HK1194586B - Instability detection and avoidance in a feedback system - Google Patents

Description

Instability detection and avoidance in feedback systems

Technical Field

The present invention relates to feedback systems, in particular to instability detection and avoidance in feedback active noise reduction systems.

Background

The presence of ambient noise in the environment can have a wide impact on human hearing. Some examples of ambient noise, such as engine noise in the passenger cabin of a jet passenger aircraft, may cause minor disturbances to passengers. Other examples of ambient noise, such as power drills on construction sites, may cause permanent hearing loss. Techniques for reducing ambient noise are an active area of research, providing benefits such as a more pleasant hearing experience and avoidance of hearing loss.

Many conventional noise reduction systems utilize active noise reduction techniques to reduce the amount of noise perceived by a user. Active noise reduction systems are typically implemented using feed-forward, feedback, or a combination of feed-forward and feedback approaches. Feedback-based systems typically measure noise sound waves, possibly in combination with other sound waves, near an area where noise reduction is desired (e.g., in an acoustic cavity such as an ear cavity). Typically, this measured signal is used to generate an "anti-noise signal," which is an inverted and scaled version of the measured noise. The anti-noise signal is provided to a noise cancellation driver that transduces the signal into sound waves that are presented to the user. When the anti-noise waves generated by the noise cancellation driver combine with the noise sound waves in the acoustic cavity, the two sound waves cancel each other due to destructive interference. The result is a reduction in the level of noise perceived by the user in the area where noise reduction is desired.

Feedback systems often have the potential to be unstable and produce distortion based on the instability. For example, as understood based on classical analysis of a feedback system, if the gain of the feedback loop is greater than 1 at a frequency at which the phase of the feedback loop is 180 °, an oscillating additive signal can be generated at that frequency. This case can be described as the phase margin of a zero or negative value system, i.e. the margin to reach 180 ° phase at a frequency with a gain of 1.

In an acoustic active noise reduction system, at least a portion of the feedback path can include an acoustic component. Although it is possible to directly control the electrical or digital components of the feedback path in an active noise reduction system, the acoustic components may be affected by variations, for example as a result of variations in the physical properties of the acoustic path.

Disclosure of Invention

In some cases, changes in the acoustic path can cause instability in the system due to changes in the resulting feedback loop gain or transfer function. For example, the acoustic component can have an acoustic transfer function between the acoustic driver and the feedback microphone. One example of a situation where the acoustic transfer function varies is when the wearer of an in-ear headphone inserts the earpiece of the headphone into the ear canal. During the insertion process, the compliant end of the earplug becomes occluded, for example, by being squeezed (snapped) or folded upon itself. Such a blocked end can change the acoustic transfer function, thereby changing the overall loop gain and possibly causing instability in the system.

There is a need for a system that can detect the characteristics of instability in a feedback noise reduction system and adjust the loop gain of the system to avoid instability.

In one aspect, in general, an active noise reduction system detects actual or potential instability by detecting characteristics of the system related to potential or actual unstable behavior (e.g., oscillation), and adapts system characteristics to mitigate such instability.

In some examples, the system adapts to changes in characteristics of the acoustic components of the feedback path that have or can cause erratic behavior to improve the acoustic experience of the user.

In one aspect, in general, a feedback-based active noise reduction system includes a feedback element and an instability detector for detecting an instability condition in the feedback element and forming a control parameter based on a result of the detection. The feedback element includes a feedback input for receiving a first feedback signal from the first sensor; a control input for accepting control parameters for adjusting gain and phase characteristics of the feedback element; and a driver output for providing a drive signal to the driver. The instability detector includes a control parameter output for providing a control parameter to a control parameter input of the feedback element; and a plurality of inputs for receiving a plurality of feedback signals from a plurality of sensors including the first sensor. Detecting the instability condition includes processing the plurality of feedback signals to determine a characteristic of an acoustic path between the driver and the first sensor.

Aspects can include one or more of the following features.

The first sensor may comprise a microphone and the driver may comprise a speaker. The feedback element may be configured to change one or both of a gain characteristic and a phase characteristic of the feedback element by a predetermined amount when providing the control parameter. The feedback element may be configured to modify the transfer functions of the feedback filter, the feedforward filter and the acoustic input filter in parallel when providing the control parameter.

