Patent Description:
Acoustic devices such as headphones can include active noise reduction (ANR) capabilities that block at least portions of ambient noise from reaching the ear of a user. Therefore, ANR devices create an acoustic isolation effect, which isolates the user, at least in part, from the environment. To mitigate the effect of such isolation, some acoustic devices can include a hear-through mode, in which the noise reduction is turned down for a period of time and the ambient sounds are allowed to be passed to the user's ears. Examples of such acoustic devices can be found in <CIT> and <CIT>.

<CIT>, <CIT>, <CIT>, <CIT> and <CIT> disclose prior art ANR devices.

The present invention relates to a method and an active noise reduction device according to the independent claims. Advantageous embodiments are set forth in dependent claims of the appended set of claims.

<FIG> is a flowchart of an example process for generating an output signal in an ANR device that includes an ANR signal flow path and a pass-through signal flow path disposed in parallel.

This document describes technology that allows the use of Active Noise Reduction (ANR) in acoustic devices while concurrently allowing a user to control the amount of ambient noise that the user would like to hear. Active Noise Reduction (ANR) devices such as ANR headphones are used for providing potentially immersive listening experiences by reducing effects of ambient noise and sounds. However, by blocking out the effect of the ambient noise, an ANR device may create an acoustic isolation from the environment, which may not be desirable in some conditions. For example, a user waiting at an airport may want to be aware of flight announcements while using ANR headphones. In another example, while using an ANR headphone to cancel out the noise of an airplane in flight, a user may wish to be able to communicate with a flight attendant without having to take off the headphone.

Some headphones offer a feature commonly called "talk-through" or "monitor," in which external microphones are used to detect external sounds that the user might want to hear. For example, the external microphones, upon detecting sounds in the voice-band or some other frequency band of interest, can allow signals in the corresponding frequency bands to be piped through the headphones. Some other headphones allow multi-mode operations, wherein in a "hear-through" mode, the ANR functionality may be switched off or at least reduced, over at least a range of frequencies, to allow relatively wide-band ambient sounds to reach the user. However, in some cases, a user may want to maintain ANR functionalities, while still being able to be aware of the ambient sounds. In addition, the user may want to control the amount of noise and ambient sounds that pass through the ANR device.

The technology described herein allows for the implementation of an ANR signal flow path in parallel with a pass-through signal flow path, wherein the gain of the pass-through signal path is controllable by the user. This may allow for implementing ANR devices where the amount of ambient noise passed through can be adjusted based on user-input (e.g., either in discrete steps, or substantially continuously) without having to turn-off or reduce the ANR provided by the device. In some cases, this may improve the overall user experience, for example, by avoiding any audible artifacts associated with switching between ANR and pass-through modes, and/or putting the user in control of the amount of ambient noise that the user wishes to hear. This in turn can make ANR devices more usable in various different applications and environments, particularly in those where a substantially continuous balance between ANR and pass-through functionalities is desirable.

An active noise reduction (ANR) device can include a configurable digital signal processor (DSP), which can be used for implementing various signal flow topologies and filter configurations. Examples of such DSPs are described in <CIT> and <CIT>. <CIT> describes an acoustic implementation of an in-ear active noise reducing (ANR) headphone, as shown in <FIG>. This headphone <NUM> includes a feedforward microphone <NUM>, a feedback microphone <NUM>, an output transducer <NUM> (which may also be referred to as an electroacoustic transducer or acoustic transducer), and a noise reduction circuit (not shown) coupled to both microphones and the output transducer to provide anti-noise signals to the output transducer based on the signals detected at both microphones. An additional input (not shown in <FIG>) to the circuit provides additional audio signals, such as music or communication signals, for playback over the output transducer <NUM> independently of the noise reduction signals.

