Patent Description:
<CIT>, <CIT>, <CIT>, <CIT> and <CIT> are background prior art references. They fail to disclose at least the features recited in the characterizing portion of the independent claims.

Aspects and examples are directed to audio systems, devices, and methods that detect instability in a feedback noise reduction system. The systems and methods operate to detect when a plant transfer function (e.g., from a driver signal to a feedback microphone) becomes similar to the reciprocal of a transfer function of a feedback filter (applied to the microphone signal) such that the closed loop system may exhibit instability by, for example, having a loop gain of unity at one or more frequencies.

The present invention relates to a headphone system and a method according to the independent claims. Advantageous embodiments are set forth in the dependent claims.

Various aspects of at least one example are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, identical or nearly identical components illustrated in various figures may be represented by identical or similar numerals. In the figures:.

Aspects of the present disclosure are directed to noise cancelling headphones, headsets, or other audio systems, and methods, that detect instability in the noise canceling system. Noise cancelling systems operate to reduce acoustic noise components heard by a user of the audio system. Noise cancelling systems may include feedforward and/or feedback characteristics. A feedforward component detects noise external to the headset (e.g., via an external microphone) and acts to provide an anti-noise signal to counter the external noise expected to be transferred through to the user's ear. A feedback component detects acoustic signals reaching the user's ear (e.g., via an internal microphone) and processes the detected signals to counteract any signal components not intended to be part of the user's acoustic experience. Examples disclosed herein may be coupled to, or placed in connection with, other systems, through wired or wireless means, or may be independent of any other systems or equipment.

The systems and methods disclosed herein may include or operate in, in some examples, headsets, headphones, hearing aids, or other personal audio devices, as well as acoustic noise reduction systems that may be applied to home, office, or automotive environments. Throughout this disclosure the terms "headset," "headphone," "earphone," and "headphone set" are used interchangeably, and no distinction is meant to be made by the use of one term over another unless the context clearly indicates otherwise. Additionally, aspects and examples in accord with those disclosed herein are applicable to various form factors, such as in-ear transducers or earbuds and on-ear or over-ear headphones, and others.

<FIG> and <FIG> illustrate two example headsets 100A, 100B. Each headset <NUM> includes a right earpiece 110a and a left earpiece 110b, intercoupled by a supporting structure <NUM> (e.g., a headband, neckband, etc.) to be worn by a user. In some examples, two earpieces <NUM> may be independent of each other, not intercoupled by a supporting structure. Each earpiece <NUM> includes one or more microphones, such as a feedforward microphone <NUM> and/or a feedback microphone <NUM>. The feedforward microphone <NUM> is configured to sense acoustic signals external to the earpiece <NUM> when properly worn, e.g., to detect acoustic signals in the surrounding environment before they reach the user's ear. The feedback microphone <NUM> is configured to sense acoustic signals internal to an acoustic volume formed with the user's ear when the earpiece <NUM> is properly worn, e.g., to detect the acoustic signals reaching the user's ear. Each earpiece also includes a driver <NUM>, which is an acoustic transducer for conversion of, e.g., an electrical signal, into an acoustic signal that the user may hear. In various examples, one or more drivers are included in an earpiece, and an earpiece may in some cases include only a feedforward microphone or only a feedback microphone.

While the reference numerals <NUM> and <NUM> are used to refer to one or more microphones, the visual elements illustrated in the figures may, in some examples, represent an acoustic port wherein acoustic signals enter to ultimately reach such microphones, which may be internal and not physically visible from the exterior. In examples, one or more of the microphones <NUM>, <NUM> may be immediately adjacent to the interior of an acoustic port, or may be removed from an acoustic port by a distance, and may include an acoustic waveguide between an acoustic port and an associated microphone.