The feedback element may be configured to change the bandwidth of the feedback element by a predetermined amount when providing the control parameter. The feedback element may include a low pass filter selectively adapted to the feedback element based on the control parameter. The plurality of sensors may include a second sensor, and the instability detector may be configured to determine a characteristic of the acoustic path between the driver and the first sensor based on a ratio of a first feedback signal associated with the first sensor relative to a second feedback signal associated with the second sensor.

The ratio of the first feedback signal to the second feedback signal may be indicative of the acoustic impedance of the acoustic path. The first sensor may comprise a pressure microphone and the second sensor may comprise a velocity microphone. The first sensor may include a pressure microphone and the second sensor may include a pressure microphone. The plurality of sensors may include a third sensor for generating a third feedback signal, and the instability detector may be configured to determine the validity of the instability condition detected by the instability detector based on the third feedback signal.

The feedback element may include a first signal input for accepting an input signal, the instability detector may include a second signal input for accepting the input signal and a driver input for accepting the drive signal, and the instability detector may be configured to detect an instability condition in the feedback element, including determining a characteristic of the feedback element based on the input signal and the drive signal. The unstable condition may include the presence of oscillations in a specified frequency range. The specified frequency range may be mutually exclusive from the frequency range in which active noise reduction occurs.

The instability detector may be configured to analyze the input signal and the drive signal to determine whether oscillations are present in the drive signal and not present in the input signal.

In another aspect, in general, a method for detecting and avoiding instability in a feedback-based active noise reduction system includes detecting an instability condition in a feedback element and forming a control parameter based on a result of the detecting. Detecting the instability condition includes receiving a plurality of feedback signals from a plurality of sensors including the first sensor and processing the plurality of feedback signals to determine a characteristic of an acoustic path between the driver and the first sensor. The method also includes providing a control parameter to the feedback element, receiving the control parameter at the feedback element, receiving a first feedback signal at the feedback element from a first sensor, adjusting a gain characteristic and a phase characteristic of the feedback element based on the control parameter, and outputting a driver output signal from the feedback element to the driver.

Aspects can include one or more of the following features.

The first sensor may comprise a microphone and the driver may comprise a speaker. Providing a control parameter to the feedback element may change one or both of the gain characteristic and the phase characteristic of the feedback element by a predetermined amount. Providing control parameters to the feedback element may cause parallel modification of the transfer functions of the feedback filter, the feedforward filter and the audio input filter. Providing a control parameter to the feedback element may cause the bandwidth of the feedback element to change by a predetermined amount. Providing the control parameter to the feedback element may cause the low pass filter to be selectively applied to the feedback element based on the provided parameter.

The plurality of sensors may include a second sensor, and determining a characteristic of the acoustic path between the driver and the first sensor may include calculating a ratio of a first feedback signal associated with the first sensor relative to a second feedback signal associated with the second sensor. The ratio of the first feedback signal to the second feedback signal may be indicative of the acoustic impedance of the acoustic path. The first sensor may comprise a pressure microphone and the second sensor may comprise a velocity microphone.

The first sensor may include a pressure microphone and the second sensor may include a pressure microphone. The plurality of sensors may include a third sensor for generating a third feedback signal, and detecting the instability condition may include determining a validity of the instability condition based on the third feedback signal.

The method may also include the step of accepting an input signal at the feedback element, wherein detecting the instability condition further includes accepting the input signal, accepting the drive signal, and determining a characteristic of the feedback element based on the input signal and the drive signal.

The unstable condition may include the presence of oscillations in a specified frequency range. The specified frequency range may be mutually exclusive from the frequency range in which active noise reduction occurs. Detecting the instability condition can include analyzing the input signal and the drive signal to determine whether oscillations are present in the drive signal and not present in the input signal.

Embodiments may have one or more of the following advantages.

Embodiments may require few electronic components, resulting in reduced costs relative to conventional conventions that include general purpose Digital Signal Processing (DSP) hardware.

Embodiments may consume very little power (e.g., microwatts) because they do not require high speed/low noise operational amplifiers.

Embodiments may respond to interference more quickly than DSP-based systems that require long measurement and computation times. In some examples, DSP-based systems do not respond fast enough to prevent loud high-pitched sounds from impinging on the eardrum for extended periods of time due to the speaker driver in the headphone device being in close proximity to the eardrum.

