Noise cancellation system and signal processing method for ear-worn playback device

Abstract

A noise cancellation system for an ear-worn playback device (HP) having a speaker (SP), a feedforward microphone (FF_MIC), and an error microphone (FB_MIC) includes a filter chain (FF_CH) for coupling the feedforward microphone (FF_MIC) to the speaker (SP), and a noise control processor (SCP). The filter chain (FF_CH) includes a coarse filter (FF_C) and a fine filter (FF_F) connected in series or in parallel. The fine filter (FF_F) is formed by a set of sub-filters having predefined frequency ranges, wherein the predefined frequency ranges of each of the sub-filters together form the effective total frequency range of the fine filter (FF_F). The noise control processor (SCP) is configured to: calculate an error signal based on a first noise signal sensed by a feedforward microphone (FF_MIC) and based on a second noise signal sensed by an error microphone (FB_MIC); perform adaptation of coarse filter parameters of a coarse filter (FF_C) based on the error signal; and perform limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein the limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q-factor limitation.

Description

[0001] The present disclosure relates to a noise cancellation system and a signal processing method, each for an ear-worn playback device (eg, headphones), the ear-worn playback device including a speaker, a feedforward microphone, and an error microphone.

[0002] Nowadays, many headphones, including earbuds, are equipped with noise cancellation technology. This type of noise cancellation is known as active noise cancellation or ambient noise cancellation, both abbreviated as ANC. ANC typically utilizes recorded ambient noise, which is processed to generate an anti-noise signal. This anti-noise signal is then combined with the desired audio signal for playback through the headphone's speakers. ANC can also be used with other audio devices, such as handsets or mobile phones.

[0003] Various ANC methods use feedback (FB) microphones, feed-forward (FF) microphones, or a combination of feedback and feed-forward microphones.

[0004] FF and FB ANC are achieved by tuning the filters based on the acoustics of a given system.

[0005] In conventional ANC systems, the filter parameters of the individual ANC filters are tuned, for example, during the production of the ANC headset, for example using calibration measurements, or by continuously adapting all filter parameters during operation of the ANC headset.

[0006] An object to be achieved is to provide an improved concept for improving the ANC performance in the feedforward part of an ANC system.

[0007] This object is achieved by the subject matter of the independent claims. Embodiments and developments of the improved concept are defined in the dependent claims.

[0008] In various embodiments, as a general example of an ear-worn playback device with ANC, a noise-canceling headphone includes a driver or speaker whose front is directly acoustically coupled to a front volume that is partially formed by the ear canal volume when the headphone is worn. The rear of the driver may be surrounded by a rear volume. There are typically front vents that acoustically couple the front volume to the surrounding environment, and rear vents that acoustically couple the rear volume to the surrounding environment. Either vent may be covered with a soundproofing mesh.

[0009] ANC headphones can have a microphone on the housing that is directly coupled to the surrounding environment and detects a negligible amount of the driver signal. The microphone signal is processed by a feedforward filter and played back from the driver, generating an anti-noise signal that is in phase with and equal in amplitude to the noise signal at the ear, thus achieving FF ANC. The attenuation achieved is typically around 20dB over the frequency band of 100Hz to 1kHz.

[0010] The noise at the ear can be represented by the environment-to-ear acoustic transfer function AE, while the anti-noise signal can be given by the environment-to-FF microphone acoustic transfer function AFFM, the FF filter response F, and the driver-to-ear acoustic transfer function DE, resulting in a residual error Err, such as

[0011] Err=AE-AFFM.F.DE.

[0012] For perfect noise cancellation, the error Err = 0, so the ideal filter shape F is given by:

[0013]

[0014] The ideal filter shape can be calculated by measuring the three transfer functions mentioned above. This is often referred to as the FF target. Therefore, if the filter differs from the FF target, noise cancellation is degraded. The goal of good FF ANC is to match the filter F to the FF target as closely as possible.

[0015] ANC headphones may also have microphones mounted close to the drivers that detect sounds from both the surrounding environment and the drivers themselves.

[0016] For a FF system to achieve 20dB ANC, the filters should be matched to the FF target with high accuracy. It has been found that if the filter phase is perfectly matched, the filter amplitude must be matched within 0.8dB, or if the filter amplitude is perfectly matched, the filter phase must be matched within 5 degrees.

