EP4133476A1

EP4133476A1 - Method, device, headphones and computer program for actively suppressing interfering noise

Info

Publication number: EP4133476A1
Application number: EP21717392.1A
Authority: EP
Inventors: Johannes Fabry; Peter Jax; Stefan Liebich
Original assignee: Rheinisch Westlische Technische Hochschuke RWTH
Current assignee: Rheinisch Westlische Technische Hochschuke RWTH
Priority date: 2020-04-07
Filing date: 2021-04-06
Publication date: 2023-02-15
Also published as: DE102020109658A1; CN115298735A; WO2021204754A1; US20230154449A1

Abstract

In the method according to the invention for actively suppressing interfering noise, a transfer function for a secondary path between a loudspeaker and an error microphone is measured (20). A transfer function for a primary path between a reference microphone and the error microphone is estimated (21) based on the measured transfer function for the secondary path. Filter coefficients for a filter unit to generate the erase signal are then determined (22) based on the estimated transfer function for the primary path.

Description

description

Method, device, headphones and computer program for active noise suppression

The present invention relates to a method for active noise suppression. The present invention also relates to an apparatus for carrying out the method. The invention also relates to headphones that are set up to carry out a method according to the invention or have a device according to the invention, and a computer program with instructions that cause a computer to carry out the steps of the method.

The high level of noise that is caused, for example, by planes, trains or cars and is perceived as ambient noise by people outside or inside these vehicles, can lead to stress and even serious psychological and physical illnesses for the people concerned. For this reason, methods for active noise cancellation ("Active Noise Cancellation", ANC) are known, in which such disruptive ambient noises are reduced, an important feature for headphones or so-called hearables.

An additional sound signal is artificially generated, which corresponds to that of the interfering sound as exactly as possible, but with opposite polarity, in order to then cancel out the disturbing noises as far as possible by superimposing the two sound signals by means of destructive interference. In the case of headphones with active noise suppression, one or more microphones integrated in the headphones measure the ambient noise and then use the headphones' acoustic transfer function to calculate the amount that would remain on the ear. For this part, the opposite polarity signal is generated in the headphones for compensation and output by means of a loudspeaker, via which the useful sound is also reproduced. Modern ANC headphones usually use fixed feedforward and feedback filters for this purpose and thus enable an attenuation of up to 30 dB at low frequencies, but the filter performance depends sensitively on the respective seat of the headphones and the respective ear shape of the user. In principle, adaptive algorithms can also be considered in order to improve the level of noise suppression. However, such adaptive algorithms require a high computing power and are therefore unsuitable at present in headphones, hearables or hearing aids. Most commercially available ANC headphones are equipped with a built-in loudspeaker and two microphones. One of the microphones is directed towards the headphone environment in order to measure a reference signal in the form of ambient noise and is often referred to as a reference microphone. The other microphone is directed towards the ear canal or eardrum of the user in order to determine an internal error signal and is also known as the error microphone.The acoustic transmission from the external reference microphone to the internal error microphone is called the primary path, the transmission from the loudspeaker to the error microphone is called the secondary path.

A measurement of these primary and secondary paths enables an individual design and thus a considerable improvement in the performance and robustness of an ANC system. The secondary path can be measured using the loudspeaker and the inner microphone, whereby the signal-to-noise ratio at the inner microphone is quite high due to the passive isolation of the headphones costly and not easy to carry out for end users.

Against this background, it is an object of the invention to provide an improved method and an improved device for active noise suppression, in particular for suppressing disturbing ambient noise in headphones, as well as corresponding headphones and a computer program for performing the method.

This object is achieved by a method with the features of claim 1, a corresponding device according to claim 8, a corresponding headphone according to claim 10 and a computer program according to claim 11. Preferred embodiments of the invention are the subject matter of the dependent claims.