Embodiments are not affected by being triggered by a separate audio signal and can reliably detect oscillations present in the audio signal.

Other features and advantages of the invention will be apparent from the following description and from the claims.

Drawings

FIG. 1 is a block diagram of a feedback noise reduction system including an oscillation detector.

Fig. 2 is an oscillation detector.

Fig. 3 is a graph showing gain and phase margin.

Fig. 4 is a circuit configured to reduce loop gain.

Fig. 4A, 4B, and 4C provide detailed views of the circuit of fig. 4.

Fig. 5 is a graph showing gain and phase margin.

Fig. 6 is a circuit configured to reduce loop gain and bandwidth.

Fig. 7 is an in-ear headphone with an occluded end.

FIG. 8 is a graph of acoustic impedance for an unblocked condition and a blocked condition.

Fig. 9 is an in-ear headphone configured to detect an occluded end.

FIG. 10 is a block diagram of feedback noise reduction including a combined oscillation/blocking end detector.

Fig. 11 is a combined oscillation/blockage end detector.

Fig. 12 is a truth table showing the logic used to calculate the output of the combined oscillation/blocking end detector.

FIG. 13 is a graph of acoustic impedance measurements for an unblocked condition and a blocked condition.

Detailed Description

1. Overview

The system described herein detects actual or potential feedback loop instability due to excessive feedback loop gain in a feedback control based active noise reduction system and mitigates the instability to return the system to a stable or more stable operating state.

The system utilizes the following knowledge:

a) as the gain of the feedback loop approaches 1 at a frequency where the phase of the feedback loop approaches 180 °, the bandwidth of the gain of the feedback loop increases. This reduces the phase margin in the system, eventually leading to an unstable feedback loop, which can lead to oscillations or damped oscillations at this frequency.

b) When the end of the earplug is occluded, a significant change in acoustic impedance occurs, which changes the feedback loop gain.

Upon detecting an instability of the feedback loop, the system mitigates the instability by adjusting a gain of the feedback loop.

2. Oscillation detector

Referring to fig. 1, for acoustically active noise reductionThe system 200 receives an input signal (e.g., an acoustic signal) x (t) and provides a modified version of the input signal to the acoustic driver 102. The acoustic driver 102 transduces the modified version of the input signal into an acoustic wave y (t) in the acoustic cavity 104. In the acoustic cavity 104, y (t) passes through an acoustic transfer function a106 between the acoustic driver 102 and the feedback microphone 108. y (t) the result of the pass through A106 is combined with the noise sound wave N (t) to generateThe feedback microphone 108 measuresThe acoustic wave is transduced into an electrical signal e (t). The signal passes along the feedback path through a feedback factor H210.

In the forward path, the input signal x (t) is provided to a first transfer function block a₁112. The output of the feedback factor H210 is then subtracted from the output of the first transfer function block 112. In some examples, a₁The output of 112 includes only (or predominantly) frequency components of x (t) within the desired active noise reduction bandwidth, while frequencies outside the desired active noise reduction bandwidth are attenuated. The result of this subtraction is supplied to a first forward path gain element G₁116。

In parallel, the input signal x (t) is supplied to a second transfer function block A₂114. First forward path gain element G₁The output of 116 is added to the output of the second transfer function block 114. In some examples, a₂The output of 114 includes only the frequency components of x (t) that are outside the desired active noise reduction bandwidth, while frequencies within the desired active noise reduction bandwidth are attenuated. The result of this addition is supplied to a second forward path gain element G₂118. Second forward path element G₂The output of 118 is provided to the acoustic driver 102.

In some examples, the purpose of injecting different components of the input signal x (t) into the forward path at different stages is to apply a higher gain to the signal being appliedThe more important components of the input signal are considered. For example, the system of fig. 1 injects frequency components of x (t) within the active noise reduction bandwidth into the system earlier than frequency components of x (t) outside the active noise reduction bandwidth. This results in more gain (i.e., G)₁116 and G₂118) are applied to frequency components within the active noise reduction bandwidth and less gain (i.e., only G)₂118) Is applied to frequency components outside the active noise reduction bandwidth. Higher feedback gain results in greater noise reduction.

