Calibration and stabilization of an active noise cancelation system

A method of calibrating an earphone may include: securing an ANC earphone to a calibration fixture, the calibration fixture including an ear model configured to support the ANC earphone, the ear model having an ear canal configured to anatomically resemble a human ear canal and a concha configured to anatomically resemble a human ear concha, the ear canal extending from the concha to an inner end of the ear canal; generating, with the ANC earphone, an audio signal based on a reference tone; determining a characteristic of the audio signal; comparing the characteristic of the audio signal to a previously determined reference characteristic; and adjusting a gain value of the ANC earphone based on the comparing. Additional methods and apparatus are also disclosed.

FIELD OF THE INVENTION

This disclosure is related to audio processing and, and, more particularly, to a system and method for calibration and stabilization of an active noise cancellation system in a headphone.

BACKGROUND

Active noise cancellation (ANC) is a conventional method of reducing an amount of undesired noise received by a user listening to audio through headphones. The noise reduction is typically achieved by playing an anti-noise signal through the headphone's speakers. The anti-noise signal is an approximation of the negative of the undesired noise signal that would be in the ear cavity in the absence of ANC. The undesired noise signal is then neutralized when combined with the anti-noise signal.

In a general noise-cancellation process, one or more microphones monitor ambient noise or residual noise in the ear cups of headphones in real-time, then the speaker plays the anti-noise signal generated from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example.

In feedforward ANC, the microphone senses ambient noise but does not appreciably sense audio played by the speaker. In other words, the feedforward microphone does not monitor the signal directly from the speaker. In feedback ANC, the microphone is placed in a position to sense the total audio signal present in the ear cavity. So, the microphone senses the sum of both the ambient noise as well as the audio played back by the speaker. A combined feedforward and feedback ANC system uses both feedforward and feedback microphones.

For optimal noise rejection performance, the filter gain values of the feedforward and the feedback ANC paths generally are precisely tuned. Even so, the gain in an ANC path may differ from one part to another. These differences may be due to variations in the sensitivity or efficiency of the speaker and microphone transducers. If the feedforward ANC gain is too high, ambient noise may bleed in to the headphone. Also, if the feedback ANC gain is too high, there may be an increased hiss noise or loud spontaneous oscillations in the audio played by the speaker. On the other hand, if the feedback ANC gain or the feedforward ANC gain is too low, there may be a reduced amount of noise cancellation.

Even after calibration, the feedback ANC gain may increase or decrease from the tuned value. If the gain increases, the feedback ANC path may spontaneously oscillate, with the amplitude of the oscillation limited only by the full scale.

Embodiments of the invention address these and other issues in the prior art.

SUMMARY OF THE DISCLOSURE

Embodiments of the disclosed subject matter determine a characteristic of an audio signal in an active noise cancellation (ANC) system of an earphone and utilize the characteristic to calibrate and reduce instability in the ANC system.

Accordingly, at least some embodiments of a fixture for calibrating an ANC earphone may include an ear model and an acoustic path. The ear model may be configured to support an ANC earphone, and the ear model may include an ear canal extending from an outer end of the ear canal to an inner end of the ear canal. The acoustic path may be external to the ear canal and may extend from, at a first end of the acoustic path, the inner end of the ear canal of the ear model to an opposite, second end of the acoustic path. The acoustic path may be configured to transmit a mechanical sound wave received from the inner end of the ear canal to a region external to the ear model and adjacent the outer end of the ear canal.

In another aspect, at least some embodiments of a method of calibrating an earphone may include: securing an active noise canceling (ANC) earphone to a calibration fixture, the calibration fixture including an ear model configured to support the ANC earphone, the ear model having an ear canal configured to anatomically resemble a human ear canal and a concha configured to anatomically resemble a human ear concha, the ear canal extending from the concha to an inner end of the ear canal; generating, with the ANC earphone, an audio signal based on a reference tone; determining a characteristic of the audio signal; comparing the characteristic of the audio signal to a previously determined reference characteristic; and adjusting a gain value of the ANC earphone based on the comparing.