The invention makes use of the knowledge that, in particular with in-ear headphones, but also with headphones with other designs, there can be a significant correlation between the frequency spectra of primary and secondary paths and this can be used to even without measuring the Primary path to achieve an optimization of the noise suppression. According to the knowledge of the inventors, in the method according to the invention for active noise suppression, a transfer function for a secondary path between a loudspeaker and an error microphone is measured. Based on the measured transfer function for the secondary path, a transfer function for a primary path between a reference microphone and the error microphone is estimated. Then, based on the estimated transfer function for the primary path, filter coefficients for filtering to generate the cancellation signal are determined.

In particular, at least one reference microphone detects interfering sound signals, a loudspeaker outputs a canceling signal and an error microphone detects the remaining signal after the canceling signal has been superimposed on the interfering sound signal.

According to one embodiment of the invention, active noise suppression is carried out when reproducing a useful audio signal using headphones, with one or more reference microphones being located on the outside of the headphones and the error microphone being located on the inside of the headphones.

The transfer function for the secondary path is preferably measured individually for a user and an individual transfer function for the primary path is estimated for the user based on the individually measured transfer function for the secondary path.

In this case, the filtering is advantageously carried out by means of a forward FIR filter or IIR filter.

According to a further embodiment of the invention, an estimation function for the primary path is determined by measuring and analyzing both the transfer function for the secondary path and the transfer function for the primary path in advance in a training process for different people and / or fits of the headphones.

It is advantageous here if a main component analysis with subsequent dimensional reduction of the measured values obtained in the training process is carried out for measured values in frequency ranges of the transfer functions in which there are deterministic changes for the primary path and the secondary path; complex gain vectors for the primary paths and the secondary paths are determined based on principal components and mean values determined by the principal component analysis; and a linear mapping which minimizes the error between the determined and the estimated gain vectors of the primary paths is determined.

Accordingly, a device according to the invention for active noise suppression comprises at least one reference microphone; a speaker; an error microphone; a digital filter for generating a cancellation signal; a digital signal processor which is set up to generate a measurement signal which can be output via the loudspeaker and to evaluate a signal detected with the error microphone in order to measure a transfer function for a secondary path between the loudspeaker and the error microphone; to estimate a transfer function for a primary path between the reference microphone and the error microphone based on the measured transfer function for the secondary path; and adapting filter coefficients for the digital filter based on the estimated transfer function for the primary path.

According to one embodiment of the invention, the digital filter is designed as an FIR filter or IIR filter.

The invention also relates to headphones that are set up to carry out the method according to the invention or have a device according to the invention, as well as a computer program with instructions which cause a computer to carry out the steps of the method according to the invention.

Further features of the present invention will become apparent from the following description and the claims in conjunction with the figures.

1 shows schematically an in-ear headphone with a primary and secondary acoustic path; 2 shows a flow chart of the method according to the invention for active noise suppression;

Fig. 3 shows a block diagram of a headphone according to the invention;

4 shows spectra of measured primary paths (a) and secondary paths (b);

5 shows a) spectra measured based on the individual secondary paths and the average primary path and b) measured spectra of the active transfer function from the reference microphone to the error microphone based on the individual secondary paths and the respectively estimated primary path;

6 shows the median of the primary path | P (z) | and the spectrum \ H (z) \ for different primary path estimates;

Figure 7 shows a box graph for the energy ratio for various primary path estimates; and

Fig. 8 shows schematically the use of headphones in connection with an external computing device

For a better understanding of the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. It goes without saying that the invention is not restricted to these embodiments and that the features described can also be combined or modified without departing from the scope of protection of the invention as defined in the claims.

The inventive method can in particular for active

Noise suppression in in-ear headphones, as shown schematically in FIG. 1, can be used. The in-ear headphones 10 are located on the ear of a user, with an ear insert 14 of the in-ear headphones being introduced into the external auditory canal 15 in order to hold them in place. The ear insert can, depending on the individual position in the ear canal, partially shield external interference noises so that they are Interfering noises then only reach the eardrum 16 of the user at a reduced level.