In some examples, x (t) =0 (i.e., no input signal is provided). In these examples, the active noise reduction system reduces ambient noise at the feedback microphone, thereby driving the signal detected at the microphone to zero.

In the system shown in fig. 1, e (t) is a measurement of the acoustic signal in the acoustic cavity at the location of the feedback microphone 108. In the frequency domain, E (t) can be expressed as E (ω) as follows:

g in the denominator₁G₂Thus, statements such as "loop equals 1 ∠ 180 °" should be understood as loop characteristics where the loop gain equals 1 at frequency and the loop phase equals 180 °.

From inspection, it can be seen that the noise term N (ω) is reduced as the gain of the first and second forward path gain elements 116, 118 becomes very large. In this way noise reduction of the system of fig. 1 is achieved using high loop gain.

It should also be noted that as the first and second forward path gain elements 116, 118 become very large, such as due to the inputPredicted by two injection points of the incoming signal x (t), G₁G₂A₁The ratio G of the influence of high loop gain on the X (omega) term₂A₂The AX (ω) term is small.

Referring to the portion shown in bold in fig. 1, the system includes an oscillation detector 202 configured to detect oscillations at a frequency with loop gain equal to 1 < 180 °. If oscillation is detected, the oscillation detector 202 can trigger a loop gain adjustment to return the feedback loop to a stable operating state.

The oscillation detector 202 receives the input signal x (t) and the output of the second forward path gain element 118And outputs the control parameter P to the adjustable feedback factor H210. This control parameter P indicates whether there is oscillation in the feedback loop due to instability, and commands the feedback factor H210 to adjust the loop gain (e.g., by output P = high) if necessary.

Referring to FIG. 2, the oscillation detector 202 processesThe processing of the signals is based on the knowledge that an oscillating signal, due to feedback loop instability, is typically present in a frequency range where the loop gain is close to 1 ∠ 180 deg. furthermore, an active noise reduction signal is typically present at a lower frequency than the oscillating signal.

Oscillation detector 202 processes in two separate pathsAnd x (t). The drive signal path 302 applies a band pass filter 304 toAnd the band pass filter 304 is expected to beThe frequency range of oscillations due to instability has a passband. The output of the filtered band pass filter 304 is rectified by a full wave rectifier 306 and smoothed by a smoothing element 308 (e.g., a low pass filter). The result of driving signal path 302 is in a frequency range where oscillations due to instability are expectedThe signal level of (c).

In the absence of the input signal x (t) (i.e. when no acoustic drive signal is provided), the drive signal path 302 is sufficient to detect oscillations due to instability in the feedback loop. However, in the presence of the input signal x (t), x (t) andand both. This is due to the fact that the input signal x (t) (e.g. an audio signal) may comprise frequency components present in the frequency range of the expected oscillation. In the presence of such an input signal, a false unstable detection result is expected.

Therefore, to improve the robustness of the system, x (t) is processed in the reference signal path 310 for the purpose of establishing a dynamic threshold reference. The reference signal path applies a band pass filter 312 to x (t), and the band pass filter 312 has a pass band at a frequency range where oscillations due to instability are expected. The output of the filtered band pass filter 312 is rectified by a full wave rectifier 314 and smoothed by a smoothing element 316 (e.g., a low pass filter).

The output of the smoothing element 316 is the signal level of x (t) in the frequency range expected to oscillate due to instability. This output is scaled by a scaling factor K318 so that the output of the reference signal path 310 is slightly larger than the output of the drive signal path 302 when x (t) is present and there is no oscillation in the feedback loop.

The output of drive signal path 302 and the output of reference signal path 310 are provided to differential detector 320, where differential detector 320 outputs P = high if the output of drive signal path 302 is greater than the output of reference signal path 310 (i.e., there is oscillation), and where differential detector 320 outputs P = low if the output of drive signal path 302 is less than the output of reference signal path 310 (i.e., there is no oscillation).