Shown in <FIG> is an example of a processing unit <NUM> that is physically housed somewhere on or within the headset <NUM>. The processing unit <NUM> includes a processor <NUM>, an audio interface <NUM>, and a battery <NUM>. The processing unit <NUM> is coupled to one or more feedforward microphone(s) <NUM>, driver(s) <NUM>, and/or feedback microphone(s) <NUM>, in various examples. In various examples, the interface <NUM> may be a wired or a wireless interface for receiving audio signals, such as a playback audio signal or program content signal, and may include further interface functionality, such as a user interface for receiving user inputs and/or configuration options. In various examples, the battery <NUM> may be replaceable and/or rechargeable. In various examples, the processing unit <NUM> may be powered via means other than or in addition to the battery <NUM>, such as by a wired power supply or the like. In some examples, a system may be designed for noise reduction only and may not include an interface <NUM> to receive a playback signal.

<FIG> illustrates a system and method of processing microphone signals to reduce noise reaching the user's ear. <FIG> presents a simplified schematic diagram to highlight features of a noise reduction system. Various examples of a complete system may include amplifiers, analog-to-digital conversion (ADC), digital-to-analog conversion (DAC), equalization, sub-band separation and synthesis, and other signal processing or the like. In some examples, a playback signal <NUM>, p(t), may be received to be rendered as an acoustic signal by the driver <NUM>. The feedforward microphone <NUM> provides a feedforward signal <NUM> that is processed by a feedforward processor <NUM>, having a feedforward transfer function <NUM>, Kff, to produce a feedforward anti-noise signal <NUM>. The feedback microphone <NUM> provides a feedback signal <NUM> that is processed by a feedback processor <NUM>, having a feedback transfer function <NUM>, Kfb, to produce a feedback anti-noise signal <NUM>. In various examples, any of the playback signal <NUM>, the feedforward anti-noise signal <NUM>, and/or the feedback anti-noise signal <NUM> may be combined, e.g., by a combiner <NUM>, to generate a driver signal <NUM>, d(t), to be provided to the driver <NUM>. In various examples, any of the playback signal <NUM>, the feedforward anti-noise signal <NUM> may be omitted and/or the components necessary to support any of these signals may not be included in a particular implementation of a system.

Various examples described herein include a feedback noise reduction system, e.g., a feedback microphone <NUM> and a feedback processor <NUM> having a feedback transfer function <NUM> to provide a feedback anti-noise signal <NUM> for inclusion in a driver signal <NUM>. The feedback microphone <NUM> is configured to detect sound within the acoustic volume that includes the user's ear and, accordingly, detects an acoustic signal <NUM> produced by the driver <NUM>, such that a loop exists. Accordingly, in various examples and/or at various times, a feedback loop may exist from the driver signal <NUM> through the driver <NUM> producing an acoustic signal <NUM> that is picked up by the feedback microphone <NUM>, processed through the feedback transfer function <NUM>, Kfb, and included in the driver signal <NUM>. Accordingly, at least some components of the feedback signal <NUM> are caused by the acoustic signal <NUM> rendered from the driver signal <NUM>. Alternately stated, the feedback signal <NUM> includes components related to the driver signal <NUM>.

The electrical and physical system shown in <FIG> exhibits a plant transfer function <NUM>, G, characterizing the transfer of the driver signal <NUM> through to the feedback signal <NUM>. In other words, the response of the feedback signal <NUM> to the driver signal <NUM> is characterized by the plant transfer function <NUM>, G. The system of the feedback noise reduction loop is therefore characterized by the combined (loop) transfer function, GKfb. If the loop transfer function, GKfb, becomes equal to unity, GKfb = <NUM>, at one or more frequencies, the loop system may diverge, causing at least one frequency component of the driver signal <NUM> to progressively increase in amplitude. This may be perceived by the user as an audible artifact, such as a tone or squealing, and may reach a limit at a maximum amplitude the driver <NUM> is capable of producing, which may be extremely loud. Accordingly, when such a condition exists, the feedback noise reduction system may be described as unstable.