[0017] The improvement idea is based on the following observation: This is a challenge for fixed FF filters because the FF target response can change based on the following factors:

[0018] -variable acoustic leakage from the front space to the surrounding environment due to the way the headphones are worn each time on the head,

[0019] - differences in ambient noise response at the ear due to variable compression of the ear pads or rubber tips,

[0020] - Component differences leading to varying driver and microphone responses,

[0021] -Noise transmission through headphones varies due to manufacturing differences.

[0022] These changes are likely very small, but even with the calibration process, the FF ANC can't reach more than 20dB.

[0023] A typical FF target contains several highly damped and difficult-to-characterize resonances, which are based on the driver response and its acoustic loading, as well as the propagation of sound through the earphone into the ear. These resonances are susceptible to change based on the factors mentioned above. Therefore, a fixed FF filter, even one with a very high order, cannot compensate for these, as it will only work for one earphone driver when worn in a specific way by the same person. This means that any small changes to the FF target and FF filter will no longer be optimal.

[0024] Therefore, these small changes in the FF target response need to be accounted for, and this process must account for variations based on manufacturing differences between units, as well as the constant subtle changes in the FF target when placed in different ways on the head. Because the FF target changes so frequently, an adaptive filter is required.

[0025] However, conventional adaptive ANC exists but has drawbacks, particularly for infinite impulse response (IIR) high-order filters, which are needed to reduce processing overhead in noise cancellation ICs. Conventional adaptation algorithms adapt the coefficients in IIR filters, which can become unstable and can cause the coefficients to compete vigorously with each other, risking false nulls and very slow or high-power adaptation, which is impractical for noise-canceling headphone ICs.

[0026] Therefore, the improved concept is based on the idea of ​​a two-stage filter chain adaptation process. The first stage is the adaptation of the coarse filter, which compensates for large variations in the FF target. The second stage is a fine adaptation, which adapts an additional high-resolution filter, or fine filter, arranged in series or parallel with the coarse filter and strictly limited to have a small impact on the entire filter chain. The fine filter has the effect of refining the overall filter response to reduce the gain and phase errors between the filter and the acoustics, thereby increasing the FF ANC by 40dB or more within the bandwidth already determined by the driver and processor speed.

[0027] For example, the fine filter is formed by a sub-filter bank, each of which has a predefined frequency range. The predefined frequency range of each sub-filter may be adjacent to or at least partially overlap with the predefined frequency range of at least one other sub-filter in the sub-filter bank. The sub-filters may be connected in series or in parallel.

[0028] Thus, an effective total frequency range can be achieved by utilizing a continuous frequency range of the fine filter. For example, the effective frequency range is selected to have the best effect of refining the filter response of the filter chain.

[0029] According to an improved concept, only limited adaptation of the filter parameters of each of the sub-filters is performed, wherein the limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q-factor limitation. Such limitations may not be directly targeted at frequency gain or Q-factor, but they may be directly targeted at the poles / zeros of the sub-filter or their coefficients, so that they have the effect of indirectly limiting the frequency, gain or Q-factor.

[0030] For example, an embodiment of a noise cancellation system for an ear-worn playback device according to an improved concept is provided. The ear-worn playback device has a speaker, a feedforward microphone, and an error microphone, wherein the feedforward microphone is configured to primarily sense ambient sound and the error microphone is configured to sense ambient sound and sound output from the speaker. The noise cancellation system includes a filter chain for coupling the feedforward microphone to the speaker, the filter chain including a coarse filter and a fine filter connected in series or in parallel. The noise cancellation system also includes a noise control processor configured to calculate an error signal based on a first noise signal sensed by the feedforward microphone and based on a second noise signal sensed by the error microphone. The noise control processor is further configured to perform adaptation (e.g., coarse adaptation) of coarse filter parameters of the coarse filter based on the error signal and to perform limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein the limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q factor limitation.

[0031] In some embodiments, the subfilters are configured to provide a plurality of filter elements, each of which is a biquad filter or a second order IIR filter. For example, at least one of the subfilters is a biquad filter or a second order IIR filter. In some embodiments, all subfilters are implemented in the same manner. Biquad filters or other second order IIR filters can be easily implemented in a signal processor. In addition, such filter can be parameterized with five or six filter parameters separately, which reduces the workload in the adaptation process in terms of computational workload and stability tracking. Especially, limiting the parameters in the limited adaptation process can reduce the workload in terms of required computing during the adaptation period.

[0032] In some embodiments, the sub-filter bank includes between six and twelve sub-filters, for example eight to ten sub-filters. For example, given the limited total frequency range of the fine filter, this allows the sub-filters to have a small predefined frequency range, thereby resulting in a higher resolution of the overall filter response for refining the filter chain.