3.Adjustable feedback factor

The parameter P (e.g., high or low output) output by the oscillation detector 202 is provided to the adjustable feedback factor H (fig. 1, element 210). In some examples, the adjustable feedback factor 210 is adjusted based on the parameter P to modify the overall feedback loop gain of the system over all or a large frequency range. In other examples, the adjustable feedback factor 210 is adjusted based on the parameter P to modify the bandwidth of the feedback loop gain, for example, by reducing the gain over a limited frequency range. In some examples, the modification of the feedback loop gain is maintained for a predetermined amount of time. After the predetermined amount of time (e.g., 3 seconds) has elapsed, the modification of the feedback loop gain is undone (reversed).

3.1 Total gain adjustment

Referring to fig. 3, an example of a feedback loop gain and phase response illustrates an unstable condition of the feedback loop of the system of fig. 1. In particular, due to the frequency ω_uThe lower solid gain curve 420 equals 1 and the solid phase curve 422 equals 180, the feedback loop is in an unstable condition. In this case, the phase margin is 0 °, resulting in instability.

In some examples, the adjustable feedback factor 210 may be configured to mitigate this instability by reducing the gain by a predetermined amount based on the parameter P received from the instability detector 202. In particular, if P indicates that the phase margin is at or near 0 ° (i.e., the instability detector outputs a high parameter value), then the feedback factor reduces the overall gain by a predetermined amount.

The dashed gain curve 424 is the result of the overall reduction in the feedback loop gain. Since the phase curve 422 is unchanged, decreasing the overall loop gain causes the phase margin 426 to increase, thereby returning the feedback loop to a stable operating condition.

Referring to fig. 4, the circuit is configured to reduce the overall loop gain delivered over P. The overall reduction in loop gain is achieved by P = high output from the instability detector 202, opening the MOSFET530 at the feedback microphone 108, thereby reducing the loop gain at the feedback microphone input 108.

3.2 Bandwidth adjustment

Referring to fig. 5, another example of feedback loop gain and phase response illustrates an unstable condition of the feedback loop of the system of fig. 1. In particular, since the first gain curve 620 is at the frequency ω_uLower has a value of 0dB, and at this frequency omega_uThe value of the first phase curve 622 is close to-180 deg., and the feedback loop is in an unstable condition. In this case, the phase margin is reduced, causing instability.

In some examples, adjustable feedback factor 210 may be configured to switch the feedback loop gain between a high bandwidth mode and a low bandwidth mode based on parameter P. The high bandwidth mode is used during normal operation of the system, while the low bandwidth mode is used when system changes place the system in a potentially unstable operating state. If the parameter P indicates that the bandwidth of the feedback loop needs to be reduced (i.e. the instability detector output P = high parameter value), the feedback factor can be adjusted to enable low pass filtering in the feedback path.

The second loop gain curve 624 shows a reduction in loop gain at high frequencies and has little effect on loop gain at low frequencies. This reduction in the bandwidth of the loop gain results in an increase in the phase margin 626 with less impact on the audio output quality of the system when compared to the overall reduction in the loop gain previously described.

Referring to fig. 6, one example of an adjustable feedback factor 210 achieves a low bandwidth mode of feedback loop gain by switching to an existing high bandwidth feedback loop in a simple pole-zero low pass network (pole-zero low pass network) 740 when a potentially unstable operating condition is detected.

For example, the parameter output P of the instability detector (element 202, figure) can be provided to MOSFET M1742 so that a high parameter value switches M1742 to the on state. When M1742 is turned on, RC networks 744, 746 are switched into the system. The RC networks 744, 746 form a low pass filter with the effective output impedance 748 of the feedback microphone 108.

The low pass filter formed by the RC networks 744, 746 and the effective impedance 748 of the feedback microphone 108 includes zero break (caused by the inclusion of the resistor R331744). The zero-break stops the phase lag in the low-pass filter at higher frequencies, resulting in a higher margin of stability.

The adjustable feedback factor 210 as described above can be implemented using analog or digital electronics. In some examples, the parametric output P of the instability detector 202 is used to switch the compensation filter into the system using transfer functions other than those described above. In some examples, different compensation filters are used based on whether analog or digital electronics (e.g., dedicated DSP hardware) are used to implement the adjustable feedback factor.

4. Occlusion tip detection

Referring to fig. 7, an earpiece 850 of an active noise reducing headphone system is configured to be inserted into an ear canal 852 of a wearer 854. When inserted, the earplug 850 presses outwardly against the inner wall of the wearer's ear canal 852, creating a closed chamber 856 within the ear canal 852. The earplug 850 includes an internal cavity 858 that extends from an acoustic driver 860 in the earplug into a closed cavity 856 within an ear canal 852.

