbext : fonction de transfert entre la surface externe de la cavité 12 où s'applique le bruit externe bext et le microphone externe 41
• PlantFB = Gadci*Gdac*Hmici*Ha est la fonction de transfert du chemin secondaire traité pris au travers de la chaine de traitement anti-bruit interne 30
• PlantFF = Gadce*Gdac*Hmice*Ha est la fonction de transfert du chemin secondaire traité pris au travers de la chaine de traitement anti-bruit externe 40
• Où :
  • Gadci et Gadce : gains des convertisseurs analogique/numérique 32 et 42 pour les microphones interne 31 et externe 41 respectivement
  • Gdac : gain de sortie du convertisseur numérique/analogique 20
  • Hmici : fonction de transfert du microphone interne 31
  • Hmice : fonction de transfert du microphone externe 41
  • Ha : fonction de transfert entre le transducteur 16 et le tympan supposé correspondre au point de mesure du microphone interne 31
  • Ha dépend des caractéristiques du transducteur, ainsi que de l'architecture acoustique autour de lui, et notamment des chambres en avant et en arrière du transducteur lorsqu'il s'agit d'un haut-parleur.

The residual noise s is expressed, in the Laplace domain, by the following expression: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{PlantFB}*\mathrm{HFB}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}+\mathrm{PlantFF}*\mathrm{HFF}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

Or:

• p: a complex variable
• HFB: filter transfer function 34
• HFF: Filter 44 transfer function
• HPA: passive attenuation transfer function of the helmet structure delimiting cavity 12
• Hbext: transfer function between the external surface of the cavity 12 where the external noise bext is applied and the external microphone 41
• PlantFB = Gadci*Gdac*Hmici*Ha is the transfer function of the processed secondary path taken through the internal anti-noise processing chain 30
• PlantFF=Gadce*Gdac*Hmice*Ha is the transfer function of the processed secondary path taken through the external anti-noise processing chain 40
• Or :
  • Gadci and Gadce: gains of the analog/digital converters 32 and 42 for the internal 31 and external 41 microphones respectively
  • Gdac: output gain of the digital/analog converter 20
  • Hmici: internal microphone transfer function 31
  • Hmice: external microphone transfer function 41
  • Ha: transfer function between the transducer 16 and the eardrum assumed to correspond to the measurement point of the internal microphone 31
  • Ha depends on the characteristics of the transducer, as well as on the acoustic architecture around it, and in particular on the chambers in front and behind the transducer when it is a loudspeaker.

• T_FB : temps de propagation de l'onde acoustique sur la distance d_FB entre le transducteur 16 et le microphone interne 31, T_FB = dFB / c, où c'est la vitesse du son (342 m/s)
• D_FB : délai de traitement entre l'entrée et la sortie du processeur de signaux numériques pour la chaine de traitement anti-bruit interne 30
• D_FF : délai de traitement entre l'entrée et la sortie du processeur de signaux numériques pour la chaine de traitement anti-bruit externe 40

Ha represents the transfer function of the secondary path, that is to say the transfer function between the transducer 16 and the positioning point of the internal microphone 31 or the eardrum without taking delays into account. In the modeling, the propagation time of the acoustic wave has been isolated in a specific term exp(-pT _FB ). Thus, the complete transfer function Hareal is expressed taking this delay into account by Hareal = Ha*exp(-pT _FB ).

• T _FB : propagation time of the acoustic wave over the distance d _FB between the transducer 16 and the internal microphone 31, T _FB = dFB / c, where it is the speed of sound (342 m/s)
• D _FB : processing delay between the input and the output of the digital signal processor for the internal anti-noise processing chain 30
• D _FF : processing delay between the input and the output of the digital signal processor for the external anti-noise processing chain 40

Consider now a first embodiment in which the external anti-noise processing chain 40 is eliminated.

In this case, the residual noise s is expressed at the level of the eardrum in the form: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{PlantFB}*\mathrm{HFB}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}\right)*\mathrm{bext}$$

It is measured and processed from the internal microphone 31 alone.

As the filter 34 is formed of two parts, namely a noise cancellation filter 34B having a transfer function denoted HFB2 and the stabilization filter 34A with a transfer function HFBcorr, we have: HFB = HFBcorr * HFB2

According to the invention, the transfer function HFBcorr is taken substantially equal to the inverse of the transfer function of the processed secondary path PlantFB, i.e.: $$\mathrm{HFBcorr}*\mathrm{PlantFB}\sim =1$$

Stabilization filter 34A is then constructed to substantially apply this transfer function HFBcorr ∼=PlantFB ^-1 .