In yet another aspect, at least some embodiments of a method of reducing feedback instability in an ANC system may include: determining a characteristic of a feedback path signal in a feedback ANC path of an ANC system; determining a characteristic of a second signal in the ANC system, the second signal being outside of the feedback ANC path; comparing the feedback path characteristic to the second signal characteristic; and adjusting a feedback gain value of the feedback ANC path based on the comparing.

In still another aspect, at least some embodiments of a method of reducing feedforward instability in an ANC system may include: determining a characteristic of a feedforward anti-noise signal in a feedforward ANC path of an ANC system; determining a characteristic of a second signal in the ANC system; comparing the feedforward anti-noise characteristic to the second signal characteristic; and adjusting a feedforward gain value of the feedforward ANC path based on the comparing.

DETAILED DESCRIPTION

In general, systems and methods according to embodiments of the invention determine a characteristic of an audio signal in an active noise cancellation (ANC) system of an earphone and utilize the characteristic to calibrate and reduce instability in the ANC system.

During calibration, the earphone may be installed in a calibration fixture, and the calibration fixture may have an acoustic path from an ear canal portion of the calibration fixture to a region near a feedforward microphone of the ANC system. Also, the characteristic determined for calibration of the earphone may be compared to a corresponding characteristic of a reference standard earphone, which was previously set to a desired performance level. The characteristic may be, for example, a power level or an energy level.

To reduce instability, a characteristic of one portion of the ANC system may be compared to a characteristic of another portion of the ANC system. And a gain value within the ANC system may be adjusted based on the comparison. For the stability analysis, the characteristics may be, for example, fast Fourier transform vectors of the one portion and the other portion of the ANC system.

FIG. 1is a diagrammatic representation showing portions of a conventional earphone used to describe aspects of the disclosed systems and methods. The earphone101may be any earphone having an active noise cancellation (ANC) system and that is configured to sit on or in a user's ear. The earphone101, as illustrated inFIG. 1, may include an earphone enclosure102, a speaker103, a feedback microphone104, and a feedforward microphone105. The earphone enclosure102generally encloses the speaker103, the feedback microphone104, and the feedforward microphone105. The feedback microphone104and the feedforward microphone105operate generally as described below forFIG. 2.

Although some of the features below are described with respect to an earphone, such as the earphone101ofFIG. 1, unless otherwise indicated, the features are equally applicable to other types of headphones, including in-ear monitors, and pad- or cup-style headphones that are used in one ear or in both ears.

FIG. 2is a functional block diagram showing portions of a conventional ANC system200used to describe aspects of the disclosed systems and methods. The ANC system200may be an ANC system of an earphone, such as the earphone101ofFIG. 1. As illustrated inFIG. 2, the ANC system200may include a feedforward gain206, a feedback gain207, a speaker203, a feedforward microphone205, a feedback microphone204, a feedforward transfer function208(HFF), a feedback transfer function209(HFB), a first mixer210, and a second mixer211.

In a feedback ANC path212, the feedback microphone204generates a feedback microphone signal213based on an audio output of the speaker203. The feedback transfer function209receives the feedback microphone signal213and outputs a transformed feedback signal214to the feedback gain207. The feedback gain207receives the transformed feedback signal214and outputs a feedback anti-noise signal215to the speaker203, which generates the audio output.

In a feedforward ANC path216, the feedforward microphone205generates a feedforward microphone signal217based on an ambient noise level. The feedforward transfer function208receives the feedforward microphone signal217and outputs a transformed feedforward signal218to the feedforward gain206. The feedforward gain206receives the transformed feedforward signal218and outputs a feedforward anti-noise signal219to the speaker203.