At the end of the interior cavity 858 of the earplug 850 opposite the acoustic driver, a blockage 862 blocks the opening of the cavity 856 where the interior cavity 858 enters the ear canal 852. Such blockage 862 typically results from the insertion of the earplug 850 by the wearer 854 into the ear canal 852, and may be referred to as an "blocked end".

Referring to fig. 8, one indication of an obstructed end is an increase in acoustic impedance in the internal cavity (fig. 7, element 858) of the earplug (fig. 7, element 850). The overhead curve (On Head curve) 970 in the figure shows the acoustic impedance of the earplug 850 without an obstructing end, and the obstructing end curve 972 in the figure shows the acoustic impedance of the earplug 850 with an obstructing end. By inspection, it is readily determined that the acoustic impedance in the case of an occluded end is significantly increased.

Referring to fig. 9, one method of detecting such a change in acoustic impedance is to use a velocity microphone 1080 in addition to a pressure microphone 1082 that has been used as the feedback microphone (fig. 1, element 108) of the active noise reduction system (i.e., the system of fig. 1).

The equation for the acoustic impedance is:

thus, acoustic impedance is determined by placing velocity microphone 1080 in close proximity to pressure microphone 1082 and calculating the ratio between the two microphone signals over a specified frequency range. If the acoustic impedance is determined to exceed a predetermined threshold, the end of the earplug may become occluded.

This approach is not affected by the nature of the sound waves (e.g., noise, voice, audio) emitted by the acoustic driver 860 inside the internal cavity 858 of the earplug 850. However, in order to calculate this ratio, there must be sufficient acoustic signal in the interior cavity 858 of the earplug 850.

To determine whether there is sufficient acoustic signal in the interior cavity 858 of the earplug, an additional pressure microphone 1084 may be included in the earplug 850 such that it is external to both the interior cavity 858 of the earplug 850 and the cavity 856 within the ear canal. This microphone 1084 may detect the pressure outside the ear chamber 856 and use it to determine whether the calculated impedance is reliable. For example, if the external pressure exceeds some predetermined threshold, the calculated impedance is considered reliable.

Combined oscillation and blocked end detector

Referring to fig. 10, oscillation detector 202 of the system of fig. 1 is enhanced using the blocked end detection algorithm as described above, resulting in a system 1100 including a combined oscillation/blocked end detector 1110.

The basic operation of the feedback loop of the system 1100 is substantially the same as described with respect to the feedback loop of the system 100 shown in fig. 1 and, therefore, will not be repeated in this paragraph.

Combined oscillation/blocking end detector 1110 outputs signals from input signal x (t), driverThe feedback pressure microphone M1108, the feedback rate microphone M21080, and the outer pressure microphone M31084 receive inputs. The output of the combined oscillation/blocked end detector 1110 is a parameter P, which is high if an oscillation or blocked end due to instability is detected. Otherwise, the value of P is low. As described above with respect to the system of fig. 1, P is provided to an adjustable feedback factor H210, which in turn adjusts the feedback loop gain or bandwidth to mitigate instability in the feedback loop.

Referring to fig. 11, a detailed block diagram of the oscillation/blocked end detector 1110 includes the oscillation detector 1202, the blocked end detector 1204, and the outside pressure detector 1206 as described above. If an oscillation or blocked end is detected, the results of oscillation detector 1202, blocked end detector 1204, and lateral pressure detector 1206 are processed using Boolean logic 1208 to produce a high parameter value. Otherwise, Boolean logic 1208 generates a low parameter value.

The blocked end detector 1204 receives as inputs a feedback pressure microphone signal M1(t) and a velocity microphone signal M2 (t). M1(t) is filtered by a first band pass filter 1210, rectified by a first full wave rectifier 1212, and smoothed by a first smoothing element 1214. M2(t) is filtered by a second bandpass filter 1216, rectified by a second full wave rectifier 1218, and smoothed by a second smoothing element 1220.

