FEED FORWARD ACTIVE NOISE CANCELLATION SYSTEM

Abstract

A feedforward active noise cancellation system comprising one or more reference microphones, one or more loudspeakers, one or more error microphones, a first filter, and a second and adaptive filter. The first filter is configured to filter a first input signal to minimize a residual error. The second and adaptive filter comprises a linear predictor of speech. The second and adaptive filter is configured to filter a second input signal based on predicted speech to compensate for a processing delay and a second propagation delay exceeding a first propagation delay.

Claims

1. A feedforward active noise cancellation system comprising: one or more reference microphones for obtaining an input audio signal indicative of a first ambient sound, one or more loudspeakers for outputting a first anti-noise sound, one or more error microphones for obtaining an error signal indicative of a second anti-noise sound and a second ambient sound, wherein the second ambient sound corresponds to the first ambient sound having travelled a first sound path having a first propagation delay from the one or more reference microphones to the one or more error microphones, and wherein the second anti-noise sound corresponds to the first anti-noise sound having travelled along a second sound path having a second propagation delay from the one or more loudspeakers to the one or more error microphones, a first filter configured to receive a first input signal, filter the first input signal, and output a first filtered audio signal, wherein the first filter is configured to filter the first input signal to minimize a residual error determined based on the input audio signal and the error signal, a second and adaptive filter configured to receive a second input signal, filter the second input signal, and output a second filtered audio signal, wherein the second and adaptive filter comprises a linear predictor of speech, wherein the second and adaptive filter is configured to filter the second input signal based on predicted speech to compensate for a processing delay and the second propagation delay exceeding the first propagation delay.

2. A feedforward active noise cancellation system according to claim 1, wherein the first filter is configured to filter the first input signal based on a primary transfer function and a secondary transfer function, wherein the primary transfer function is determined based on estimated characteristics of the first sound path, and wherein the secondary transfer function is determined based on estimated characteristics of the second sound path.

3. A feedforward active noise cancellation system according to claim 2, wherein the first filter is configured to filter the first input signal based on a minimum phase part of the primary transfer function, and a minimum phase part of the secondary transfer function.

4. A feedforward active noise cancellation system according to claim 1, wherein the linear predictor of speech comprises a high-order sparse linear predictor.

5. A feedforward active noise cancellation system according to claim 1, wherein the second and adaptive filter is adapted based on an improved proportionate normalised least mean squares algorithm.

6. A feedforward active noise cancellation system according to claim 1, wherein the first filter is a fixed filter.

7. An audio device comprising a feedforward active noise cancellation system according to claim 1.

8. An audio device according to claim 7, wherein the audio device comprises a headset or a set of earbuds.

9. A method for feedforward active noise cancellation comprising: obtaining an input audio signal indicative of ambient sounds, filtering a first input signal to provide a first filtered audio signal, wherein the first input signal is based on the input audio signal, wherein the filtering is based on a primary transfer and a secondary transfer function, wherein the primary transfer function is determined based on estimated characteristics of a first sound path a first propagation delay from one or more reference microphones to one or more error microphones, and wherein the secondary transfer function is determined based on estimated characteristics of a second sound path having a second propagation delay from one or more loudspeakers to the one or more error microphones, filtering a second input signal to provide a second filtered audio signal, wherein the second input signal is based on the first filtered audio signal, wherein filtering the second input signal comprises to predict speech to compensate for a processing delay and the second propagation delay exceeding the first propagation delay, wherein speech is predicted by a linear predictor of speech.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The above and other features and advantages of the present invention will become readily apparent to those skilled in the art by the following detailed description of example embodiments thereof with reference to the attached drawings, in which:

[0053] FIG. 1 is a schematic drawing of a feedforward active noise cancellation system according to the present disclosure.

[0054] FIG. 2 is a schematic drawing of a signal flow diagram of a feedforward active noise cancellation system according to the present disclosure.

DETAILED DESCRIPTION

[0055] Various example embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described.

[0056] Referring initially to FIG. 1 which depicts a schematic drawing of a feedforward active noise cancellation system according to the present disclosure. The system comprises one or more reference microphones 10. The one or more reference microphones 10 are for obtaining an input audio signal 3 indicative of a first ambient sound 1. The first ambient sound 1 may speech or other noise desired to be cancelled by the system. The input audio signal is sent to a processing unit 20.

[0057] The system comprises the processing unit 20. The processing unit 20 may perform any number of processing steps on the input audio signal 3. The input audio signal 3 or a processed version of the input audio signal is sent to a first filter 21 in the processing unit 20. The signal 3 sent to the first filter 21 will now be referred to as the first input signal 3.