The first mixer210is configured to combine the feedback anti-noise signal215, the feedforward anti-noise signal219, and a first audio signal220. The second mixer211is configured to combine the feedback microphone signal213and a second audio signal221. The first audio signal220may be, for example, a signal characteristic of the desired audio to be played through the speaker203as an audio playback signal. Typically, the first audio signal220is generated by or derived from an audio source such as a test instrument, a media player, a computer, a radio, a mobile phone, a CD player, or a game console during audio play. The second audio signal221may be, for example, the same as the first audio signal220, derived by filtering the first audio signal220, or derived by filtering the audio source from which the first audio signal220was derived.

In general, the acoustic properties of an earphone depend significantly on the physical characteristics of the ear or the ear model with which it is used.FIG. 3is a diagrammatic representation showing material portions of an embodiment of a calibration fixture300for an earphone301, or earbud. As illustrated inFIG. 3, a calibration fixture300for an earphone301may include an ear model322, a feedforward acoustic path323, and a damping partition324.

The ear model322is configured to support an earphone, such as the earphone101ofFIG. 1, during calibration and testing of the earphone301. The ear model322is also configured to resemble all or part of the human ear. Thus, the ear model322may include a pinna325configured to anatomically resemble a human ear pinna, a concha326configured to anatomically resemble a human ear concha, and an ear canal327configured to anatomically resemble a human ear canal. The ear canal327extends from an outer end353of the ear canal327, at the concha326, to an inner end of the ear canal327. Preferably, the ear model322is configured to resemble all or part of the human ear with respect to contour and air volume between the earphone301and the ear. For example, the ear canal327may have a volume of around 1 mL to 2 mL, such as about 1.5 mL, which may approximate the volume of a typical human ear canal.

The feedforward acoustic path323has a first end354and a second end355. The feedforward acoustic path323is configured to provide an acoustic path from the inner end352of the ear canal327of the ear model322to the feedforward microphone105of the earphone301under test. The feedforward microphone105of the earphone301under test may be, for example, in a region external to the ear model322and adjacent to the concha326of the ear model322, for example, as shown inFIG. 3.

The damping partition324is configured to acoustically negate or reduce the effect of the additional air volume of the feedforward acoustic path323. This is because coupling the feedforward acoustic path323to the ear canal327may change the air volume within the ear model322, resulting in a degraded speaker response. With the damping partition324, however, the response of the earphone's speaker may be substantially the same as it would be in an ear model322that does not include the feedforward acoustic path323. Accordingly, the damping partition324may allow the user to match an impedance of the ear canal327to an impedance of a typical human ear canal. As examples, the damping partition324may be made from or include resistive cloth or foam.

FIG. 4is a functional block diagram showing material portions of a feedback ANC path400for calibration, according to embodiments of the invention. The feedback ANC path400for calibration may be a portion of the ANC system200ofFIG. 2. Also, the feedback ANC path400for calibration may be a feedback ANC path400of an earphone under calibration, such as the earphone101ofFIG. 1, installed in a calibration fixture, such as the calibration fixture300ofFIG. 3. As illustrated inFIG. 4, a feedback ANC path400for calibration may include a feedback gain407, a speaker403, a feedback microphone404, and a feedback transfer function409, HFB. The speaker403and the feedback microphone404may correspond, respectively, to the speaker103and the feedback microphone104ofFIG. 1.

The feedback microphone404generates a feedback microphone signal413based on an audio output of the speaker403. The feedback transfer function409receives the feedback microphone signal413and outputs a transformed feedback signal414to the feedback gain407. The feedback gain407receives the transformed feedback signal414and outputs a feedback anti-noise signal415to the speaker403, which generates the audio output. Preferably, the feedback gain407is a variable gain stage. The feedback gain407may be a standalone gain stage, or the feedback gain407may be combined with another gain stage in the feedback ANC path400.

As illustrated inFIG. 4, a gain or level ratio, TFB, from an input side428of the speaker403to a feedback microphone output429may be calculated by setting the feedback gain407, GFB, to zero, playing a reference tone at the speaker403, determining a level, XSPK, at the input side428of the speaker403, and determining a level, YMFB, at the feedback microphone output429.