Bandpass filtering, rectification, and smoothing of the microphone input signals M1(t) and M2(t) yields estimates of the signal level in the frequencies of interest (e.g., frequencies known to significantly increase the acoustic impedance at the end of occlusion). The processed version of M1(t) is divided by the processed version of M2(t) to produce an estimate of the acoustic impedance near the microphone (fig. 10, elements 108, 1080). The estimated value of the acoustic impedance is compared with an acoustic impedance threshold value V_{Z_Ref}And (6) comparing. If the estimate of acoustic impedance is greater than the reference threshold, the blocked end detector 1204 outputs a high value indicating that the end may be blocked. Otherwise, the blocked end detector outputs a low value.

The outer pressure level detector 1206 receives as input an outer pressure microphone signal M3 (t). M3(t) is filtered by a third bandpass filter 1222, rectified by a third full wave rectifier 1224, and smoothed by a third smoothing element 1226. The output of the third smoothing element 1226 is an estimate of the sound pressure level outside the ear cavity. The sound pressure level estimated value outside the ear cavity and the outside pressure threshold value V are compared_{Pout_Ref}And (6) comparing. If the estimated value of the acoustic pressure level outside the ear cavity is greater than the outside pressure threshold, then the outside acoustic pressure level detector 1206 outputs a high value indicating that the result of the obstructed end detector 1204 is valid. Otherwise, the outside acoustic pressure level detector 1206 outputs a low value indicating that the result of the obstruction end detector 1204 is invalid.

The high or low outputs of the occlusion tip detector 1204, oscillation detector 1202, and outside acoustic pressure level detector 1206 are used as inputs to boolean logic 1208, which boolean logic 1208 determines the output P of the occlusion tip/oscillation detector 1110.

Referring to fig. 12, a truth table shows the results of applying the following boolean logic to the outputs of the blocked end detector 1204, the oscillation detector 1202, and the outside pressure level detector 1206:

6. alternative examples

In some examples, a microcontroller can be used to interpret the output of one or more of the oscillation detector, the blocked end detector, and the outside pressure level detector, and take action to reduce the loop gain.

In some examples, a dedicated digital signal processor or microcontroller performs bandpass filtering, peak detection, comparator functions, and gain reduction functions.

Referring to fig. 13, in some examples, instead of using a velocity microphone in conjunction with a feedback pressure microphone to calculate acoustic impedance, a second pressure microphone is placed inside the cavity (e.g., near the end of the mouthpiece). The acoustic impedance may be calculated as the ratio P1/(P1-P2). Fig. 13 shows an impedance curve calculated using this method. Curve 1402 is an impedance curve representing an unobstructed end. Curve 1404 is an impedance curve representing the occluded end.

In some examples, the change in acoustic impedance is detected by monitoring the electrical input impedance at the driver. In some examples, the acousto-electric conversion ratio is relatively low due to the characteristics of the driver, resulting in a poor signal-to-noise ratio. However, the characteristics of the driver can be adjusted to produce a greater ratio of acousto-electric conversion, resulting in an improved signal-to-noise ratio.

The above description has focused on a single channel of an in-ear headphone system. It should be noted, however, that the system as described above can be extended to two or more channels.

Just as the oscillation detector can be used to detect instabilities without using a stuck end detector, the stuck end detector can be used alone to detect potential instabilities without using an oscillation detector. Neither of which depend on each other and each can be used independently and effectively.

Although described in the context of an in-ear active noise cancellation system, the methods described above can be applied to other scenarios. For example, the method can be applied to an over-the-ear (over-the-ear) noise canceling headphone. More generally, the method may be applied to other audio feedback situations, in particular when the characteristics of the audio component of the feedback path may vary, e.g. the audio characteristics of a room or a vehicle cabin may change (e.g. when a door or window is opened). Furthermore, the method of oscillation and impedance detection described above may be applied to a motion control system, where the oscillation and mechanical impedance (e.g., velocity/force) of the feedback loop can be detected and measured.

In the above description, the feedback loop gain is adjusted by modifying the feedback factor in the feedback path. In some examples, instead of adjusting the feedback loop gain in the feedback path, the forward path gain element can be adjusted.

In some examples, circuitry implementing the methods described above is integrated into a housing that includes a driver and a microphone. In other examples, the circuitry is provided separately and may be configured to fit different housings and arrangements of drivers and microphones.