[0058] The system comprises the first filter 21. The first filter 21 is configured to receive the first input signal 3, filter the first input signal 3, and output a first filtered audio signal 4. Filtering of the first input signal 3 may comprise a any number steps. Filtering of the first input signal 3 may comprise changing or otherwise modulating the first input signal 3. The first filter 21 is configured to filter the first input signal 3 based on a primary transfer function and a secondary transfer function. The primary transfer function is determined based on estimated characteristics of a first sound path. The secondary transfer function is determined based on estimated characteristics of a second sound path. The first sound path corresponds to a sound path where sound has travelled from the one or more reference microphones 10 to one or more error microphones 40. The second sound path corresponds to a sound path where sound has travelled from one or more loudspeakers 30 to the one or more error microphones 40. The first sound path has a first propagation delay associated with it. The first propagation delay being indicative of the time taken for sound to travel from the one or more reference microphones 10 to the one or more error microphones 40. The second sound path has a second propagation delay associated with it. The second propagation delay being indicative of the time taken for sound to travel from the one or more loudspeakers 30 to the one or more error microphones 40. The first filter 21 may be configured to filter the first input signal 3 based on a minimum phase part of primary transfer function, and a minimum phase part of the secondary transfer function. The first filter 21 may be a fixed filter. The first filtered audio signal 4 may undergo further processing before being sent to a second and adaptive filter 22 or be sent to the second and adaptive filter 22. The signal 4 sent to the second and adaptive filter 22 will now be referred to as a second input signal 4.

[0059] The system comprises the second and adaptive filter 22. The second and adaptive filter 22 is configured to receive the second input signal 4, filter the second input signal 4, and output a second filtered audio signal 5. The second and adaptive 22 filter comprises a linear predictor of speech. The second and adaptive filter 22 is configured to filter the second input signal 4 based on predicted speech to compensate for a processing delay and the second propagation delay exceeding the first propagation delay. The linear predictor of speech may comprise a high-order sparse linear predictor. The second and adaptive filter 22 may be adapted based on an improved proportionate normalised least mean squares algorithm. The second filtered audio signal 5 may undergo further processing before being sent to the one or more loudspeakers 30 or be sent to the one or more loudspeakers. The signal 5 sent to the one or more loudspeakers will now be referred to as a first anti-noise signal 5.

[0060] The system comprises one or more loudspeakers 30 for outputting a first anti-noise sound 6. The first anti-noise sound 6 is based on the first anti-noise signal 5. The first anti-noise signal 5 is based on the second filtered audio signal 5.

[0061] The system comprises one or more error microphones 40 for obtaining an error signal 7 indicative of a second anti-noise sound and a second ambient sound 2. The second ambient sound corresponds to the first ambient sound having travelled the first sound path from the one or more reference microphones 10 to the one or more error microphones 40. The second anti-noise sound corresponds to the first anti-noise sound 6 having travelled along a second sound path from the one or more loudspeakers 30 to the one or more error microphones 40. The error signal 7 may be used for determining a residual error. The second adaptive filter 21 may be adapted to minimize the residual error determined.

[0062] Referring to FIG. 2 depicting a schematic drawing of a signal flow diagram of a feedforward active noise cancellation system according to the present disclosure. The incoming noise x(n) propagates through two paths, one denoted as the first sound path running between the one or more reference microphones 10 and the one or more error microphones 40 and having a first propagation delay, the other being denoted as the second first sound path running between the one or more loudspeakers 30 and the one or more error microphones 40 and having a second propagation delay. However, before being outputted by the one or more loudspeakers processing takes place introducing a processing delay, which is then associated with the second sound path. Sound traveling via the first sound path may be described as being transformed according to the primary transfer function denoted P(z). Sound traveling via the second sound path may be described as being transformed according to the secondary transfer function denoted S(z). The optimal transfer function W.sub.0(z) for a feedforward active noise cancellation filter is

[00001]$W_0\left(z\right)=\frac{P\left(z\right)}{S\left(z\right)}$

[0063] However, since P(z) and S(z) include sound paths and S(z) also has the processing delay associated with it, they are non-minimum phase and can be expressed as

[00002]$P\left(z\right)=P_{\min }\left(z\right)z^{-D_1}\mathrm{and}S\left(z\right)=S_{\min }\left(z\right)z^{-\left(D_2+D_3\right)}$

where (.Math.) min denotes the minimum-phase part, D.sub.1 is the first propagation delay, D.sub.2 is the second propagation delay, and D.sub.3 is the processing delay. Hence, a causal W.sub.0(z) can only be realized if P(z) contain a delay of at least equal length as S(z). The additional delay D associated with S(z) may be expressed as

[00003]$D=D_3+D_2-D_1$

[0064] The additional delay in S(z) will require the optimal filter to be non-causal, which creates a need for prediction. In the presented example a high order sparse linear predictor, HOSpLP, is used for the prediction. The first filter 21, W (z), may then be determined as a causal fixed filter by taking the minimum-phase part of P(z) and S(z), i.e.,

[00004]$W\left(z\right)=\frac{P_{\min }\left(z\right)}{S_{\min }\left(z\right)}$

[0065] The delay Dis then compensated for by the second filter 22, W.sub.HOSpLP, utilizing the HOSpLP which predicts x.sub.w(n) D samples ahead in time, resulting in d(n) and x.sub.w(n) being aligned in time at the cancellation point.