Various examples of an earpiece <NUM> with a driver <NUM> and a feedback microphone <NUM> are designed to avoid feedback instability, e.g., by designing to avoid or minimize the chances of the loop transfer function, GKfb, having undesirable characteristics. Despite various quality designs, a loop transfer function, GKfb, may nonetheless exhibit instability at various times or under certain conditions, e.g., by action of the plant transfer function <NUM>, G, changing due to movement or handling of the earpiece <NUM> by the user, such as when putting a headset on or off, or adjusting the earpiece <NUM> while worn. In some cases, a fit of the earpiece <NUM> may be less than optimal or may be out of the norm and may provide differing coupling between the driver <NUM> and the feedback microphone <NUM> than anticipated. Accordingly, the plant transfer function <NUM>, G, may change at various times to cause an instability in the feedback noise reduction loop. In some examples, processing by the feedback processor <NUM> may include active processing that may change a response or transfer function, such as by including one or more adaptive filters or other processing that may change the feedback transfer function, Kfb, at various times. Such changes as these may cause (or remedy) an instability in the feedback noise reduction loop.

Accordingly, various example systems and methods described herein operate to monitor for a condition in which a loop transfer function, GKfb, becomes equal to unity, GKfb = <NUM>, and to indicate that a feedback instability exists when so. With continued reference to <FIG>, when the loop transfer function equals unity, such is equivalently expressed as the plant transfer function <NUM>, G, being the inverse (e.g., reciprocal) of the feedback transfer function <NUM>, Kfb, thereby satisfying the expression, G = Kfb-<NUM>. Accordingly, a feedback noise reduction system is unstable when a plant transfer function (e.g., <NUM>) is the inverse of a feedback transfer function (e.g., <NUM>).

As discussed previously, the feedback signal <NUM> may include components of the driver signal <NUM>. When a feedback instability exists, components of the feedback signal <NUM> may be related to the driver signal <NUM> by the inverse of the feedback transfer function <NUM>, because during an instable condition the plant transfer function <NUM> may be inversely related to the feedback transfer function <NUM>. Various systems and methods in accord with those described herein detect feedback instability by monitoring for components in the feedback signal <NUM> being related to the driver signal <NUM> such that the relationship is the inverse of the feedback transfer function <NUM>. In some examples, the driver signal <NUM> is filtered by the inverse of the feedback transfer function <NUM> and the resulting signal is compared to the feedback signal <NUM>. A threshold level of similarity may indicate that the plant transfer function <NUM> is nearly equal to the inverse of the feedback transfer function <NUM>, and thus indicates that a feedback instability exists.

With reference to <FIG>, an example system and method is shown wherein the feedback signal <NUM> is compared to the driver signal <NUM> by a comparator <NUM>, and if their relationship is similar to the inverse of the feedback transfer function <NUM>, an instability indicator <NUM> is provided. The instability indicator <NUM> may be, for example, a flag, indicator, or logic level signal (e.g., having high and low output levels) to indicate the presence or absence of instability, or may be any suitable type of signal for interpretation by various other components. For example, other components may receive the instability indicator <NUM> and may take action in response to an instability, such as reducing a gain in the feedback transfer function <NUM> (e.g., at one or more frequencies or frequency ranges).

With reference to <FIG>, at least one example of a comparator <NUM> is illustrated, suitable for comparing whether the feedback signal <NUM> is related to the driver signal <NUM> by an inverse of the feedback transfer function <NUM>. The driver signal <NUM> is received and processed by a filter <NUM> having a transfer function, Kfb-<NUM>, that is the inverse of the feedback transfer function <NUM> to provide a reference signal <NUM>. In some examples, a delay may be applied to the feedback signal <NUM> to align the feedback signal <NUM> with the reference signal <NUM> (e.g., to match a delay added by the filter <NUM>). A correlation measurement <NUM> is made between the feedback signal <NUM> and the reference signal <NUM>, to quantify their similarity, and if their similarity meets a threshold <NUM>, an instability is indicated by the instability indicator <NUM>, which is an output signal of the comparator <NUM>. In various examples, the correlation measurement <NUM> may be any of various measurements to correlate signals. In some examples, a cross-correlation may be calculated between the feedback signal <NUM> and the reference signal <NUM>. In various examples, signal envelopes and/or signal energies in various sub-bands may be measured and compared, and/or various smoothing and/or weighting may be applied in various instances, and/or other processing to quantify a relationship between the feedback signal <NUM> and the reference signal <NUM>. In various examples, the threshold <NUM> may apply a threshold level (e.g., of the quantified similarity) necessary to decide that an instability exists, and may also apply a threshold timeframe, such as an amount of time the similarity must remain above the threshold level. In some examples, an amount of time and/or a delay before indicating that an instability exists may be defined by a minimum number of samples, e.g., of the correlation of sampled signals in a digital domain, meeting the threshold level.