The reference tone may be a single tone that, for example, has a frequency indicative of the overall gain of the feedforward microphone and the speaker403. The reference tone also may be a Brown noise. Preferably, the reference tone is a multi-tone, having individual components placed in important bands and weighted differently. For example, the multi-tone may include three tones: a first tone at around 200 Hz and about −20 dBFS, a second tone at around 1000 Hz and about −10 dBFS, and a third tone of around 5000 Hz and about −10 dBFS. These values are just examples, though, and other values may be used, particularly since the values strongly depend on the precise ANC system being calibrated.

From the determined levels XSPKand YMFB, TFBmay be given by:

Using Equation 1, the gain TFBof a reference standard may be calculated by determining the level, XSPK, at the input side428of the speaker403of the reference standard, and determining the level, YMFB, at the feedback microphone output429of the reference standard. For purposes of this discussion, the gain TFBof the reference standard is referred to as TFB_REF.

Preferably, the reference standard is an earphone, such as the earphone101ofFIG. 1, whose feedback ANC path400and feedforward ANC path500(seeFIG. 5) were previously tuned for optimal performance or otherwise set to a desired performance level. For example, the reference standard may have been manually tuned to a desired performance level. The reference device has a tuned feedback gain407that is non-zero and is denoted as GFB_REF.

Accordingly, the calibrated feedback gain407may be determined by:

In Equation 2, GTOLis a tolerance applied to the equation to indicate that, excluding GTOL, the right side of Equation 2 need not exactly equal the left side of Equation 2. Even so, GTOLmay be set to zero in some embodiments. In other embodiments, GTOLmay be preset to another value, such as 0.05 dB or 0.1 dB. Other values, positive or negative, could also be used.

In this way, the feedback gain may be calibrated without a speaker external to the earphone or a microphone external to the earphone. Even so, in some embodiments an external speaker or external microphone, or both, could also be used.

FIG. 5is a functional block diagram showing material portions of a feedforward ANC path500for calibration with a calibration fixture, according to embodiments of the invention. The feedforward ANC path500for calibration may be a portion of the ANC system200ofFIG. 2. Also, the feedforward ANC path500for calibration may be a feedforward ANC path of the earphone under calibration discussed above forFIG. 4, installed in a calibration fixture, such as the calibration fixture300ofFIG. 3. As illustrated inFIG. 5, a feedforward ANC path500for calibration may include a feedforward gain506, a speaker503, a feedforward microphone505, and a feedforward transfer function508, HFF. The speaker503and the feedforward microphone505may correspond, respectively, to the speaker103and the feedforward microphone105ofFIG. 1.

The feedforward microphone505generates a feedforward microphone signal517based on an ambient noise level. The feedforward transfer function508receives the feedforward microphone signal517and outputs a transformed feedforward signal518to the feedforward gain506. The feedforward gain506receives the transformed feedforward signal518and outputs a feedforward anti-noise signal519to the speaker503. Preferably, the feedforward gain506is a variable gain stage. The feedforward gain506may be a standalone gain stage, or the feedforward gain506may be combined with another gain stage in the feedforward ANC path500.

With the setup ofFIG. 5and a calibration fixture having a feedforward acoustic path, such as the calibration fixture300ofFIG. 3, a gain or level ratio, TFF, from an input side528of the speaker503to a feedforward microphone output530may be calculated by setting the feedforward gain506, GFF, to zero, playing the reference tone at the speaker503, determining a level, XSPK, at the input side528of the speaker503, and determining a level, YMFF, at the feedforward microphone output530. The reference tone is generally as described above forFIG. 4.

From the determined levels XSPKand YMFF, TFFmay be given by:

Using Equation 3, the gain TFFof the reference standard may be calculated by determining the level, XSPK, at the input side528of the speaker503of the reference standard, and determining the level, YMFF, at the feedforward microphone output530of the reference standard. For purposes of this discussion, the gain TFFof the reference standard is referred to as TFF_REF. The reference device has a tuned feedforward gain506that is non-zero and is denoted as GFF_REF.

