Patent Application
20230054927
This application claims the benefit of priority to Patent Application No. 202110944442.1, filed in China on Aug. 17, 2021; the entirety of which is incorporated herein by reference for all purposes.
The disclosure generally relates to a technology of noise cancellation and, more particularly, to an adaptive active noise cancellation apparatus and an audio playback system using the same.
General noise reduction techniques for headphones include passive noise cancellation (PNC) and active noise cancellation (ANC). The passive noise cancellation mainly isolate noise as much as possible through headphone sound-insulation materials or special structures, which generally are in-ear headphones or over-ear headphones. Wearing these two-types headphones for a long period of time cause ear pain, and excessive sound pressure may even cause users' hearing loss. The active noise cancellation means that a special noise cancellation circuit is set in headphones. Generally, an audio receiver (such as a miniature microphone) and an anti-noise output chip are used to receive and analyze frequency of external noise and generate an anti-noise sound in inverted phase. By the destructive interference, the external noise would be canceled.
Further, implementation of noise reduction of ANC is divided into factory preset ANC filters and adaptive ANC filters. The adaptive ANC filter basically generates different noise cancellation transfer functions according to environmental noise. With time of the ANC filter operation, the error between the environmental noise and the generated anti-noise sound is gradually compared and converged, and the environmental noise is canceled thereby. In fact, for different environmental noises, the conventional ANC filter exhibits different capabilities of noise cancellation, such that the conventional ANC filter is relatively unreliable. How to reduce the impact of environmental noise on the capability of noise cancellation has become a crucial issue in this field.
In view of this, how to reduce or eliminate the deficiencies in the above-mentioned prior art field and how to suppress noise in a manner that conforms to environmental noise by adaptive active noise elimination filtering technology are the issues to be solved.
The present invention provides an audio playback system for outputting an anti-phase noise audio signal according to an anti-phase noise signal, wherein the audio playback system includes an error microphone, and an adaptive active noise cancellation apparatus. The error microphone receives an environmental noise and the anti-phase noise audio signal, to generate an error signal. The adaptive active noise cancellation apparatus includes an automatic noise shaping circuit, an adaptive active noise filtering unit, a first transmission channel simulation unit and a coefficient adjustment unit. The automatic noise shaping circuit receives the error signal, shaping an interference signal to a shaped interference signal and the error signal to a shaped error signal according to a preset noise shape and outputting the shaped interference signal and the shaped error signal. The adaptive active noise filtering unit receives the interference signal, outputting the anti-phase noise signal for generating the anti-phase noise audio signal. The first transmission channel simulation unit receives the shaped interference signal, for generating a simulated shaped interference signal according to a channel transfer function. The coefficient adjustment unit receives the simulated shaped interference signal and the shaped error signal, adjusting a filter parameter of the adaptive active noise filtering unit by an adaptive algorithm according to the simulated shaped interference signal and the shaped error signal.
In accordance with a preferred embodiment of the present invention, when the audio playback system is a feedback active noise cancellation headphone, the interference signal is the restored environmental noise signal. In the other preferred embodiment of the present invention, when the audio playback system is a feedforward active noise cancellation headphone, the audio playback system further includes an external noise receiving microphone for receiving an external audio noise to convert the external audio noise to the interference signal.
The spirit of the present invention is to shape the received error signal and the received interference signal according to an ideal noise shape. Afterward, the shaped interference signal and the shaped error signal is transmitted to the coefficient adjustment unit to perform adaptive parameter algorithm, such that the adaptive active noise filtering unit is not only can affectively suppress the external noise and the noise in the ear canal to minimize the error signal, but also can suppress specific frequencies to which the human ear is sensitive.
The other advantages of the present invention will be explained in more detail in conjunction with the following description and drawings.
Both the foregoing general description and the following detailed description are examples and explanatory only, and are not restrictive of the invention as claimed.
Reference is made in detail to embodiments of the invention, which are illustrated in the accompanying drawings. The same reference numbers may be used throughout the drawings to refer to the same or like parts, components, or operations.
The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto and is only limited by the claims. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having the same name (but for use of the ordinal term) to distinguish the claim elements.
It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent.” etc.)
A wireless communication protocol, such as A2DP (advanced audio distribution profile) Bluetooth package, can be used to transmit the user's speech signal or music package between the mobile device 303 and the left wireless earbud 301 and between the mobile device 303 and the right wireless earbud 302.