[0033] For example, the effective total frequency range of the fine filter is 80 Hz to 2000 Hz, for example 80 Hz to 1000 Hz. It has been found that such a frequency range has a good influence on the overall frequency response of the filter chain.

[0034] Limiting the gain of each sub-filter results in less exposure to stability issues; similarly, limiting the Q factor of a sub-filter results in limited changes in the shape of the corresponding filter response and can also be used to support the stability of the sub-filter during the adaptation process. For example, in addition to limiting to a predefined frequency range, gain limiting and Q factor limiting are also applied.

[0035] For example, each sub-filter is one of a peaking filter and a notch filter. For example, during the adaptation process, a specific sub-filter can be changed from a peaking filter to a notch filter, and vice versa, through the adaptation process. If the result of the sub-filter is a peaking filter, the total gain within the predefined frequency range can be increased, while if the result of the sub-filter is a notch filter, the total gain can be reduced.

[0036] As mentioned previously, the calculated and / or measured target response function F (without taking into account variations during operation) forms the basis for the coarse filter of the filter chain, which may also include a non-minimum phase portion. In other words, it can be assumed that the fine filter does not require a significant amount of delay, as this is compensated for by the coarse filter. Therefore, it may be sufficient if each subfilter is a minimum phase filter.

[0037] In various embodiments, the limited self-adaptation of the sub-filters is based on an error minimization algorithm, such as a least mean square (LMS) algorithm. For example, a u-filtered LMS algorithm can be used to adapt the fine filter parameters of the sub-filters.

[0038] In various embodiments, the limited adaptation of the sub-filters includes adapting the gain, center frequency, and Q factor of at least one of the sub-filters. Thus, the fine filter parameters of the corresponding sub-filter can be calculated based on the adapted gain, center frequency, and Q factor.

[0039] Additionally or alternatively, the limited adaptation of the sub-filters may comprise directly adapting fine filter parameters of at least one of the sub-filters and checking limits of the limited adaptation of the adapted fine filter parameters.Each of the embodiments allows for an efficient adaptation process.

[0040] As described above, the coarse filter may have an initial state that is tuned to match a gold reference headphone to achieve approximately 20 dB or more of noise cancellation. For each individual headphone, the coarse filter may be calibrated to match as best as possible to compensate for component and manufacturing tolerances.

[0041] Depending on the fit of the earphones, the coarse filter will adapt to achieve approximately 20dB of ANC. This adaptation can be relatively simple, employing, for example, adaptation of the noise control processor's coarse filter gain and / or low-pass filter cutoff frequency. Due to variations in fit, the primary coarse change can be leakage between the earpad and the user's head, which can cause most noise to enter the ear via this low-acoustic impedance path rather than through the earphone vents and housing. This significantly alters the earphone's driver response and ultimately changes the low-pass characteristics of the AFFM portion of the AE section relative to the FF target. In most earphone examples, varying the coarse filter gain and low-pass characteristics can provide significantly better amplitude and phase matching.

[0042] In various embodiments of the system, the noise control processor is configured to perform a coarse adaptation before and / or during the finite adaptation at a slower rate than the finite adaptation. Adapting the fine filter then requires only small changes. These small changes are typically not smooth. This means that the fine filter may adapt to a "fluctuating" amplitude and phase response. To match these fluctuations, a relatively high-order filter is likely used, as described above.

[0043] With conventional adaptation of ANC, fully adapting the coefficients would be complex, time consuming, and risk falling into false zeros for high order filters. Therefore, the adaptation process according to the improved concept is simplified by imposing constraints on adapting the fine filter, i.e. within a limited adaptation.

[0044] The fine filter or sub-filters of the fine filter do not require large gain or phase differences, so the adaptation can be constrained or limited to a specific range defined during the tuning or factory calibration phase or defined by the coarse filter parameters.

[0045] The error signal calculated from the first and second noise signals can represent a normalized measure of the residual ambient noise at the ear. For example, by calculating the ratio of the residual noise at the ear to the ambient noise measured by the feedforward microphone, a measure of noise cancellation performance can be achieved. However, other calculation methods are not excluded. This can be used to manipulate the adaptation algorithm.

[0046] The noise cancellation system according to one of the above embodiments can be used in an ear-worn playback device (such as headphones or handsets). Therefore, the ear-worn playback device includes the above noise cancellation system, a speaker, a feedforward microphone, and an error microphone located near the speaker.

[0047] In other embodiments, the noise cancellation system according to one of the above embodiments can be constituted by an audio player. For example, the audio player is provided with a corresponding microphone signal from a headset or the like, and provides a corresponding speaker signal for the headset.