Accordingly, the calibrated feedforward gain506may be determined by:

GTOLis generally as described above for Equation 2. Preferably, GFFis determined after determining GFBfor the earphone under calibration, for example, by using the operations discussed above forFIG. 4.

In this way, the feedforward gain may be calibrated without a speaker or a microphone external to the earphone. Even so, in alternative embodiments an external speaker or external microphone, or both, could also be used.

FIG. 6is a functional block diagram showing material portions of an ANC system600for calibration, according to embodiments of the invention. The ANC system600for calibration may be an ANC system of the earphone101ofFIG. 1. In contrast to what is discussed above forFIG. 5, the setup illustrated inFIG. 6is generally for an earphone installed in a calibration fixture or an ear model that does not have the feedforward acoustic path described above forFIG. 3.

As illustrated inFIG. 6, the ANC system600for calibration may include a feedforward gain606, a feedback gain607, a speaker603, a feedforward microphone605, a feedback microphone604, a feedforward transfer function608(HFF), a feedback transfer function609(HFB), and mixer610. These components are generally as described above forFIG. 2and may be part of an earphone, such as the earphone101ofFIG. 1. The ANC system600for calibration may also include a noise source631, or speaker, that is external to the earphone.

With the setup ofFIG. 6, the feedforward gain606, GFF, may be determined by first determining the feedback gain607, GFB, for example, as described above forFIG. 4; playing the reference tone on the external noise source631; and, while the reference tone is playing, determining the level YMFBat a feedback microphone output629and the level YMFFat a feedforward microphone output630. Preferably, the level YMFBand the level YMFFare determined substantially simultaneously.

Similar to what is described above forFIGS. 4 and 5, a reference standard, which was previously tuned for optimal performance or otherwise set to a desired performance level, has a tuned feedback gain607denoted as GFB_REFand a tuned feedback gain607denoted as GFF_REF. The reference standard further has a determined level, YMFB_REF, at the feedback microphone output629of the reference standard and a determined level, YMFF_REF, at the feedforward microphone output630of the reference standard.

Accordingly, the calibrated feedforward gain606may be given by Equation 5, where GTOLis generally as described above for Equation 2:

The levels discussed with regard toFIGS. 4, 5, and 6may be, for example a power level or an energy level. In some embodiments, the levels may be estimated or determined by mean-square methods. In embodiments using a Brown noise, a fast Fourier transform (FFT) may be used to estimate the levels in various bands.

Accordingly, referring back to the descriptions ofFIGS. 1 to 6, a method of calibrating an earphone may include securing an ANC earphone to a calibration fixture; generating, with the ANC earphone, an audio signal based on a reference tone; determining a characteristic of the audio signal; comparing the characteristic of the audio signal to a previously determined reference characteristic; and adjusting a gain value, of the ANC earphone based on the comparing. The calibration fixture may include an ear model configured to support the ANC earphone. The ear model may have an ear canal configured to anatomically resemble a human ear canal and a concha configured to anatomically resemble a human ear concha. The ear canal may extend from the concha to an inner end of the ear canal.

The operation of determining a characteristic of the audio signal may include setting a feedback gain value to zero; playing the reference tone at a speaker of the ANC earphone while generating the audio signal; and determining a level-ratio between an output of a feedback microphone of the ANC earphone and an input side of the speaker.

The calibration fixture may also include an acoustic path configured to transmit a mechanical sound wave received from the inner end of the ear canal to a region external to the ear model and adjacent the concha of the ear model. In such embodiments, the operation of determining a characteristic of the audio signal may include setting a feedforward gain value to zero; playing the reference tone at a speaker of the ANC earphone while generating the audio signal; and determining a level-ratio from an input side of the speaker to an output of a feedforward microphone of the ANC earphone.

Once calibration is completed, it may be important to detect oscillations in the feedback ANC path and implement instability control measures.FIG. 7is a functional block diagram showing material portions of an enhanced ANC system700having feedback instability control, according to embodiments of the invention. As illustrated inFIG. 7, a feedback microphone704generates a feedback microphone signal703based on an audio output of a speaker703. A feedback transfer function709receives the feedback microphone signal703and outputs a transformed feedback signal714to a feedback gain707. The feedback gain707receives the transformed feedback signal714and outputs a feedback anti-noise signal715to the speaker703, which generates the audio output.