The term headphone, which is interchangeably used herein with the term headset, includes various types of personal acoustic devices such as in-ear, around-ear or over-the-ear headsets, earphones, and hearing aids. The headsets or headphones can include an earbud or ear cup for each ear. The earbuds or ear cups may be physically tethered to each other, for example, by a cord, an over-the-head bridge or headband, or a behind-the-head retaining structure. In some implementations, the earbuds or ear cups of a headphone may be connected to one another via a wireless link.

Various signal flow topologies can be implemented in an ANR device to enable functionalities such as audio equalization, feedback noise cancellation, feedforward noise cancellation, etc. For example, as shown in the example block diagram of an ANR device <NUM> in <FIG>, the signal flow topologies can include a feedforward signal flow path <NUM> that drives the output transducer <NUM> to generate an anti-noise signal (using, for example, a feedforward compensator <NUM>) to reduce the effects of a noise signal picked up by the feedforward microphone <NUM>. In another example, the signal flow topologies can include a feedback signal flow path <NUM> that drives the output transducer <NUM> to generate an anti-noise signal (using, for example, a feedback compensator <NUM>) to reduce the effects of a noise signal picked up by the feedback microphone <NUM>. The signal flow topologies can also include an audio path <NUM> that includes circuitry (e.g., equalizer <NUM>) for processing input audio signals <NUM> such as music or communication signals, for playback over the output transducer <NUM>.

Other configurations of signal flow topologies are also possible. <FIG> is a block diagram of another example configuration <NUM> of an ANR device. For the sake of brevity, the example configuration <NUM> does not show an audio path akin to the audio path <NUM> shown in <FIG>. The configuration <NUM> also shows the transfer function Gsd that represents the acoustic path between the acoustic transducer <NUM> and the feedback microphone <NUM> (which may also be referred to as the system microphone or sensor s). The transfer function Ged represents the acoustic path between the driver d (or the acoustic transducer <NUM>) and the microphone e disposed proximate to the ear of the user. The microphone e measures the noise at the ear of the user. The microphone may be inserted in the ear canal of a user during the system design process, but may not be a part of the ANR device itself. The noise n represents an input to the configuration <NUM>. The transfer function between the noise source <NUM> and the feedforward microphone <NUM> is represented by Gon, such that the noise, as captured by the feedforward microphone <NUM>, is represented as n x Gon. The transfer functions of the acoustic paths between (i) the noise source <NUM> and the feedback microphone <NUM>, and (ii) the noise source and the ear e are represented as Gsn and Gen, respectively.

The relationships between the various sensors or microphones, and the two sources of audio (the noise source <NUM> and the acoustic transducer <NUM>) can therefore be expressed using the following equations: <MAT> <MAT> <MAT> <MAT>.

Therefore, the ratio of noise measured at the feedback microphone <NUM> relative to the noise n is given by: <MAT>.

Similarly, the noise measured at the ear (e) relative to the disturbance noise n is given by: <MAT>.

As a reference, the open-ear response to the noise can be defined as: <MAT>.

The total performance of the ANR device (e.g., an ANR headphone) can be expressed in terms of a target Insertion Gain (IG), which is the ratio of: (i) the noise at the ear relative to the noise when the device is active and being worn by a user, and (ii) the reference open-ear response. This is given by: <MAT> where the passive insertion gain (PIG) is defined as the purely passive response of the ANR device when it is worn by the user. The PIG is given by: <MAT>.

In some implementations, where the noise is measured at a point with an omni-directional reference microphone, the expressions in equations (<NUM>) and (<NUM>) may be evaluated as energy ratios (e.g., without considering the phase) measured at the ear microphone before and after the user wearing the ANR device, with the ANR device in either active or passive mode, respectively.

In some implementations, the various noise disturbance terms may be expressed as normalized cross spectra between the available microphones as: <MAT>.

Using these expressions, equation (<NUM>) may be rewritten as: <MAT>.