[0048] According to another embodiment embodying the improved concept, a signal processing method for an ear-worn playback device is provided. The ear-worn playback device includes a speaker, a feedforward microphone configured to primarily sense ambient sound, and an error microphone configured to sense both ambient sound and sound output from the speaker. The feedforward microphone is coupled to the speaker via a filter chain comprising a coarse filter and a fine filter connected in series. The fine filter is formed from a subfilter bank, each of which has a predefined frequency range, and the predefined frequency range of each subfilter at least partially overlaps with the predefined frequency range of at least one other subfilter in the subfilter bank. The method includes calculating an error signal based on a first noise signal sensed by the feedforward microphone and a second noise signal sensed by the error microphone. The method also includes coarsely adapting coarse filter parameters of the coarse filter based on the error signal, and performing limited adaptation of fine filter parameters of each subfilter based on the error signal. The limited adaptation is limited by the predefined frequency range of the subfilter and at least one of a gain limit and a Q factor limit.

[0049] From the various embodiments of the noise cancellation system described above, further embodiments of the method will be apparent to those skilled in the art.

[0050] The method may be implemented in hardware or software, for example using a signal processor (such as the noise control processor described above).

[0051] In all of the above-described embodiments, ANC can be performed using both digital and / or analog filters. All audio systems may also include feedback ANC. In these embodiments, for example, the system also includes a feedback noise filter that couples the error microphone to the loudspeaker. The processing and recording of the various signals is preferably performed in the digital domain.

[0052] The improved concept is described in more detail below with the aid of the accompanying drawings. In all the accompanying drawings, elements with the same or similar functions use the same reference numerals. Therefore, there is no need to repeat their description in the following drawings.

[0053] In the attached figure:

[0054] Figure 1 shows a schematic diagram of a headset;

[0055] Figure 2 A block diagram of an example adaptive ANC system is shown;

[0056] Figure 3 An example representation of a "leaky" type earbud headphone is shown;

[0057] Figure 4An exemplary headset worn by a user is shown, the headset having multiple sound paths from ambient sound sources;

[0058] Figure 5 An example representation of an ANC-enabled handset is shown;

[0059] Figure 6 An exemplary embodiment of a fine filter according to an improved concept is shown;

[0060] Figure 7 shows an example frequency diagram of multiple frequency ranges with sub-filters according to the improved concept;

[0061] Figure 8 Several example zero / pole plots are shown; and

[0062] Figure 9 A block diagram of another example adaptive ANC system is shown.

[0063] Figure 1 A schematic diagram shows an ANC-enabled playback device in the form of headphones HP, which in this example are designed as supra-aural or circumaural headphones. Only a portion of the headphones HP corresponding to a single audio channel is shown. However, extension to stereo headphones will be readily apparent to those skilled in the art from this and subsequent disclosures. The headphones HP include a housing HS that houses a speaker SP, a feedback noise microphone or error microphone FB_MIC, and an ambient noise microphone or feedforward microphone FF_MIC. The error microphone FB_MIC is specifically oriented or arranged so that it records both the sound played back by the speaker SP and ambient noise. Preferably, the error microphone FB_MIC is positioned close to the speaker, for example, near the edge of the speaker SP or near the speaker's membrane, so that the speaker sound can be the primary source for recording. The ambient noise / feedforward microphone FF_MIC is specifically directed or arranged so that it primarily records ambient noise originating from outside the headphones HP. However, a negligible portion of the speaker sound may reach the microphone FF_MIC.

[0064] exist Figure 1 In an embodiment of the present invention, the noise control processor SCP is located within the headset HP for performing various signal processing operations, examples of which will be described in the following disclosure. The noise control processor SCP can also be located external to the headset HP (e.g., in an external device located in a mobile handset or phone) or in the wires of the headset HP.

[0065] Figure 2A block diagram of an example adaptive ANC system is shown. The system includes an error microphone FB_MIC and a feedforward microphone FF_MIC, both of which provide their output signals to a noise control processor SCP. The noise signal n1 recorded by the feedforward microphone FF_MIC is also provided to a feedforward filter chain FF_CH for generating an anti-noise signal output via a loudspeaker SP. The filter chain FF_CH comprises a coarse filter FF_C and a fine filter FF_F connected in series, both of which are adaptable by the noise control processor SCP.