A feedforward microphone705generates a feedforward microphone signal717based on an ambient noise level. A feedforward transfer function708receives the feedforward microphone signal717and outputs a transformed feedforward signal718to a feedforward gain706. The feedforward gain706receives the transformed feedforward signal718and outputs a feedforward anti-noise signal719to the speaker703.

A first mixer710is configured to combine the feedback anti-noise signal715, the feedforward anti-noise signal719, and a first audio signal720. A second mixer711is configured to combine the feedback microphone signal703and a second audio signal721. The first audio signal720and the second audio signal721are generally as describe above forFIG. 2.

Preferably, the feedback microphone704, the feedforward microphone705, the speaker703, the feedback transfer function709, the feedforward transfer function, the feedback gain707, the feedforward gain706, the first mixer710, and the second mixer711are part of an ANC subsystem736of an earphone, such as the earphone101ofFIG. 1.

A first decimator737receives the feedforward microphone signal717from the feedforward microphone705and reduces the sampling rate of the feedforward microphone signal717. For example, the first decimator737may reduce the sampling rate of the feedforward microphone signal717to about 48 kHz. The reduced feedforward microphone signal717is then temporarily stored in a first buffer738. A first fast Fourier transform (FFT) transfer function739then receives the buffered feedforward microphone signal717and determines a discrete Fourier transform of the buffered feedforward microphone signal717. The output of the first FFT transfer function739is referred to in this disclosure as a feedforward noise FFT vector740.

A second decimator741receives the feedback anti-noise signal715from the feedback gain707and reduces the sampling rate of the feedback anti-noise signal715. For example, the second decimator741may reduce the sampling rate of the feedback anti-noise signal715to about 48 kHz. The reduced feedback anti-noise signal715is then temporarily stored in a second buffer742. A second FFT transfer function743then receives the buffered feedback anti-noise signal715and determines a discrete Fourier transform of the buffered feedback anti-noise signal715. The output of the second FFT transfer function743is referred to in this disclosure as a feedback anti-noise FFT vector744.

The second decimator741preferably receives the feedback anti-noise signal715. Alternatively, the second decimator741may instead receive and reduce the sampling rate of the feedback microphone signal703or the transformed feedback signal714, which is then temporarily stored in the second buffer742and acted on by the second FFT transfer function743as described here.

The first audio signal720is temporarily stored in a third buffer745. A third FFT transfer function746then receives the buffered first audio signal720and determines a discrete Fourier transform of the buffered first audio signal720. The output of the third FFT transfer function746is referred to in this disclosure as a forward audio FFT vector747. Although not shown inFIG. 7, the first audio signal720may also be decimated before being acted upon by the third FFT transfer function746.

Preferably, the first buffer738, the second buffer742, and the third buffer745are each configured to store 256 samples. Thus, where the first decimator737and the second decimator741each provide samples at about 48 kHz, the first buffer738and the second buffer742may include a delay of about 5.3 milliseconds to store the 256 samples. Preferably, a window, such as a triangular window, a Hanning window, or a Hamming window, is applied to the buffered feedforward microphone signal717, the buffered feedback anti-noise signal715, and the buffered first audio signal720before its respective discrete Fourier transform is determined. Additionally, where the first buffer738, the second buffer742, and the third buffer745are each configured to store 256 samples, the first FFT transfer function739, the second FFT transfer function743, and the third FFT transfer function746are preferably each configured to perform a 256-point FFT.