To build the filter 34A, one proceeds first, on a complete helmet but without programming the internal processing filter 34 in the digital signal processor (DSP in English), to the measurement of the transfer function PlantFB of the secondary path processed by implementing an excitation of the transducer 16 by a sine function of variable frequency over the entire audio range and measurement of the signal obtained by an artificial ear 14, which makes it possible to determine the value of Ha. The values of the other terms are known, these being the commercially documented component transfer functions.

The inverse PlantFB ^-1 of this transfer function is then numerically calculated.

Since the PlantFB transfer function does not integrate the delays exp(-pT _FB ) and exp(-pD _FB ), it is invertible and its inverse being causal can be achieved by a filter in a real-time system. This is why the delays are not taken into account in the transfer function of the processed secondary path. Without this, the terms constituting delays would be invertible but would not allow the construction of a filter implementing its inverse in a real-time system, this filter then having to be anti-causal.

A stabilization filter 34A is then constructed, with a transfer function HFBcorr reproducing as faithfully as possible the transfer function PlantFB ^-1 .

In practice, the stabilization filter 34A is constructed so that its transfer function is substantially equal to the inverse of the transfer function of the secondary path, over the entire audio range, and in particular from 5 Hz to 50 Hz and from 1 kHz to 10 kHz, to within an error on the gain of 5 dB, advantageously of 1 dB, and with a phase shift of +45 to -45° on the corrected phase of the linear phase due to the pure delay resulting from the propagation in air and processor delay.

The filter is programmed and implemented in the digital signal processor (DSP). It is advantageously produced by a combination of several cascaded filters.

The action of the stabilization filter 34A is applied over the entire range of frequencies authorized by the sampling frequency (Fs) of the digital signal processor (DSP). By way of example, if Fs=384 kHz, the correction range of the filter 34A is from 0 Hz to 192 kHz.

The second part of the filter 34, consisting of the noise cancellation filter 34B with transfer function HFB2, is designed so that stability is ensured for all frequencies, while applying the highest possible gain in the audio band and in particular greater than 20dB to provide maximum noise cancellation performance. The filter 34B is advantageously formed of a proportional integral (PI) filter or of a stepped filter known under the terms of shelving filter in English.

As the residual noise of the internal anti-noise processing chain 30 with only the internal microphone 31 is: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{PlantFB}*\mathrm{HFB}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}\right)*\mathrm{bext}$$

By incorporating [Math 3] into [Math 2], we get: $$\mathrm{s}\left(\mathrm{p}\right)\sim =\mathrm{1}/\left(\mathrm{1}-\mathrm{<figure-callout\; id="2"\; label="HFB"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFB</figure-callout>}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}\right)*\mathrm{bext}\mathrm{.}$$

It is understood that the phase of the denominator is only dependent on a pure delay, which is the sum of the time T _FB of physical propagation of the acoustic wave and the time D _FB of processing of the digital signal processor (DSP in English), and the phase of the noise cancellation filter HFB2 pushing back into the frequency band, a cancellation of the denominator, which is at the origin of the feedback. This is thus avoided, even if the anti-noise has a high gain over a wide frequency range.

The result is shown on the figure 2 and 3 .

In these figures, the inverse of the transfer function PlantFB ^-1 is represented by a thin continuous line. It is the exact mathematical inverse of the PlantFB transfer function measured experimentally on the helmet.

The actual HFBcorr transfer function of the implemented stabilization filter 34A is shown as a dotted line. These two curves are very close as explained previously.

$${\mathrm{T}}_{\mathrm{FB}}\sim =\mathrm{6}\mathrm{\mu s}$$

Thus, the difference equal to PlantFB * HFBcorr shown in bold solid lines corresponds to a practically flat transfer function in terms of gain ( figure 2 ) and phase ( picture 3 ), the phase difference at high frequencies being mainly due to pure system delays, when considering: $${\mathrm{D}}_{\mathrm{FB}}=\mathrm{11}\mathrm{\mu s}$$

$${\mathrm{T}}_{\mathrm{FB}}\sim =\mathrm{6}\mathrm{\mu s}$$

We now consider a second embodiment in which the internal anti-noise processing chain 30 is eliminated and only the external anti-noise processing chain 40 is present.