[0066] The second filter 22 in the presented example is updated based on an IPNLMS algorithm, based on the signal obtained by the one or more reference microphones 10, and the one or more error microphones 40.

[0067] Examples of audio devices and related methods according to the disclosure is set out in the following items:

[0068] The use of the terms first, second, third and fourth, primary, secondary, tertiary etc. does not imply any particular order, but are included to identify individual elements. Moreover, the use of the terms first, second, third and fourth, primary, secondary, tertiary etc. does not denote any order or importance, but rather the terms first, second, third and fourth, primary, secondary, tertiary etc. are used to distinguish one element from another. Note that the words first, second, third and fourth, primary, secondary, tertiary etc. are used here and elsewhere for labelling purposes only and are not intended to denote any specific spatial or temporal ordering.

[0069] Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa.

[0070] It is to be noted that the word comprising does not necessarily exclude the presence of other elements or steps than those listed.

[0071] It is to be noted that the words a or an preceding an element do not exclude the presence of a plurality of such elements.

[0072] It should further be noted that any reference signs do not limit the scope of the claims, that the example embodiments may be implemented at least in part by means of both hardware and software, and that several means, units or devices may be represented by the same item of hardware.

[0073] The various example methods, devices, and systems described herein are described in the general context of method steps processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform specified tasks or implement specific abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

[0074] Although features have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed invention. The specification and drawings are, accordingly to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover all alternatives, modifications, and equivalents.

Items

[0075] 1. A feedforward active noise cancellation system comprising: [0076] one or more reference microphones for obtaining an input audio signal indicative of a first ambient sound, [0077] one or more loudspeakers for outputting a first anti-noise sound, [0078] one or more error microphones for obtaining an error signal indicative of a second anti-noise sound and a second ambient sound, wherein the second ambient sound corresponds to the first ambient sound having travelled a first sound path having a first propagation delay from the one or more reference microphones to the one or more error microphones, and wherein the second anti-noise sound corresponds to the first anti-noise sound having travelled along a second sound path having a second propagation delay from the one or more loudspeakers to the one or more error microphones, [0079] a first filter configured to receive a first input signal, filter the first input signal, and output a first filtered audio signal, wherein the first filter is configured to filter the first input signal to minimize a residual error determined based on the input audio signal and the error signal, [0080] a second and adaptive filter configured to receive a second input signal, filter the second input signal, and output a second filtered audio signal, wherein the second and adaptive filter comprises a linear predictor of speech, wherein the second and adaptive filter is configured to filter the second input signal based on predicted speech to compensate for a processing delay and the second propagation delay exceeding the first propagation delay.

[0081] 2. A feedforward active noise cancellation system according to item 1, wherein the first filter is configured to filter the first input signal based on a primary transfer function and a secondary transfer function, wherein the primary transfer function is determined based on estimated characteristics of the first sound path, and wherein the secondary transfer function is determined based on estimated characteristics of the second sound path.

[0082] 3. A feedforward active noise cancellation system according to item 2, wherein the first filter is configured to filter the first input signal based on a minimum phase part of the primary transfer function, and a minimum phase part of the secondary transfer function.

[0083] 4. A feedforward active noise cancellation system according to any of the preceding items, wherein the linear predictor of speech comprises a high-order sparse linear predictor.

[0084] 5. A feedforward active noise cancellation system according to any of the preceding items, wherein the second and adaptive filter is adapted based on an improved proportionate normalised least mean squares algorithm.

[0085] 6. A feedforward active noise cancellation system according to any of the preceding items, wherein the first filter is a fixed filter.

[0086] 7. An audio device comprising a feedforward active noise cancellation system according to any of the preceding items.

[0087] 8. An audio device according to item 7, wherein the audio device comprises a headset or a set of earbuds.

[0088] 9. A method for feedforward active noise cancellation comprising: [0089] obtaining an input audio signal indicative of ambient sounds, filtering a first input signal to provide a first filtered audio signal, wherein the first input signal is based on the input audio signal, wherein the filtering is based on a primary transfer and a secondary transfer function, wherein the primary transfer function is determined based on estimated characteristics of a first sound path a first propagation delay from one or more reference microphones to one or more error microphones, and wherein the secondary transfer function is determined based on estimated characteristics of a second sound path having a second propagation delay from one or more loudspeakers to the one or more error microphones, [0090] filtering a second input signal to provide a second filtered audio signal, wherein the second input signal is based on the first filtered audio signal, wherein filtering the second input signal comprises to predict speech to compensate for a processing delay and the second propagation delay exceeding the first propagation delay, wherein speech is predicted by a linear predictor of speech.