In some examples, multiple correlation measurements may be made, each of which may be compared to a threshold, any one or more of which may be deemed required to indicate an instability. For example, two distinct correlation measurements may be implemented in certain examples, and both may be required to meet a threshold to indicate an instability. In further examples, if one of the two distinct correlation measurements exceeds a higher threshold, such may be sufficient to indicate an instability even though the other of the two distinct correlation measurements fails to meet its threshold. In yet further examples, a third correlation measurement having its own threshold may confirm and/or over-ride the indication of instability generated by the first two correlation measurements, and the like.

Referring to <FIG>, a further example of a comparator 510A is illustrated. As above, with reference to <FIG>, the driver signal <NUM> is filtered (e.g., by filter <NUM>) through an inverse transfer function, Kfb-<NUM>, of the feedback transfer function <NUM>, and the resulting reference signal <NUM> is compared to the feedback signal <NUM>. In some examples, the reference signal <NUM> may be a predictive signal, in that it may predict the feedback signal <NUM> during times of feedback instability (as discussed previously), such that comparison of the feedback signal <NUM> to the reference signal <NUM> may be used to detect that instability exists.

With reference to <FIG>, the example comparator 510A includes a combiner <NUM> that adds the reference signal <NUM> to the feedback signal <NUM> to provide a summed signal <NUM>, and a combiner <NUM> that subtracts the reference signal <NUM> from the feedback signal <NUM> (or vice versa, in other examples) to provide a difference signal <NUM>. As described above, a feedback instability may exist when G = Kfb-<NUM>, causing the reference signal <NUM> to be predictive of the feedback signal <NUM>. Accordingly, when the feedback signal <NUM> is similar to the reference signal <NUM>, an instability may exist. Further, when the feedback signal <NUM> is similar to the reference signal <NUM>, the summed signal <NUM> may be expected to have relatively large amplitude and signal energy and the difference signal <NUM> may be expected to have relatively small amplitude and signal energy.

In some examples, each of the summed signal <NUM> and the difference signal <NUM> may be processed by a squaring block <NUM> and a smoothing block <NUM>. For example, squaring a signal yields an output that is always positive and may be considered indicative of a signal energy. Smoothing a signal mitigates rapid changes in the signal, which may be considered low pass filtering, which may provide or be considered a signal envelope. Smoothing may be applied in various ways. At least one example may include alpha smoothing, in which each new signal sample, s[n], received over time (e.g., in a digital domain) is added to a running average of the prior samples, s_avg[n-<NUM>], according to a weighting factor, α, as illustrated by equation (<NUM>). <MAT> The weighting factor, α, may be considered a tunable time constant, for example. It should be recognized that various signal processing may be performed in either of an analog or digital domain in various examples, and that various signals may be equivalently expressed with either of a time parameter, t, or a digital sample index, n. In various examples, the weighting factor, α, may be the same in the two smoothing blocks <NUM>. In other examples, the weighting factor, α, may be different for the two smoothing blocks <NUM>.