Equation (<NUM>) relates the total insertion gain (which may be referred to as the target insertion gain) of an ANR device to the measured acoustics of the system, and the associated feedback compensator <NUM> and feedforward compensator <NUM>, Kfb and Kff, respectively. In some implementations, for a given fixed feedback compensator <NUM>, equation (<NUM>) may therefore be used to compute corresponding feedforward compensators <NUM> for specified values of target insertion gains and the other parameters. For example, the target insertion gain can be set to <NUM> to obtain a feedforward compensator <NUM> configured to provide full ANR (maximum noise cancellation) for the given device. Such a filter or feedforward compensator may be denoted as KANR. Conversely, the target insertion gain can be set to <NUM> to obtain a feedforward compensator <NUM> that passes the signals captured by the feedforward microphone <NUM> with unity gain. Such a filter or feedforward compensator is referred to herein as an "aware mode" or "pass-through" filter, and is denoted as KAware.

In some implementations, to allow for intermediate target insertion gains between <NUM> and <NUM>, and allow a user to control the amount of ambient noise passed through the device, the two filters KANR and KAware can be disposed in parallel in the feedforward signal flow path, as shown in <FIG>. The example configuration of <FIG> shows a feedforward compensator <NUM> where an ANR filter <NUM> and a pass-through filter <NUM> are disposed in parallel, with the gain of the pass-through filter being adjustable by a factor C. The adjustable gain C may be implemented using a variable gain amplifier (VGA) disposed in the pass-through signal flow path of the feedforward compensator <NUM>. The overall transfer function of the feedforward compensator <NUM> may be represented as: <MAT>.

The parallel structure of the ANR filter and the pass-through filter may be implemented in various ways. In some implementations, each of the ANR filter and the pass-through filter can be substantially fixed, and the adjustable factor can be based on user-input indicative of an amount of ambient noise and sounds that the user intends to hear. This may represent an efficient and low complexity implementation, particularly for applications where the contribution of one of the signal flow paths (the ANR signal flow path or the pass-through signal flow path) is expected to dominate the final output. This can happen, for example, when the value of C is expected to be close to either <NUM> or <NUM>. In such cases, the magnitude responses of the individual paths may not deviate significantly from corresponding design values. For example, the magnitude response of each of the ANR signal flow path and the pass-through signal flow path may be designed in accordance with a set of target spectral characteristics (e.g., spectral flatness), and when one of the paths dominate the output, the paths may not deviate significantly from the corresponding target flatness.

When the individual gains of the ANR path and the pass-through path approach one another, the phase responses of the individual paths may interfere constructively or destructively, thereby potentially making the corresponding magnitude responses deviate significantly from the design values. For example, the interference of the phase responses of the two paths may, in some cases, degrade the target flatness of the corresponding magnitude responses. This in turn may degrade the performance of the ANR device.

In the invention, the effect of interference between the phase responses of the two paths are mitigated by using a filter bank in at least the ANR signal flow path. In the invention, the ANR filter <NUM> includes a filter bank that includes a plurality of selectable digital filters, wherein each digital filter in the filter bank corresponds to a particular value of C. In some implementations, the pass-through filter <NUM> may include a similar filter bank. In such cases, a change in the value of C can prompt a change in the ANR filter <NUM> and the pass-through filter <NUM>.

The ANR filter is selected (or computed in real time based on the value of C), such that any interference between the resulting phase responses do not degrade the spectral characteristics (e.g., flatness) of the magnitude response beyond a target tolerance limit.

In some implementations, instead of obtaining a KANR and a KAware separately for two different values of insertion gain, and adding the two filters together, the insertion gain can be kept as a free parameter to obtain two separate filters that are independent of any particular insertion gain. For example, solving for Kff using equation (<NUM>) yields: <MAT> which may be represented as: <MAT>.