[0066] At the error microphone FB_MIC, the sound output from the loudspeaker SP is combined with the ambient noise and recorded as a second noise signal n2 comprising the remainder of the ambient noise after ANC. The noise control processor SCP uses the first noise signal n1 and the second noise signal n2 to calculate an error signal, which is then used to adjust the filter response of the feedforward filter chain FF_CH, in particular by adjusting the coarse filter FF_C and the fine filter FF_F, respectively.

[0067] Figure 3 An example representation of a "leaky" type earphone is shown, i.e. an earphone characterized by some acoustic leakage between the surroundings and the ear canal EC. In particular, there is a sound path between the surroundings and the ear canal EC, which is indicated as "acoustic leakage" in the figure.

[0068] Figure 4 An example configuration of headphones HP with multiple sound paths worn by a user is shown. Figure 4 The earphones HP shown in the figure represent an example of any ear-worn playback device of an audio system with noise cancellation enabled, and can, for example, include in-ear headphones or earbuds, on-ear headphones or on-ear headphones. In addition to earphones, the ear-worn playback device can also be a mobile phone or similar device.

[0069] The headset HP in this example has a loudspeaker SP, a feedback noise microphone FB_MIC and a feedforward microphone FF_MIC, which is designed as a feedforward noise cancellation microphone, for example. For a better overview, the internal processing details of the headset HP are not shown here.

[0070] For example, the earphone HP has a front space that is directly acoustically coupled to the user's ear canal space, a driver or speaker SP facing the front space, and a rear space surrounding the back of the driver SP. The rear space can have a vent with an acoustic resistor to allow some pressure to be relieved from the back of the driver SP. The front space can also have a vent with an acoustic resistor to allow some pressure to be relieved in front of the driver SP. The ear pad can surround the front of the driver SP and constitute part of the front space.

[0071] In normal operation, the earphone is placed on the user's head such that a full or partial seal is formed between the ear cushion and the user's head, thereby at least partially acoustically coupling the front space to the ear canal space.

[0072] exist Figure 4 In the configuration shown, there are multiple sound paths, each of which can be represented by its own acoustic response function or acoustic transfer function. For example, the first acoustic transfer function DFBM represents the sound path between the speaker SP and the feedback noise microphone FB_MIC, and can be referred to as a driver-to-feedback response function. The first acoustic transfer function DFBM may include the response of the speaker SP itself. The second acoustic transfer function DE represents the acoustic sound path between the speaker SP of the headset (which may include the response of the speaker SP itself) and the eardrum ED of the user exposed to the speaker SP, and can be referred to as a driver-to-ear response function. The third acoustic transfer function AE represents the acoustic sound path between the ambient sound source and the eardrum ED through the user's ear canal EC, and can be referred to as an environment-to-ear response function. The fourth acoustic transfer function AFBM represents the acoustic sound path between the ambient sound source and the feedback noise microphone FB_MIC, and can be referred to as an environment-to-feedback response function.

[0073] The fifth acoustic transfer function AFFM represents the acoustic sound path between the ambient sound source and the feedforward microphone FF_MIC and may be referred to as an ambient-to-feedforward response function.

[0074] In particular, a response function or transfer function of the earphone HP between the microphones FB_MIC and FF_MIC and the loudspeaker SP can be used together with a feedback filter function B and a feedforward filter function F which can be parameterized as noise cancellation filters during operation.

[0075] The headset HP as an example of an ear-worn playback device can be embodied with both microphones FB_MIC and FF_MIC activated or enabled to be able to perform hybrid ANC, or as a FF ANC device, where for the purpose of FB ANC only the feedforward microphone FF_MIC is activated while the error or feedback noise microphone FB_MIC is not activated.

[0076] For a better overview, Figure 4Any processing of the microphone signal or any signal transmission is omitted. However, the processing of the microphone signal to perform ANC can be implemented in a processor located within the headphones or other ear-worn playback device, or in a dedicated processing unit external to the headphones. The processor or processing unit may be referred to as a noise control processor. If the processing unit is integrated into the playback device, the playback device itself may form an audio system with noise cancellation enabled. If the processing is performed externally, the external device or processor together with the playback device may form an audio system with noise cancellation enabled. For example, the processing may be performed in a mobile device such as a mobile phone or mobile audio player, to which the headphones are connected either wired or wirelessly.

[0077] Now refer to Figure 5 , presents another example of an audio system with noise cancellation enabled. In this example embodiment, the system is formed by a mobile device (such as a mobile phone MP) comprising a playback device having a loudspeaker SP, an error microphone FB_MIC, an ambient noise or feedforward microphone FF_MIC, and a noise control processor SCP for performing, in particular, ANC and / or other signal processing during operation.