With continued reference to <FIG>, squaring and smoothing the summed signal <NUM> provides a primary signal <NUM> that is expected to have a relatively large value when an instability exists. By contrast, the difference signal <NUM> is expected to have relatively low amplitude, such that a squared and smoothed version is expected to have a relatively low value. In some examples, a ratio <NUM> may be taken, to provide a relative signal <NUM>, which provides a single signal indicative of the extent to which both the summed signal <NUM> is large and the difference signal <NUM> is small, relative to each other. Accordingly, the relative signal <NUM> is expected to have a relatively large value when an instability exists.

Each of the primary signal <NUM> and the relative signal <NUM> may be tested against a respective threshold <NUM>, each of which may apply varying thresholds, including a quantity threshold and optionally a time threshold (e.g., the amount of time, or number of digital samples, that a quantity threshold must be met). In various examples, a threshold 760a for the primary signal <NUM> may be a fixed or variable threshold, selected based upon various aspects and/or settings (e.g., gain) related to various components of the system overall, such as a level of the driver signal <NUM>. The threshold 760b for the relative signal <NUM>, may also be a fixed or variable threshold selected based upon various aspects, components, and/or settings of the system. In various examples, either or both of the thresholds <NUM> may be selected based upon testing and characterization of the system as a whole, under conditions that cause instability and conditions that don't cause instability. In some examples, the threshold 760b is a fixed threshold in a range of <NUM> to <NUM> dB. In certain examples, the threshold 760b is a fixed threshold in a range of <NUM> to <NUM> dB, and in particular examples may be <NUM> dB, <NUM> dB, <NUM> dB, or other values.

With continued reference to <FIG>, a logic <NUM> may combine outputs from the thresholds <NUM>. In the example of <FIG>, the logic <NUM> applies AND logic, requiring both of the primary signal <NUM> and the relative signal <NUM> to meet its respective threshold 760a, 760b. In some examples, a minimum time and/or number of digital samples may be applied by the logic <NUM>, e.g., a minimum number of samples that each of the primary signal <NUM> and the relative signal <NUM>, potentially in combination, must meet its respective threshold <NUM>, 760b. Various examples may user other combinations for logic <NUM>, which may also incorporate signals from additional processing. In some examples, either of the primary signal <NUM> or the relative signal <NUM> meeting the respective threshold <NUM> may be deemed sufficient to produce the output instability indicator <NUM>. In some examples, additional thresholds <NUM> may be applied to the signals shown and/or other signals. For instance, an additional threshold may be applied to the relative signal <NUM> that, when met, may be incorporated by the logic <NUM> to produce the output instability indicator <NUM> even if the primary signal <NUM> fails to meet the threshold 760a.

According to some examples, a system may be tested and characterized and may be determined to be more likely to exhibit feedback instability at one or more frequencies and/or one or more frequency sub-bands. Accordingly, in some examples, the various processing illustrated, e.g., in <FIG>, may be performed within a range of frequencies and/or one or more sub-bands in which the instability is likely to occur. Additionally or alternately, each of a number of sub-bands or frequency ranges may have differing parameters applied by the various processing. For example, a threshold 760b may be a fixed value for one sub-band of the relative signal <NUM> and a different fixed value for another sub-band of the relative signal <NUM>.

According to some examples, a system may be tested and characterized and may be determined to be more likely to exhibit high signal energies at one or more frequencies and/or one or more frequency sub-bands even though no feedback instability exists. Accordingly, in some examples, the various processing illustrated, e.g., in <FIG>, may be configured to omit or ignore one or more sub-bands and/or range of frequencies.

According to some examples, a system may be tested and characterized and may be determined that more complex or less complex signal processing and/or logic may be beneficially applied to one or more sub-bands or frequency ranges than to others. Accordingly, in some examples, the various processing illustrated, e.g., in <FIG>, may vary significantly for differing ranges of frequencies and/or one or more sub-bands.