In equation (<NUM>), Knc equals the first term in the right hand side of equation (<NUM>), and represents a noise cancellation filter. Kaw equals the second term in the right hand side of equation (<NUM>) and represents a pass-through filter. <FIG> is a block diagram of an example configuration <NUM> of an ANR device that includes an ANR signal flow path disposed in parallel to a pass-through signal flow path in accordance with equation (<NUM>) within a feedforward compensator <NUM>. Specifically, the ANR signal flow path includes the ANR filter <NUM> and the pass-through signal flow path includes the pass-through filter <NUM>, wherein the filters <NUM> and <NUM> are obtained in accordance with equations (<NUM>) and (<NUM>). The transfer functions Neo and Nso are defined above in equation (<NUM>).

In some implementations, the feedforward compensator <NUM> shown in <FIG> may provide one or more advantages. For example, because the filters <NUM> and <NUM> can be implemented as fixed coefficient filters, the need for any filter bank may be obviated. This in turn may allow for the feedforward compensator <NUM> to be implemented using lower processing power and/or storage requirements. This may be particularly advantageous in smaller form-factor ANR devices that have limited processing power and/or storage space on-board. Further, because the phase responses of the two parallel paths are not dependent on the insertion gain, the magnitude responses may remain substantially invariant to the insertion gain IG. For example, the insertion gain may not significantly affect the flatness or other spectral characteristics of the magnitude responses associated with the two parallel paths when the insertion gains are varied over a range. In some implementations, the feedforward compensator can be configured to support arbitrary values of the insertion gain IG, including for example, values large than unity that can be used to amplify the ambient sounds. This can be useful, for example, in devices such as hearing aids, and/or to hear ambient sounds that may not be otherwise audible. For example, in order to better hear audio emanating from a distant source, a user may temporarily turn up the gain such that the IG value is more than unity.

<FIG> is a flowchart of an example process <NUM> for generating an output signal in an ANR device that includes an ANR signal flow path and a pass-through signal flow path disposed in parallel. At least a portion of the process <NUM> can be implemented using one or more processing devices such as DSPs described in <CIT> and <CIT>. Operations of the process <NUM> include receiving an input signal captured using one or more sensors associated with an ANR device (<NUM>). In some implementations, the one or more sensors include a feedforward microphone of an ANR device such as an ANR headphone. In some implementations, the ANR device can be an in-ear headphone such as one described with reference to <FIG>. In some implementations, the ANR device can include, for example, around-the-ear headphones, over-the-ear headphones, open headphones, hearing aids, or other personal acoustic devices. In some implementations, the feedforward microphone can be a part of an array of microphones.

Operations of the process <NUM> also include processing the input signal using a first filter disposed in an ANR signal flow path to generate a first signal for an acoustic transducer of the ANR device (<NUM>). The ANR signal flow path can be disposed in a feedforward signal flow path of the ANR device, the feedforward signal flow path being disposed between a feedforward microphone and an acoustic transducer of the ANR device. In some implementations, the first filter can be substantially similar to the ANR filters <NUM> and <NUM> described above with reference to <FIG> and <FIG>, respectively. In the invention, the first signal includes an anti-noise signal generated in response to a noise detected by a feedforward microphone, wherein the anti-noise signal is configured to cancel or at least reduce the effect of the noise. In the invention, the first filter is provided as a filter bank that includes a plurality of selectable digital filters, each digital filter in the filter bank corresponding to a value of a variable gain associated with a pass-through signal flow path disposed in parallel to the ANR signal flow path.

Operations of the process <NUM> further include processing the input signal in the pass-through signal flow path to generate a second signal for the acoustic transducer, wherein the pass-through signal flow path is configured to allow at least a portion of the input signal to pass through to the acoustic transducer in accordance with the variable gain (<NUM>). The pass-through signal flow path can include a second digital filter. The second digital filter can be substantially similar to the pass-through filter <NUM> and <NUM> described above with reference to <FIG> and <FIG>, respectively. In some implementations, the second filter may be implemented as a fixed-coefficient filter. In some implementations, the coefficients of the second filter may be determined substantially independently of a set of coefficients of the first filter. For example, both the first and second filter may be determined independently using equation (<NUM>), but with different values of insertion gain. In some implementations, the second filter may be provided as a bank of selectable filters.