In other embodiments, Wi-Fi Direct or other P2P (Peer-to-peer) protocols can also be adopted between the mobile device 303 and the left wireless earbud 301 and between the mobile device 303 and the right wireless earbud 302. The present invention is not limited thereto. In the abovementioned embodiment, although wireless earbuds are taken as an example of the ANC audio playback system, people having ordinary skill in the art should know that the ANC audio playback system may also be wired headphones (earbuds or headset) as preferred embodiments, and the present invention is not limited thereto.
It is noted that the audio playback system involves both the acoustic domain and the electrical domain. For example, the symbols d(n) and y(n) indicated in
The audio channel response schematic block 42 can be seen as a primary path, for representing a transmission path from a reference microphone (i.e., the external noise receiving microphone 411 in the embodiment in
The transmission channel 40 can be referred to as a secondary path, for representing a transmission path from the adaptive active noise filtering unit 414 to the error microphone 412, thereby the conversion of the electrical signal, which is output by the adaptive active noise filtering unit 414 and passes through the transmission path, is analyzed, wherein the channel transfer function S(z) represents the simulation result of the conversion. In some possible implementations, the external noise source is removed, and the transfer function S(z) is evaluated based on the signal y′(n) output by the adaptive active noise filtering unit 414 and the signal e(n) obtained through the error microphone 412. Due to the absence of external noise sources, the signal d(n) doesn't exist and the signal e(n) is substantially the same as the signal y(n).
The external noise receiving microphone 411 receives an external audio noise (e.g., environmental noise), and converts the external audio noise into a digital interference signal x(n). The adaptive active noise filtering unit 414 receives the interference signal x(n), and outputs an anti-phase noise signal y′(n) based on the interference signal x(n).
The error microphone 412 receives the environmental noise d(n) and an anti-phase noise audio signal y(n) in the ear canal, and converts them to a digital error signal e(n) accordingly, wherein the external audio noise is converted into the environmental noise d(n) through the audio channel response schematic block 42. Since both environmental noise d(n) and anti-phase noise audio signal y(n) are analog acoustic signals, in the acoustic domain, the above environmental noise d(n) and the anti-phase noise audio signal y(n) would interfere with each other in the ear canal. For the convenience of description, an adder symbol 43 is especially illustrated in the drawings. People having ordinary skill in the art should know that the adder symbol 43 is not a physical element, and is only used to represent the interference phenomenon of two analog acoustic signals.
The adaptive active noise filtering unit 414 is used to generate an anti-phase noise signal y′(n) based on the interference signal x(n) and the error signal e(n), and the anti-phase noise signal y′(n) is converted into the anti-phase noise audio signal y(n) through the transmission channel 40. In another embodiment, the adaptive active noise filtering unit 414 includes a finite impulse response (FIR) filter. In detail, in the embodiment of the present invention, the adaptive active noise filtering unit 414 is an adaptive filter that adjusts coefficients through adaptive algorithm of iterative method. In an ideal situation, after the audio signals interfere with each other, the anti-phase noise audio signal y(n) can almost entirely eliminate the environmental noise d(n) such that the error signal e(n) would approach to zero. However, in real noise eliminating process, due to different filter designs and different filter coefficient algorithms, it is difficult to eliminate noise in a specific frequency band, especially the low-frequency noise, to which the human ears are more sensitive. Further, in the real environment, due to the changing of external audio noise, the actual noise cancellation result would be also changed accordingly, such that the noise heard by user would be sometimes louder and sometimes lower.
Accordingly, ideally, a possible way is to shape the external audio noise to reduce the degree of variation in environmental noise, so that a noise reduction filter can generate an effective anti-phase noise signal based on shaped external audio noise, which has the lower variation than the unshaped external audio noise, provided by the microphone. However, in practice, it is difficult to shape the external audio noise propagating in the air. An alternative method is to shape the interference signal x(n) generated by the external noise receiving microphone 411 and the error signal e(n) generated by the error microphone 412.
In addition, since the external audio noise may change at any time, it is necessary to dynamically adjust shaping means. Since the environmental noise d(n) is derived from the external audio noise, the degree of variation in the external audio noise can be identified by analyzing the environmental noise d(n).
The automatic noise shaping circuit 413 in the embodiment of the present invention is designed based on the above reasons, such that it can effectively suppress the environmental noise d(n), and can suppress the specific frequencies (generally low frequencies) to which the human ears are more sensitive. The detailed description is as follows.
In this embodiment, the automatic noise shaping circuit 413 includes a second transmission channel simulation unit 417, a first adder circuit 418, a shaping filter parameter generation unit 419, a first shaping filter 420, a second adder circuit 421 and a second shaping filter 422. In another preferred embodiment, the automatic noise shaping circuit 413 can be implemented by a digital signal processor (DSP).