An instability controller748may collect the feedforward noise FFT vector740, the feedback anti-noise FFT vector744, and the forward audio FFT vector747, and also make an instability determination based on one or more of those collected vectors. For example, the instability controller748may perform a bin-wise comparison of the feedforward noise FFT vector740to the feedback anti-noise FFT vector744. As another example, the instability controller748may determine that an instability exists if, during a bin-wise comparison of the feedforward noise FFT vector740to the feedback anti-noise FFT vector744, a bin of the feedforward noise FFT vector740exceeds the feedback anti-noise FFT vector744in a corresponding bin plus a first threshold vector. In other words, if the instability controller748is comparing bin number24, then an instability is determined to be present if the value in bin number24of the feedforward noise FFT vector740exceeds the sum of the first threshold vector plus the value in bin number24of the feedback anti-noise FFT vector744. In some embodiments, though, the comparison may be made without adding the first threshold vector to the feedback anti-noise FFT vector744or by setting the first threshold vector to zero.

Alternatively or additionally, the instability controller748may perform a bin-wise comparison of the forward audio FFT vector747to the feedback anti-noise FFT vector744. For example, the instability controller748may determine that an instability exists if, during a bin-wise comparison of the forward audio FFT vector747to the feedback anti-noise FFT vector744, a bin of the forward audio FFT vector747exceeds the feedback anti-noise FFT vector744in a corresponding bin plus a second threshold vector. In some embodiments, though, the comparison may be made without adding the second threshold vector to the feedback anti-noise FFT vector744or by setting the second threshold vector to zero. Preferably, the second threshold vector is not identical to the first threshold vector.

If the instability controller748determines that an instability exists, then the instability controller748may output instructions749to the feedback gain707to reduce a feedback gain707value. In this way, instability control may be provided to the feedback ANC path of the ANC system.

Preferably, the second decimator741, the first buffer738, the second buffer742, the third buffer745, the first FFT transfer function739, the second FFT transfer function743, the third FFT transfer function746, and the instability controller748are part of a digital signal processor750. The digital signal processor750may reside, for example, in an earphone, such as the earphone101ofFIG. 1.

FIG. 8is a functional block diagram showing material portions of an enhanced ANC system800having feedforward instability control, according to embodiments of the invention. As illustrated inFIG. 8, a feedback microphone804generates a feedback microphone signal813based on an audio output of a speaker803. A feedback transfer function809receives the feedback microphone signal813and outputs a transformed feedback signal814to a feedback gain807. The feedback gain807receives the transformed feedback signal814and outputs a feedback anti-noise signal815to the speaker803, which generates the audio output.

A feedforward microphone805generates a feedforward microphone signal817based on an ambient noise level. A feedforward transfer function808receives the feedforward microphone signal817and outputs a transformed feedforward signal818to a feedforward gain806. The feedforward gain806receives the transformed feedforward signal818and outputs a feedforward anti-noise signal819to the speaker803.

A first mixer810is configured to combine the feedback anti-noise signal815, the feedforward anti-noise signal819, and a first audio signal820. A second mixer811is configured to combine the feedback microphone signal813and a second audio signal821. The first audio signal820and the second audio signal821are generally as describe above forFIG. 2.

Preferably, the feedback microphone804, the feedforward microphone805, the speaker803, the feedback transfer function809, the feedforward transfer function, the feedback gain807, the feedforward gain806, the first mixer810, and the second mixer811are part of an ANC subsystem836of an earphone, such as the earphone101ofFIG. 1.

A first decimator837receives the feedforward microphone signal817from the feedforward microphone805and reduces the sampling rate of the feedforward microphone signal817. The reduced feedforward microphone signal817is then temporarily stored in a first buffer838. A first fast Fourier transform (FFT) transfer839function then receives the buffered feedforward microphone signal817and determines a discrete Fourier transform of the buffered feedforward microphone signal817. The output of the first FFT transfer function839is referred to in this disclosure as the feedforward noise FFT vector840.

A second decimator841receives the feedforward anti-noise signal819from the feedforward gain806and reduces the sampling rate of the feedforward anti-noise signal819. The reduced feedforward anti-noise signal819is then temporarily stored in a second buffer842. A second FFT transfer function843then receives the buffered feedforward anti-noise signal819and determines a discrete Fourier transform of the buffered feedforward anti-noise signal819. The output of the second FFT transfer function843is referred to in this disclosure as the feedforward anti-noise FFT vector851.