An interfering sound signal x (t) arriving from the environment on the headphones is recorded with a reference microphone 11, which is directed away from the ear canal 13, which is located in the vicinity of the error microphone 12, on. A cancellation signal y (t) can be output by means of the loudspeaker 13. The error microphone 12 detects the remaining signal e (t) after a superposition of the cancellation signal y (t) with the interfering sound signal x (t). The primary acoustic path P _a (s) describes the transfer function from the reference microphone 11 to the error microphone 12 _, while the secondary acoustic _{path S a} (s) describes the transfer function from the loudspeaker 13 to the error microphone 12. The in-ear headphones shown only have one reference microphone, but several reference microphones can also be used, for each of which a separate primary path exists

FIG. 2 shows schematically the basic concept for a method for active noise suppression, as it can be carried out, for example, with such in-ear headphones. Here, in a first step 20, a transfer function for a secondary path between the loudspeaker and the error microphone is measured. In a subsequent step 21, based on the measured transfer function for the secondary path, a transfer function for a primary path between the reference microphone and the error microphone is then estimated. For this purpose, use is made of the relationships between the primary path and secondary path in the present headphones, which are determined in a training phase, which will be described below. In a further step 22, the estimated transfer function then allows filter coefficients to be determined for a filter for generating the cancellation signal. In this way, the filter can then be adapted so that the output cancellation signal enables the best possible compensation of the interference signal to prevent or at least reduce impairment of the user's perception by interfering noises when a useful audio signal is played back by means of the in-ear headphones. The user can also suppress background noise without playing a Useful audio signal can be perceived as more pleasant, for example when it is traveling by train or plane and the volume level is reduced as a result.

FIG. 3 shows a block diagram of a device according to the invention, the analog unit 30 with the hardware components from FIG the loudspeaker 13 is connected. The electronic backend comprises a digital filter unit 34 and a processor unit 35.

The device according to the invention can be fully integrated into an ANC headset or also partially part of an external device, such as a smartphone. For example, the processor unit 35 can be part of such an external device.

The processor unit 35 here has one or more digital signal processors, but can also contain processors of other types or combinations thereof. The digital filter 34 is designed as a time-invariant FIR forward filter W (z), which receives the digitally converted interference signal x (n) and generates the cancellation signal y (n). However, the digital filter 34 can also be designed as an IIR filter, usually as a biquad filter. The digital signal processor 35 generates a measurement signal m (n) and evaluates the digitized error signal e (n) in order to measure the secondary path. Furthermore, the filter coefficients of the digital filter W (z) are adapted by the digital signal processor Procedure to be carried out.

The overall transfer function H (s) describes the transfer function from reference microphone 11 to error microphone 12 and, in contrast to the primary path, includes the influence of the ANC system. The primary path P (z) and the secondary path S (z) contain the influence of the analog-digital converter and the digital-analog converter, the loudspeaker and the microphones.

The overall transmission path is then defined as

H (z) = P (z) - W (z) S (z). Here s and z denote the complex frequency parameters of the Laplace and z transformation and n denotes a discrete time index.

In the following, it is first derived how the filter quotients for the FIR forward filter W (z) can be selected based on the individually measured secondary path. An estimator for the primary path is then presented, which is trained based on a series of previously measured primary and secondary paths. After the training phase, measured values of an individual secondary path can then be fed to this estimator in order to estimate the individual primary path.

Let T = {p _j , S _j e ^L \ j = 1, ...,]} be the set of measured impulse responses of length L. The optimal FIR forward filter iv minimizes the average of the energy of the entire transmission path, such as by the the following cost function is defined: with the primary path vector expanded by zeros and convolution matrix s _; for the

Secondary path.

The optimal FIR forward filter iv with respect to the average is given by

In order to individually optimize the FIR forward filter iv, however, precise knowledge of the respective primary and secondary path is required.