[0078] In another embodiment not shown, the headset HP (e.g. Figure 1 or Figure 4 The headset (HP) shown can be connected to a mobile phone MP, wherein the signals from the microphones FB_MIC, FF_MIC are transmitted from the headset to the mobile phone MP, in particular to the processor PROC of the mobile phone, for generating an audio signal to be played on the loudspeaker of the headset. For example, depending on whether the headset is connected to a mobile phone, ANC is performed using the internal components of the mobile phone (i.e., the loudspeaker and microphone) or using the loudspeaker and microphone of the headset, whereby a different set of filter parameters is used in each case.

[0079] Hereinafter, several embodiments of the improved concept will be described in conjunction with specific usage examples. However, it is obvious to those skilled in the art that the details described for one embodiment can still be applied to one or more of the other embodiments.

[0080] Back to Figure 2 , the signal from the FF microphone FF_MIC passes through a filter chain FF_CH formed by a coarse adaptive filter FF_C and a constrained high-resolution adaptive fine filter FF_F.

[0081] The coarse filter FF_C can be composed of multiple biquad or second-order IIR filters that are seeded by matching the following acoustic transfer function

[0082]

[0083] For example, the coarse filter FF_C can be formed by 4 to 10 (e.g., 6 to 8) such second-order IIR filters. The coarse adaptive filter FF_C is matched to the acoustic transfer function so that after adaptation, its amplitude error is, for example, less than 1 dB and its phase error is less than 8 degrees in the specified FF ANC bandwidth.

[0084] Coarse filter can be adapted routinely by the coefficient of adaptive filter, or this coarse filter can be adapted by several parameters such as gain and low-pass cut-off frequency of adaptation.These parameters can be converted into coefficient and write filter then.Can apply EP 17189001.5 by implementing ams that coarse filter is adapted, produce coarse filter response by the interpolation of two or more parallel filters thus.Especially, noise control processor SCP can be configured to carry out interpolation between high leakage and low leakage filter according to the leakage condition of describing in detail in the mentioned ams application.

[0085] Now refer to Figure 6 , shows a possible implementation of fine filter FF_F. Fine filter FF_F is formed by, for example, a sub-filter bank connected in series. Each of sub-filters BQ_1, BQ_2, ..., BQ_N has a predefined frequency range, wherein the predefined frequency range of each of sub-filters BQ_1, BQ_2, ..., BQ_N overlaps at least partially with the predefined frequency range of at least one other sub-filter in the sub-filter bank. For example, fine filter FF_F is formed by a peak level and / or a notch level, each of which is represented by a single biquad or second-order IIR filter, which is, for example, set to the last known good state. The sub-filter bank can include six to twelve (for example, eight to ten) sub-filters. The effective total frequency range of fine filter FF_F can be 80Hz to 2000Hz, for example, 80Hz to 1000Hz.

[0086] Now refer to Figure 7 , shows the total frequency range of an example embodiment of a fine filter FF_F having eight sub-filters, which is formed by a single predefined frequency range for each of the sub-filters marked with a black frame. It can be seen that in this example, each sub-filter has a 50% overlap with the adjacent sub-filter with respect to the frequency range. However, smaller or larger overlaps are still possible.

[0087] Back to Figure 2 , the noise control processor SCP not only performs an adaptation of the coarse filter parameters of the coarse filter FF_C based on the error signal, but also performs an adaptation of the (eg subsequent) fine filter FF_F.

[0088] In particular, the noise control processor performs limited adaptation of the fine filter parameters of each of the sub-filters BQ_1, BQ_2, ..., BQ_N based on the error signal. The limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q factor limitation. For example, the sub-filter is implemented using a peak level and / or a notch level that is limited to, for example, a maximum gain of + / - 1 dB. This results in a maximum gain factor of approximately 1.26 and a minimum gain factor of 0.79. For example, the Q factor can be limited to 0.1 to 2. For example, the center frequency of each sub-filter can be limited to a predefined frequency range. Thus, the adaptation of the fine filter FF_F can be performed in a conventional manner, for example using a filtered-u LMS algorithm to adapt the IIR coefficients and with checking and limiting the resulting response of each sub-filter, or the LMS loop can adapt the poles and zeros again with checking and limiting the poles and zeros or the resulting response, or the LMS loop can adapt the fine filter parameters, i.e. the gain, Q factor and frequency of each sub-filter within a set range of a predefined topology.