In this case, the residual noise s is expressed at the level of the eardrum in the form: $$\mathrm{s}\left(\mathrm{p}\right)=\left(\mathrm{HPA}+\mathrm{PlantFF}*\mathrm{HFF}*\exp \left(-\mathrm{pTFB}\right)*\exp \left(-\mathrm{pDFF}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

It is measured and processed from the single external microphone 41.

Similarly for the external anti-noise processing chain 40, the noise cancellation filter 44B having a transfer function denoted HFF2 and the stabilization filter 44A having a transfer function HFFcorr, we have: HFF = HFFcorr * HFF2

According to the invention, the transfer function HFFcorr is taken substantially equal to the inverse of PlantFF, so that: $$\mathrm{FFcorr}*\mathrm{PlantFF}\sim =1\mathrm{ie\; HFFcorr}\sim ={\mathrm{PlantFF}}^{-1}\mathrm{.}$$

In this case, the secondary path processed is given by: $$\mathrm{PlantFF}=\mathrm{Gadce}*\mathrm{gdac}*\mathrm{Hmice}*\mathrm{Ha}$$

To construct the filter 44A, one proceeds first, on a complete helmet in the absence of the internal processing filter 44, to the measurement of the transfer function PlantFF of the secondary path processed by subjecting the transducer 16 to a variable frequency sweeping the audio range and measuring the signal obtained by an artificial ear 14

The inverse PlantFF ^-1 of this transfer function is then numerically calculated.

In practice, the stabilization filter 44A is constructed so that, as in the previous embodiment, its transfer function is substantially equal to the inverse of the transfer function of the secondary path, from 5 Hz to 50 Hz and from 1 kHz to 10 kHz, and advantageously over the entire audio range, to within an error on the gain of 5 dB, advantageously 1 dB, and with a phase shift of +45 to -45° on the phase without taking the phase into account linear due to the pure delay resulting from the propagation in the air and the delay due to the processor.

The residual noise is for the only external anti-noise processing chain 40 with only the external microphone 41 is written in the form: $$\mathrm{s}\left(\mathrm{p}\right)=\left(\mathrm{HPA}+\mathrm{PlantFF}*\mathrm{HFF}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

By incorporating [Math 5] into [Math 4], we get: $$\mathrm{s}\left(\mathrm{p}\right)=\left(\mathrm{HPA}+\mathrm{<figure-callout\; id="2"\; label="HFF"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFF</figure-callout>}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

This form makes it possible to define the HFF2 filter without having to take into account constraints on the other elements of the system, these being reduced in the expression above to a simple delay exp(-pT _FF ) * exp(-pD _FF ).

The optimal transfer function filter 44 denoted HFFopt corresponding to the cancellation of the residual noise expressed by [Math 4] is: $$\mathrm{HFOpt}=-\left(\mathrm{HPA}/\mathrm{Hbext}\right)/\left(\mathrm{PlantFF}\right)*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)$$

The noise cancellation filter 44B is chosen so that its transfer function is equal to HFF2 = - HPA / Hbext, so that thus the residual noise is: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{HPA}*\left(\mathrm{1}-\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)\right)*\mathrm{bext}$$

The delays T _FB and D _FF being small, the product of the two exponentials - exp(-pT _FB ) * exp(-pD _FF ) is close to 1 over a wide frequency range, so that (1 - exp(-pT _FB ) * exp(-pD _FF )) is very close to zero, which corresponds to very strong noise attenuation over a wide frequency range.

We now consider a third embodiment in which both the internal anti-noise processing chain 30 and the external anti-noise processing chain 40 are present.

The residual noise s is expressed, in the Laplace domain, by the expression: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{PlantFB}*\mathrm{HFB}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}+\mathrm{PlantFF}*\mathrm{HFF}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

et $$\mathrm{HFF}=\mathrm{HFFcorr}*\mathrm{HFF}2$$

avec HFFcorr construite telle que HFFcorr * PlantFF ∼= 1In this case, and as before, the transfer functions of the processing filters 34 and 44 are expressed in the form: $$\mathrm{HFB}=\mathrm{HFBcorr}*\mathrm{<figure-callout\; id="2"\; label="HFB"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFB</figure-callout>}2$$

and $$\mathrm{HFF}=\mathrm{HFFcorr}*\mathrm{<figure-callout\; id="2"\; label="HFF"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFF</figure-callout>}2$$

with HFFcorr constructed such that HFFcorr * PlantFF ∼= 1

et avantageusement en choisissant $$\mathrm{HFF}2=-\mathrm{HPA}/\mathrm{Hbext}$$

comme expliqué précédemment, on a : $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{HFB}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\mathrm{HPA}*\left(\mathrm{1}-\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)\right)*\mathrm{bext}$$