In some implementations, pass through signal path can include a VGA, which may be adjusted in accordance with one or more user-inputs indicative of an adjustable gain associated with the pass-through signal path. In some implementations, coefficients of at least one of the first filter and the second filter are determined in accordance with the one or more user-inputs indicative of the gain associated with the pass-through signal path.

In some implementations, the coefficients of the at least one of the first filter and the second filter are determined in accordance with a target spectral characteristic of the corresponding filter. In some implementations, the target spectral characteristic can be spectral flatness. For example, the filters <NUM> and <NUM> described above with reference to <FIG> may be designed in accordance with target spectral flatness of the corresponding filters. In some implementations, the first filter and the second filter may be implemented using two different processing devices running at different speeds. In such cases, the latencies associated with the two filters can be substantially different from one another. For example, the latency associated with the first filter can be <NUM>-<NUM>, whereas the latency associated with the second filter is <NUM>. If the two filters are independently determined (e.g., as in the configuration of <FIG>), a large latency difference between the filters can cause the overall magnitude response of the feedforward compensator to deviate significantly from the target flatness. In some implementations, where the latency difference is large, using the gain-agnostic feedforward compensator of <FIG> may be advantageous in maintaining a target spectral flatness of the feedforward compensator.

The operations of the process <NUM> also includes generating an output signal for the acoustic transducer based on combining the first signal and the second signal (<NUM>). In some implementations, the output signal may be combined with one or more additional signals (e.g., a signal produced by a feedback compensator of an ANR device, a signal produced in an audio path of the ANR device, etc.) before being provided to the acoustic transducer. The audio output of the acoustic transducer may therefore represent a noise-reduced audio combined with audio representing the ambience as adjusted in accordance with user-preference.

The functionality described herein, or portions thereof, and its various modifications (hereinafter "the functions") can be implemented, at least in part, via a computer program product, e.g., a computer program tangibly embodied in an information carrier, such as one or more non-transitory machine-readable media or storage device, for execution by, or to control the operation of, one or more data processing apparatus, e.g., a programmable processor, a computer, multiple computers, and/or programmable logic components.

Actions associated with implementing all or part of the functions can be performed by one or more programmable processors executing one or more computer programs to perform the functions of the calibration process. All or part of the functions can be implemented as, special purpose logic circuitry, e.g., an FPGA and/or an ASIC (application-specific integrated circuit). In some implementations, at least a portion of the functions may also be executed on a floating point or fixed point digital signal processor (DSP) such as the Super Harvard Architecture Single-Chip Computer (SHARC) developed by Analog Devices Inc.

Claim 1:
A method comprising:
receiving (<NUM>) an input signal captured by one or more sensors (<NUM>) associated with an active noise reduction, ANR, device such as headphones, the one or more sensors comprising a feedforward microphone of the ANR device;
processing (<NUM>) the input signal using a first filter (<NUM>) disposed in an ANR signal flow path to generate a first signal for an acoustic transducer (<NUM>) of the ANR device;
processing (<NUM>) the input signal in a pass-through signal flow path disposed in parallel with the ANR signal flow path to generate a second signal for the acoustic transducer, wherein the pass-through signal flow path is configured to allow at least a portion of the input signal to pass through to the acoustic transducer in accordance with a variable gain associated with the pass-through signal flow path;
generating (<NUM>) an output signal for the acoustic transducer based on combining the first signal with the second signal;
characterized in that
the first filter comprises a filter bank that includes a plurality of selectable digital filters, each digital filter in the filter bank corresponding to a value of the variable gain associated with the pass-through signal flow path;
a digital filter of the plurality of selectable digital filters is selected or computed in real time based on the value of the variable gain associated with the pass-through signal flow path, and
the digital filter is selected such that any interference between resulting phase responses of the ANR signal flow path and the pass-through signal flow path do not degrade spectral characteristics of a magnitude response beyond a target tolerance limit.