As mentioned above, the automatic noise shaping circuit 413 is used to shape the interference signal x(n) generated by the external noise receiving microphone 411 and the error signal e(n) generated by the error microphone 412, and provide the shaped signals to the coefficient adjustment unit 416. As such, input signals received by the coefficient adjustment unit 416 can maintain the characteristic of the current environmental noise, and the frequency distribution of the input signal is modified as well. Therefore, the anti-phase noise signal y′(n) generated by the adaptive active noise filtering unit 414 can effectively suppress the environmental noise d(n), and can also suppress specific frequencies (generally low frequencies) to which the human ears are more sensitive.
Therefore, in this embodiment, the first shaping filter 420 is used to shape the interference signal x(n) to generate a shaped interference signal x′(n), and the second shaping filter 422 is used to shape a restored error signal e(n), which can be regarded as the error signal e(n), to generate a shaped error signal ê′(n). The first shaping filter 420 and the second shaping filter 422 are, for example, digital filters (or equalizers), and shaping filter parameters for each of the two shaping filters 420 and 422 are generated by a shaping filter parameter generation unit 419, wherein the first shaping filter 420 receives a first shaping filter parameter and the second shaping filter 422 receives a second shaping filter parameter.
In order to generate an effective shaping filter parameter, the shaping filter parameter generation unit 419 is used to analyze the environmental noise d(n), thereby identifying the variation degree of the external audio noise. It can be observed from the circuit block diagram of
Therefore, the automatic noise shaping circuit 413 includes the second transmission channel simulation unit 417. The second transmission channel simulation unit 417 is used to simulate the channel transfer function S(z) of the transmission channel 40 in the electrical domain, and accordingly convert the anti-phase noise signal y′(n) to a simulated anti-phase noise signal ŷ(n) that is substantially equal to the anti-phase noise audio signal y(n). After subtracting the simulated anti-phase noise signal ŷ(n) from the error signal e(n), the environmental noise d(n) is restored. The simulated anti-phase noise signal ŷ(n) is similar to an electrical signal corresponding to the anti-phase noise audio signal y(n), the difference is that the anti-phase noise audio signal y(n) belongs to the acoustic domain and the simulated anti-phase noise signal ŷ(n) belongs to the electrical domain. Therefore, in the embodiment, the transfer function of the second transmission channel simulation unit 417 is labeled as Ŝ(z) to distinguish between the acoustic channel transfer function S(z) and the electrical channel transfer function Ŝ(z).
To this end, the first adder circuit 418 receives the simulated anti-phase noise signal ŷ(n) and the error signal e(n) to deduct the simulated anti-phase noise signal ŷ(n) from the error signal e(n) to generate a restored environmental noise signal {circumflex over (d)}(n). The restored environmental noise signal {circumflex over (d)}(n) can be regarded as the same signal as the environmental noise d(n). Likewise, the environmental noise d(n) belongs to the acoustic domain, while restored environmental noise signal d(n) belongs to the electrical domain. The shaping filter parameter generation unit 419 receives the restored environmental noise signal {circumflex over (d)}(n), and generates the first shaping filter parameter for the first shaping filter 420 and the second shaping filter parameter for the shaping filter 422 according to a stored preset noise shape and the restored environmental noise signal {circumflex over (d)}(n).
In addition, the second adder circuit 421 receives the simulated anti-phase noise signal ŷ(n) and the restored environmental noise signal {circumflex over (d)}(n), and generates the restored error signal ê(n). Similarly, the restored environmental noise signal {circumflex over (d)}(n) is obtained by subtracting the simulated anti-phase noise signal ŷ(n) from the error signal e(n). Therefore, in this embodiment, the restored environmental noise signal {circumflex over (d)}(n) is added to the simulated anti-phase noise signal ŷ(n) such that the original error signal e(n) can be approximately restored. In order to distinguish different signals, the restored error signal is represented by ê(n). The second shaping filter 422 receives the restored error signal ê(n), and shapes the restored error signal ê(n) to obtain the shaped error signal ê′(n), which is input to the coefficient adjustment unit 416.
On the other hand, the interference signal x(n) outputted by the external noise receiving microphone 411 will also be filtered by the first shaping filter 420. In this embodiment, the adaptive active noise cancellation apparatus 41 adopts a filtered-X least mean square (FxLMS) algorithm. In another embodiment, the adaptive active noise cancellation apparatus 41 may utilize another algorithm. According to the FxLMS algorithm, the shaped interference signal x′(n) output by the first shaping filter 420 is also required to process through the first transmission channel simulation unit 415. The first transmission channel simulation unit 415 is also used to simulate the channel transfer function S(z) of the transmission channel 40 in the electrical field, and converts the shaped interference signal x′(n) into the simulated shaped interference signal {circumflex over (x)}′(n). It should be noted that according to the mathematical principle of the linear system, a position of the first transmission channel simulation unit 415 and a position of the first shaping filter 420 are interchangeable in circuit structures.