As already mentioned, the individual secondary path can be measured with the loudspeaker and the internal error microphone of the headphones. Then if the individual secondary paths for all S _{j are replaced} in the above formula and the average of the primary paths in T, ie is used as an estimate for p, one obtains for the optimal filter for a given individual secondary path:

Since both the primary path and the secondary path depend on the fit of the headphones and the physiology of the user's ear, this correlation can be used to establish an estimator for an individual primary path based on the characteristics of a measured individual secondary path. For this purpose, the frequency ranges of the transfer functions that are affected by deterministic changes are _{extracted with window functions Q p} (z) and Q _s (z) in the z domain

A principal component analysis (PCA) makes the first K _p , K _s principal components , as well as the mean values of a set of windowed, complex frequency domain vectors of the primary path and secondary path are extracted from the set T.

The complex gain vectors g _{p, j} and g _{s, j} minimize the Euclidean distance between the reconstructed frequency domain vectors based on the main components and the frequency domain vectors of the primary path and secondary path. A linear map is then used that represents the Gain vectors g _{p, j of} the primary path are projected onto the gain vectors of the secondary path g _{s, j}

After the individual secondary path has been measured, the window function Q _s (z) in the z-range is applied to the measured secondary path and then the gain vector g _{s, j} for the secondary path is calculated using the main components and the mean value of the secondary path. _{Thereupon the gain vector g p, j} for the primary path is then first of all determined by means of the linear mapping estimated, followed by an estimate of the primary path based on the principal components as well as the mean of the primary path and the estimated gain vector g _{p, j} for the primary path. Finally, by replacing with the estimate of the the individual forward filter for each primary path. The effectiveness of the proposed estimator was checked with simulations, the results of which are presented below. For this purpose, measurements were carried out on in-ear headphones for 25 test persons and different fits, with a sampling rate of 48 kHz being used. The set M of measured primary and secondary paths comprises a total of J = 173 pairs of impulse responses.

FIG. 4 shows the spectra of the measured primary paths (a) and secondary paths (b). The shaded frequency range 40 indicates the range of the selected frequency range window. The length of the primary path and secondary bike was chosen with Z = 1024, the length of the forward filter is L _w = 64. The set of measured primary and secondary paths was randomly divided into two subsets, with a training set 80% and a validation set the other 20% of the set of measured paths. The training set was used to train the estimator as described above. _{Furthermore, K p} = 1 and K _s = 3 were chosen for the number of main components. The performance of the estimator was then validated by testing the overall transfer path, with the measurement 100 times being random divided subsets was repeated.

FIG. 5 shows the measured magnitude spectra \ H (z) \, the filter design in a) being based on the individual secondary paths and the average primary path and in b) on the individual secondary paths and the respective estimated primary path 50% percentiles 52 and 90% percentiles 53 of \ H (z) \ also the median 51 of the primary path | P (z) | to indicate the passive attenuation of the headphones.

FIG. 6 shows the median of the primary path | P (z) | and of the spectrum \ H (z) \ for different primary path estimates. Here, H _avg (z) is based on the mean value of the primary paths of the training set, H _est (z) is based on a primary path estimate, as is H _ppg (z), but with a perfect PCA gain _{vector (PPG) g p} instead of it Estimate is used, and finally, H _opt (z) is based on the actual primary path. The shaded area 60 in which | Q _p (z) | > 0, marks the frequency range in which H (z) is influenced by the primary path estimator. The figure shows that the median of the spectrum | H (z) | decreases by up to 7 dB between 250 Hz and 2.5 kHz and approaches the median based on the individual primary path. The box graph in FIG. 7 shows the energy ratio in dB for the various primary path estimates from FIG. 6 (a) mean, b) estimate, c) estimate with PPG, d) optimum when the actual primary path is known). Here, the energy ratio e of the total windowed transmission path and the primary path using Q _p (z) is defined as

For the various primary path estimates, the median and the minimum, the so-called lower whisker, and the maximum, the so-called upper whisker, are shown as horizontal lines and the lower quartile and upper quartile as a rectangle surrounding the median

As can be seen from the figure, the energy ratio e is reduced by 3.1 dB compared to the use of the mean value (a) when using the estimator (b) of the median, while the difference between the maximum values, the so-called upper whiskers, is 5.0 dB

FIG. 8 schematically shows the use of headphones 10, such as a so-called hearable, in connection with an external computer device 80. The external computer device 80 can in particular be a mobile terminal that is suitable for audio playback , a so-called wearable, such as a smartwatch, a fitness bracelet or data glasses, or a computer tablet are connected to the headphones.