[0089] Setting limits on gain, Q factor, and frequency range, along with fine topology and sub-filter shapes (i.e., peaking / notching), eliminates a significant amount of redundancy in the adaptation process, reducing the risk of false nulls and / or slow adaptation. In contrast, conventional adaptive filters adapt coefficients without this constrained topology, allowing each coefficient to represent a pole or zero across the entire complex space, thus reducing protection against instability issues.

[0090] In another embodiment, the arrangement of the sub-filters is the same, but the noise control processor SCP adapts the coefficients of each of the sub-filters individually, while imposing equivalent constraints on their gain, Q factor, center frequency and shape. This will be described in more detail below.

[0091] For example, given the desired gain factor dBgain (in dB), the center frequency f0, and the Q factor Q of the corresponding sub-filter, the filter coefficients of the associated second-order IIR filter can be calculated, where F S is the sampling frequency, A and alpha are intermediate parameters, and ω0 is the normalized center frequency.

[0092]

[0093]

[0094]

[0095] Based on the above equations, the filter function of each sub-filter can be expressed in the Laplace domain as

[0096]

[0097] Or alternatively, in the Z domain,

[0098]

[0099] The following parameters

[0100] b0=1+alpha·A

[0101] b1=-2·cos(ω0)

[0102] b2=1-alpha·A

[0103]

[0104] a1=-2·cos(ω0)

[0105]

[0106] Using this calculation method, the resulting filter shape will produce a peak if the gain is greater than 1, and a notch if the gain is less than 1. Therefore, adapting the gain will inherently select a peak filter or a notch filter. It will be apparent to those skilled in the art that the normalization method with only five filter coefficients per sub-filter can also be derived from the above explanation. Constraining the sub-filters to one shape ensures that each sub-filter is inherently stable. Alternatively, constraints imposed directly on the poles and zeros or even the coefficients can also ensure a specific filter shape or ensure that each sub-filter is stable.

[0107] Now refer to Figure 8 , imposes restrictions on adapting fine filters, in particular on their shape, gain range, Q factor range, and frequency range, essentially limiting the possible pole and zero locations to a very small range. Peaking / notch filter stages with minimum and maximum gain, Q factor, and frequency can only have poles and zeros within a very small range. Figure 8 The maximum range of pole and zero locations under these constraints is shown. Since there are three variables (gain, Q, and frequency), there are 2 3 An extreme case. Figure 8 As shown, all of this lies within a very small region of the complex plane.

[0108] Thus, it can be seen that restricting the adaptation process to adapt the coarse filter FF_C and the fine filter FF_F separately, and further restricting the fine filter FF_F as described, significantly reduces the allowed variation in poles and zeros, thereby significantly speeding up the adaptation process and ensuring stability. Traditional adaptation algorithms adapt the coefficients, thus requiring additional steps to ensure stability. Furthermore, they can apply the coefficients over a wider range. Both of these scenarios result in slower adaptation and, more importantly, the risk of the adaptation falling into false nulls.

[0109] Now refer to Figure 9 , shows a block diagram of another example adaptive ANC system based on Figure 2 In particular, in addition to the feedforward path with the filter chain FF_CH, a feedback noise filter FB_B is used to couple the error microphone FB_MIC to the loudspeaker SP to implement FB ANC. This hybrid ANC approach combined with the adaptive filter chain FF_CH can achieve an ANC performance of approximately 60dB.