Thereby, the coefficient adjustment unit 416 can obtain the filter coefficient W(z) of the adaptive active noise filtering unit 414 according to the shaped error signal ê′(n) and the simulated shaped interference signal {circumflex over (x)}′(n), by utilizing the least mean square (LSM) operation, and continuously modify the output filter coefficient W(z) according to the shaped error signal ê′(n) and the simulated shaped interference signal {circumflex over (x)}′(n) to minimize the above-mentioned error signal e(n).
In the embodiment of the present invention, since the adaptive active noise cancellation apparatus 41 utilize the FxLMS algorithm for example, based on the algorithm structure of the FxLMS, the adaptive active noise cancellation apparatus 41 should take an external noise to serve as the input. In this embodiment, the external noise receiving microphone 411 is absent, the restored environmental noise signal {circumflex over (d)}(n) output by the first adder circuit 418 can be served as the external noise. According to the description of the above-mentioned
Compared with the first shaping filter 420 of the embodiment in
The feedforward noise cancellation circuit 1001 at least includes a feedforward adaptive active noise filtering unit 1004 (the filter coefficient in the figure is represented as WFF), a third shaping filter 1006, a third transmission channel simulation unit 1010, and a second coefficient adjustment unit 1020. In the operation of the feedforward noise cancellation circuit 1001, the signal processing for the interference signal x(n) is the same as that depicting in
In addition, in
In this embodiment, since the feedforward noise cancellation circuit 1001 and the feedback noise cancellation circuit 1002 are both adaptive noise cancellation circuits, in this embodiment, the interference signal x(n) and the error signal e(n) still need to be shaped through shaping filters (such as the shaping filters 420, 422 and 1006). Then, in the feedforward noise cancellation circuit 1001 and the feedback noise cancellation circuit 1002, adaptive algorithm operations are performed to obtain the filter parameter WFF(Z) of the feedforward active noise filtering unit 1004 and the filter parameter WFB(Z) of the feedback adaptive active noise filtering unit 1005. In the embodiment of the present invention, filter coefficients are, for example, calculated by the iterative operation of the Least Mean Square Method (LMS). However, the present invention is not limited thereto.
However, in the hybrid active noise cancellation headphone, only the feedback noise cancellation circuit 1102 adopts the active noise cancellation, and the feedforward noise cancellation circuit 1101 adopts static noise cancellation. Since static noise cancellation is adopted, the coefficient adjustment unit 1020 and its related functional blocks, such as the third transmission channel simulation unit 1010, have been removed in this embodiment compared with the feedforward active noise cancellation circuit 1001 of
However, the hybrid active noise cancellation headphone only has the feedforward noise cancellation circuit 1201 adopting adaptive noise cancellation. The feedback noise cancellation circuit 1202 adopts static active noise cancellation. The operation of the feedforward noise elimination circuit 1202 can refer to the embodiment in
Although fast Fourier transform and division operation are taken an example in the above embodiment, people having ordinary skill in the art should know that to multiply two signals in the frequency domain is equivalent to convolution two discrete signals in time domain. Thus, the above-mentioned operations of the above embodiment can be implemented by different mathematical operations to obtain the shaping filter parameter W(z) in other embodiments. Therefore, the present invention is not limited thereto.
It is noted that, in the above-mentioned embodiments, the number of shaping filters is at least two, and in order to have the same shaping filtering effect on all noise or interference signals, the shaping filter parameter generation unit outputs the same filter parameters to each shaping filter for example. However, people having ordinary skill in the art should be able to infer that, in practical circuit design applications, in order to match the circuit design, the filter parameters output by the shaping filter parameter generation unit to each shaping filter may also be different. Moreover, in the actual circuit design application, the number of shaping filters of the adaptive active noise cancellation apparatus may also be only one, so the present invention does not limit the number of shaping filters and the design of filtering parameters of the shaping filters.
In summary, the spirit of the present invention is to shape the received error signal and the received interference signal according to an ideal noise shape. Afterward, the shaped interference signal and the shaped error signal is transmitted to the coefficient adjustment unit to perform adaptive algorithm, such that the adaptive active noise filtering unit is not only can affectively suppress the external noise and the noise in the ear canal to minimize the error signal, but also can suppress specific frequencies to which the human ear is sensitive.
Although the embodiment has been described as having specific elements in
While the invention has been described by way of example and in terms of the preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202110944442.1 | Aug 2021 | CN | national |