The devices communicate wirelessly via a radio link such as Bluetooth. After the connection has been established, audio signals can be transmitted from the external computer device 80 to the headphones 10 and then reproduced in a conventional manner with one or more loudspeakers integrated in the headphones.

In addition, the active noise suppression according to the invention can also be carried out by means of the external computer device 80. For this purpose, the external computer device 80, in particular when a user is using the headphones 10 for the first time, can transmit a measurement signal to the headphones, which is then output by a loudspeaker integrated in the headphones. One integrated in the headphone 10 The error microphone then detects the error signal, which is transmitted to the external computer device 80. Based on this, the external computer device 80 calculates the secondary path, estimates the primary path and then determines the filter coefficients for the filter for generating the cancellation signal. The filter coefficients are then sent via the wireless connection from the external computer device 80 to the headphones 10, in which the filter is adapted accordingly, so that interfering noises are largely suppressed when the audio signals are reproduced.

The invention can be used for active noise suppression in any areas of audio reproduction technology.

Claims

1. A method for active noise suppression, in which a transfer function for a secondary path between a loudspeaker and an error microphone is measured (20); based on the measured transfer function for the secondary path, a transfer function for a primary path between a reference microphone and the error microphone is estimated (21); and based on the estimated transfer function for the primary path, filter coefficients for filtering to generate a cancellation signal are determined (22).

2. The method according to claim 1, wherein at least one reference microphone (11) detects interfering sound signals, a loudspeaker (13) outputs a cancellation signal and an error microphone (12) detects the remaining signal after a superposition of the cancellation signal with the interference sound signal.

3. The method according to claim 2, wherein the active noise suppression is carried out during the reproduction of a useful audio signal by means of headphones (10) and one or more reference microphones (11) are on the outside of the headphones and the error microphone (12) on the inside of the headphones is located.

4. The method according to any one of the preceding claims, wherein the transfer function for the secondary path is measured individually for a user; an individual transfer function for the primary path is estimated for the user based on the individually measured transfer function for the secondary path.

5. The method according to any one of the preceding claims, wherein the filtering is carried out by means of a forward FIR filter or IIR filter (35)

6. The method according to any one of claims 3 to 5, wherein an estimator for the primary path is determined by beforehand in a training process for different people and / or fits of the headphones in each case both the transmission function for the secondary path as well as the transfer function for the primary path is measured and analyzed.

7. The method according to claim 6, wherein a principal component analysis with subsequent dimensional reduction of the measured values obtained in the training process is carried out for measured values in frequency ranges of the transfer functions in which there are deterministic changes for the primary path and the secondary path; complex gain vectors for the primary paths and the secondary paths are determined based on principal components and mean values determined by the principal component analysis; and a linear mapping which minimizes the error between the determined and the estimated gain vectors of the primary paths is determined.

8. Device for active noise suppression, with at least one reference microphone (11); a speaker (13); an error microphone (12); a digital filter (34) for generating a cancellation signal; a digital signal processor (35) which is set up to generate a measurement signal which can be output via the loudspeaker and to evaluate a signal detected with the error microphone in order to measure a transfer function for a secondary path between the loudspeaker and the error microphone; to estimate a transfer function for a primary path between the reference microphone and the error microphone based on the measured transfer function for the secondary path; and adapting filter coefficients for the digital filter based on the estimated transfer function for the primary path.

9. The device according to claim 8, wherein the digital filter (34) is designed as a forward FIR filter or IIR filter

10 headphones (10), which is set up to carry out a method according to one of claims 1 to 7 or a device according to claim 8 or 9

11. Computer program with instructions that cause a computer to carry out the steps of a method according to one of claims 1 to 7.