[0110] Reference Signs List

[0111] HP Headphones

[0112] HS housing

[0113] SP Speaker

[0114] FB_MIC Error or feedback microphone

[0115] FF_MIC Feedforward Microphone

[0116] EC ear canal

[0117] ED eardrum

[0118] SCP Noise Control Processor

[0119] FF_CH filter chain

[0120] FF_C Coarse filter

[0121] FF_F fine filter

[0122] BQ_1, BQ_2, ..., BQ_N sub-filters

[0123] FB_B Feedback noise filter

[0124] DFBM drive to feedback response function

[0125] DE Driver to Ear Response Function

[0126] AE Environment to Ear Response Function

[0127] AFBM environment to feedback response function

[0128] AFFM Environment to Feedforward Response Function

[0129] MP Mobile Phone

Claims

1. A noise cancellation system for an ear-worn playback device (HP), the ear-worn playback device having a speaker (SP), a feed-forward microphone (FF_MIC), and an error microphone (FB_MIC), wherein the feed-forward microphone is configured to primarily sense ambient sound, and the error microphone is configured to sense ambient sound and sound output from the speaker (SP), the noise cancellation system comprising: a filter chain (FF_CH) for coupling the feedforward microphone (FF_MIC) to the loudspeaker (SP), the filter chain (FF_CH) comprising a coarse filter (FF_C) and a fine filter (FF_F) connected in series or in parallel; and -Noise Control Processor (SCP); in - the fine filter (FF_F) is formed by a sub-filter bank; - Each of the sub-filters has a predefined frequency range; - the predefined frequency range of each of the sub-filters together forms the effective total frequency range of the fine filter (FF_F); and - the noise control processor (SCP) is configured to - calculate an error signal based on a first noise signal sensed by the feedforward microphone (FF_MIC) and based on a second noise signal sensed by the error microphone (FB_MIC); - performing an adaptation of coarse filter parameters of said coarse filter (FF_C) based on said error signal; and - performing limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein the limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q-factor limitation, wherein the gain limitation limits the gain range of the corresponding sub-filter and the Q-factor limitation limits the Q-factor range of the corresponding sub-filter.

2. The noise cancellation system according to claim 1, wherein: The predefined frequency range of each of the sub-filters is adjacent to or at least partially overlaps with the predefined frequency range of at least one other sub-filter in the sub-filter bank.

3. The noise cancellation system according to claim 1, wherein: The sub-filter group includes 6 to 12 sub-filters, in particular 8 to 10 sub-filters.

4. The noise cancellation system according to claim 1, wherein: The effective total frequency range of the fine filter (FF_F) is 80 Hz to 2000 Hz, in particular 80 Hz to 1000 Hz.

5. The noise cancellation system according to claim 1, wherein: Each sub-filter is one of a peak filter and a notch filter.

6. The noise cancellation system according to claim 1, wherein: Each sub-filter is a minimum phase filter.

7. The noise cancellation system according to claim 1, wherein: The limited adaptation of the sub-filters is based on an error minimization algorithm, in particular a least mean square LMS algorithm.

8. The noise cancellation system according to claim 1, wherein: The limited adaptation of the sub-filters includes adapting the gain, center frequency and Q factor of at least one of the sub-filters.

9. The noise cancellation system according to claim 1, wherein: The limited adaptation of the sub-filters comprises directly adapting fine filter parameters of at least one of the sub-filters and checking limits of the limited adaptation of the adapted fine filter parameters.

10. The noise cancellation system according to claim 1, wherein: The noise control processor (SCP) is configured to perform coarse adaptation prior to or at a different adaptation rate than the finite adaptation.

11. The noise cancellation system according to claim 1, wherein: The noise control processor (SCP) is configured to perform a coarse adaptation by adapting a gain factor and / or a cutoff frequency of the coarse filter (FF_C).

12. The noise cancellation system of claim 1, further comprising a feedback noise filter (FB_B) coupling the error microphone (FB_MIC) to the loudspeaker (SP).

13. An ear-worn playback device, in particular a headset (HP) or a handset, comprising the noise cancellation system according to one of claims 1 to 12, a loudspeaker (SP), a feedforward microphone (FF_MIC) and an error microphone (FB_MIC) located near the loudspeaker (SP).

14. An audio player comprising the noise cancellation system according to any one of claims 1 to 12.

15. A signal processing method for an ear-worn playback device (HP), the ear-worn playback device having a speaker (SP), a feedforward microphone (FF_MIC), and an error microphone (FB_MIC), wherein the feedforward microphone is configured to primarily sense ambient sound, and the error microphone is configured to sense ambient sound and sound output from the speaker (SP), wherein: The feedforward microphone (FF_MIC) is coupled to the loudspeaker (SP) via a filter chain (FF_CH), the filter chain (FF_CH) comprising a coarse filter (FF_C) and a fine filter (FF_F) connected in series or in parallel, wherein the fine filter (FF_F) is formed by a bank of sub-filters, each of the sub-filters having a predefined frequency range, and the predefined frequency range of each of the sub-filters together forming an effective total frequency range of the fine filter (FF_F), the method comprising - calculating an error signal based on a first noise signal sensed by the feedforward microphone (FF_MIC) and based on a second noise signal sensed by the error microphone (FB_MIC); - performing an adaptation of coarse filter parameters of said coarse filter (FF_C) based on said error signal; and - performing limited adaptation of fine filter parameters of each of the sub-filters based on the error signal, wherein the limitations of the limited adaptation include a predefined frequency range of the sub-filter and at least one of a gain limitation and a Q-factor limitation, wherein the gain limitation limits the gain range of the corresponding sub-filter and the Q-factor limitation limits the Q-factor range of the corresponding sub-filter.

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