The second decimator841preferably receives the feedforward anti-noise signal819. Alternatively, the second decimator841may instead receive and reduce the sampling rate of the feedforward microphone signal817or the transformed feedforward signal818, which is then temporarily stored in the second buffer842and acted on by the second FFT transfer function843.

The first audio signal820is temporarily stored in a third buffer845. A third FFT transfer function846then receives the buffered first audio signal820and determines a discrete Fourier transform of the buffered first audio signal820. The output of the third FFT transfer function846is referred to in this disclosure as the forward audio FFT vector847.

Preferably, the first buffer838, the second buffer842, and the third buffer845are each configured to store 256 samples. Preferably, a window, such as a triangular window, a Hanning window, or a Hamming window, is applied to the buffered feedforward microphone signal817, the buffered feedforward anti-noise signal819, and the buffered first audio signal820before its respective discrete Fourier transform is determined.

An instability controller848may collect the feedforward noise FFT vector840, the feedforward anti-noise FFT vector851, and the forward audio FFT vector847, and also make an instability determination. For example, the instability controller848may perform a bin-wise comparison of the feedforward noise FFT vector840to the feedforward anti-noise FFT vector851. As another example, the instability controller848may determine that an instability exists if, during a bin-wise comparison of the feedforward noise FFT vector840to the feedforward anti-noise FFT vector851, a bin of the feedforward noise FFT vector840exceeds the feedforward anti-noise FFT vector851in a corresponding bin plus a first feedforward threshold vector. In other words, if the instability controller848is comparing bin number77, then an instability is determined to exist if the value in bin number77of the feedforward noise FFT vector840exceeds the sum of the first feedforward threshold vector plus the value in bin number77of the feedforward anti-noise FFT vector851.

Alternatively or additionally, the instability controller848may perform a bin-wise comparison of the forward audio FFT vector847to the feedforward anti-noise FFT vector851. For example, the instability controller848may determine that an instability exists if, during a bin-wise comparison of the forward audio FFT vector847to the feedforward anti-noise FFT vector851, a bin of the forward audio FFT vector847exceeds the feedforward anti-noise FFT vector851in a corresponding bin plus a second feedforward threshold vector. Preferably, the second feedforward threshold vector is not identical to the first feedforward threshold vector.

If the instability controller848determines that an instability exists, then the instability controller848may output instructions849to the feedforward gain806to reduce a feedforward gain806value. In this way, instability control may be provided to the feedforward ANC path of the ANC system.

Preferably, the second decimator841, the first buffer838, the second buffer842, the third buffer845, the first FFT transfer function839, the second FFT transfer function843, the third FFT transfer function846, and the instability controller848are part of a digital signal processor850. The digital signal processor850may reside, for example, in an earphone, such as the earphone101ofFIG. 1.

Although shown separately inFIGS. 7 and 8, in some embodiments an ANC system may have both feedback instability control and feedforward instability control. Additionally, although the discussion ofFIGS. 7 and 8focuses on FFT transfer functions, other signal processing methods may also be used if the signal processing method can resolve the signal into different components or characteristics. As an example, a signal may be processed in the time domain by using signal correlation.

The previously described versions of the disclosed subject matter have many advantages that were either described or would be apparent to a person of ordinary skill. Even so, all of these advantages or features are not required in all versions of the disclosed apparatus, systems, or methods.

Additionally, this written description makes reference to particular features. It is to be understood that the disclosure in this specification includes all possible combinations of those particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment, that feature can also be used, to the extent possible, in the context of other aspects and embodiments.

Furthermore, the term “comprises” and its grammatical equivalents are used in this application to mean that other components, features, steps, processes, operations, etc. are optionally present. For example, an article “comprising” or “which comprises” components A, B, and C can contain only components A, B, and C, or it can contain components A, B, and C along with one or more other components.

Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.