The residual noise s is then written: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{<figure-callout\; id="2"\; label="HFB"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFB</figure-callout>}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\left(\mathrm{HPA}+\mathrm{<figure-callout\; id="2"\; label="HFF"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFF</figure-callout>}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)*\mathrm{Hbext}\right)*\mathrm{bext}$$

and advantageously by choosing $$\mathrm{<figure-callout\; id="2"\; label="HFF"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFF</figure-callout>}2=-\mathrm{HPA}/\mathrm{Hbext}$$

as explained above, we have: $$\mathrm{s}\left(\mathrm{p}\right)=\mathrm{1}/\left(\mathrm{1}-\mathrm{<figure-callout\; id="2"\; label="HFB"\; filenames="imgf0001.png"\; state="\{\{state\}\}">HFB</figure-callout>}\mathrm{2}*\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FB}}\right)\right)*\mathrm{HPA}*\left(\mathrm{1}-\exp \left(-{\mathrm{pT}}_{\mathrm{FB}}\right)*\exp \left(-{\mathrm{pD}}_{\mathrm{FF}}\right)\right)*\mathrm{bext}$$

In this embodiment, the advantages of the two previous embodiments are combined.

Claims (5)

Noise reduction headphones (10) comprising: - an electro-acoustic transducer (16) placed in a sound reproduction cavity (12); - at least one anti-noise processing chain (30, 40) comprising: - a microphone (31, 41) for capturing ambient sound; - an anti-noise processing filter (34, 44) of the signal coming from the microphone (31, 41) to produce an anti-noise signal; - means (18, 20) for applying the anti-noise signal for the excitation of the electro-acoustic transducer (16)
in which, for the or each anti-noise processing chain (30, 40), the processing filter (34, 44) comprises in series: - a stabilization filter (34A, 44A) whose transfer function is substantially equal to the inverse (HFBcorr, HFFcorr) of the transfer function of the processed secondary path (Plant FB, Plant FF), and - a noise cancellation filter (34B, 44B) whose transfer function is a noise cancellation transfer function (HFB2, HFF2), the secondary path being formed between the electro-acoustic transducer (16) and the user's eardrum, and the transfer function of the processed secondary path being the transfer function of the secondary path affected by the transfer functions of the various components ensuring the processing up to the transducer (16) in the anti-noise processing chain (30, 40), at the processing filter exception (34, 44), characterized in that it comprises an external anti-noise processing chain (40) having an external microphone (41) placed outside the sound reproduction cavity (12); and in that , in the external anti-noise processing chain (40), the noise cancellation transfer function (HFF2) is substantially equal to the opposite of the quotient of the transfer function corresponding to the passive attenuation of the cavity (12) by the transfer function between the external envelope of the cavity (12) and the external microphone (41). Noise reduction audio headset (10) according to Claim 1, characterized in that the stabilization filter (34A, 44A) is constructed so that its transfer function (HFBcorr, HFFcorr) is substantially equal to the inverse of the transfer function of the processed secondary path (Plant FB, Plant FF), from 5 Hz to 50 Hz and from 1 kHz to 10 kHz, up to an error on the gain of 5 dB and with a phase shift of +45 to -45° on phase corrected for the linear phase due to the pure delay resulting from the propagation in the air and the delay due to the processor. Noise reduction audio headset (10) according to claim 1 or 2, characterized in that it comprises an internal anti-noise processing chain (30) having an internal microphone (31) placed in the sound reproduction cavity ( 12). Noise reduction audio headset (10) according to Claim 3, characterized in that , in the internal anti-noise processing chain (30), the noise cancellation transfer function (HFB2) has a gain greater than 20 dB over the entire audio range. Noise reduction audio headset (10) according to Claim 3 or 4, characterized in that , in the internal anti-noise processing chain (30), the stabilization filter (32A) is a proportional-integral filter or a stepped filter .

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