In various examples, as described above, detection of a feedback instability is accomplished by analyzing a relationship between a feedback microphone signal and a driver signal (e.g., by comparison of the feedback signal <NUM> to the driver signal <NUM>) and an instability indicator <NUM> is provided. When the instability indicator <NUM> indicates that a feedback instability is detected, various systems and methods in accord with aspects and examples herein may take varying actions in response to the feedback instability, e.g., to mitigate or remove the feedback instability and/or the undesirable consequences of the instability. For example, an audio system in accord with those described may alter or replace the feedback transfer function <NUM>, alter a feedback controller or feedback processor <NUM>, change to a less aggressive form of feedback noise reduction, alter various parameters of the noise reduction system to be less aggressive, alter a driver signal amplitude (e.g., mute, reduce, or limit the driver signal <NUM>), alter a processing phase response, e.g., of the driver signal <NUM> and/or feedback signal <NUM>, in an attempt to disrupt the instability, provide an indicator to a user (e.g., an audible or vocal message, an indicator light, etc.), and/or other actions.

The above described aspects and examples provide numerous potential benefits to a personal audio device that includes feedback noise reduction. Stability criteria for feedback control may be defined by an engineer at the controller design stage, and various considerations assume a limited range of variation (of system characteristics) over the lifetime of the system. For example, driver output and microphone sensitivity may vary over time and contribute to the electroacoustic transfer function between the driver and the feedback microphone. Further variability may impact design criteria, such as production variation, head-to-head variation, variation in user handling, and environmental factors. Any such variations may cause stability constraints to be violated, and designers must conventionally take a conservative approach to feedback system design to ensure that instability is avoided. Such an instability may cause the noise reduction system to add undesired signal components rather than reduce them, thus conventional design practices may take highly conservative approaches to avoid an instability occurring, potentially at severe costs to system performance.

However, aspects and examples of detecting feedback instability, as described herein, allow corrective action to be taken to remove the instability when such condition occurs, allowing system designers to design systems that operate under conditions nearer to a boundary of instability, and thus achieve improved performance over a wider feedback bandwidth. Aspects and examples herein allow reliable detection if or when the instability boundary is crossed. For example, in an in-ear noise cancelling headphone, a user's handling may commonly block the "nozzle" of an earbud (e.g., a finger momentarily covering the audio port), which may cause an extreme physical change to the electroacoustic coupling between the driver and the feedback microphone. Conventional systems need to be designed to avoid instability even with a blocked nozzle, but instability detection in accord with aspects and examples described herein allow the feedback controller or processor to be designed without the "blocked nozzle" condition as a constraint. Accordingly, systems and methods herein may more than double the range of bandwidth in which noise reduction by a feedback processor may be effective.

In various examples, any of the functions of the systems and methods described herein may be implemented or carried out in a digital signal processor (DSP), a microprocessor, a logic controller, logic circuits, and the like, or any combination of these, and may include analog circuit components and/or other components with respect to any particular implementation. Functions and components disclosed herein may operate in the digital domain and certain examples include analog-to-digital (ADC) conversion of analog signals generated by microphones, despite the lack of illustration of ADC's in the various figures. Such ADC functionality may be incorporated in or otherwise internal to a signal processor. Any suitable hardware and/or software, including firmware and the like, may be configured to carry out or implement components of the aspects and examples disclosed herein, and various implementations of aspects and examples may include components and/or functionality in addition to those disclosed.

Claim 1:
A headphone system (<NUM>) comprising:
an acoustic transducer (<NUM>) to convert a driver signal (<NUM>) into an acoustic signal (<NUM>);
a microphone (<NUM>) to provide a feedback signal (<NUM>);
a first processing (<NUM>) component configured to process the feedback signal through a feedback transfer function (<NUM>) to provide an anti-noise signal (<NUM>), the driver signal being based at least in part upon the anti-noise signal;
characterized in that the headphone system further comprises:
a filter (<NUM>) to filter the driver signal and provide a reference signal (<NUM>), the filter configured to have a second transfer function that is inverse of the feedback transfer function; and
a second processing component (<NUM>) to compare the feedback signal to the reference signal to determine a feedback instability based upon the comparison.