In some examples, in an active noise reduction system including feedback, feedforward, and sound input filtering, it is desirable to modify the filter transfer functions of all three filters (i.e., the audio input filter, feedforward filter, and feedback filter) in parallel when the instability/oscillation detector is activated. Modifying the transfer functions of all three filters in parallel compensates for the overall system response due to changes in the feedback loop gain response. This modification of the filter transfer function can occur in both analog hardware or DSP-based systems.

It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.

Claims (28)

1. A feedback-based active noise reduction system, comprising:

a feedback element comprising

A feedback input for accepting a first feedback signal from the first sensor,

a control input for accepting control parameters for adjusting gain and phase characteristics of the feedback element, an

A driver output for providing a drive signal to the driver; and

an instability detector for detecting an instability condition in the feedback element and forming the control parameter based on a result of the detection, the instability detector comprising

A control parameter output for providing said control parameter to said control parameter input of said feedback element, an

A plurality of inputs for accepting a plurality of feedback signals from a plurality of sensors including the first sensor and the second sensor,

wherein detecting the instability condition comprises processing the plurality of feedback signals to determine a characteristic of an acoustic path between the driver and the first sensor based on a ratio of the first feedback signal associated with the first sensor relative to a second feedback signal associated with the second sensor.

2. The system of claim 1, wherein the first sensor comprises a microphone and the driver comprises a speaker.

3. The system of claim 1, wherein the feedback element is configured to change one or both of the gain characteristic and the phase characteristic of the feedback element by a predetermined amount when providing the control parameter.

4. The system of claim 1, wherein the feedback element is configured to modify transfer functions of a feedback filter, a feedforward filter, and an audio input filter in parallel when providing the control parameter.

5. The system of claim 1, wherein the feedback element is configured to change a bandwidth of the feedback element by a predetermined amount when providing the control parameter.

6. The system of claim 1, wherein the feedback element further comprises a low pass filter selectably adaptable to the feedback element as a function of the control parameter.

7. The system of claim 1, wherein the ratio of the first feedback signal to the second feedback signal is representative of an acoustic impedance of the acoustic path.

8. The system of claim 1, wherein the first sensor comprises a pressure microphone and the second sensor comprises a velocity microphone.

9. The system of claim 1, wherein the first sensor comprises a pressure microphone and the second sensor comprises a pressure microphone.

10. The system of claim 1, wherein the plurality of sensors includes a third sensor for generating a third feedback signal, and the instability detector is configured to determine the validity of the instability condition detected by the instability detector based on the third feedback signal.

11. The system of claim 1, wherein the feedback element further comprises a first signal input for accepting an input signal, the instability detector further comprises a second signal input for accepting the input signal and a driver input for accepting the drive signal, and the instability detector is configured to detect the instability condition in the feedback element, including determining a characteristic of the feedback element based on the input signal and the drive signal.

12. The system of claim 11, wherein the unstable condition comprises the presence of oscillations in a specified frequency range.

13. The system of claim 12, wherein the specified frequency range is mutually exclusive from a frequency range in which active noise reduction occurs.

14. The system of claim 12, wherein the instability detector is configured to analyze the input signal and a drive signal to determine whether the oscillation is present in the drive signal and not present in the input signal.

15. A method for detecting and avoiding instability in a feedback-based active noise reduction system, the method comprising:

detecting an unstable condition in a feedback element and forming a control parameter based on a result of the detecting, the detecting the unstable condition comprising

Receiving a plurality of feedback signals from a plurality of sensors including a first sensor and a second sensor; and

processing the plurality of feedback signals to determine a characteristic of an acoustic path between a driver and the first sensor by calculating a ratio of a first feedback signal associated with the first sensor relative to a second feedback signal associated with the second sensor;

providing the control parameter to the feedback element;

receiving the control parameter at the feedback element;

receiving a first feedback signal from the first sensor at the feedback element;

adjusting a gain characteristic and a phase characteristic of the feedback element based on the control parameter; and

a driver signal is output from the feedback element to a driver.

16. The method of claim 15, wherein the first sensor comprises a microphone and the driver comprises a speaker.

17. The method of claim 15, wherein providing the control parameter to the feedback element changes one or both of the gain characteristic and the phase characteristic of the feedback element by a predetermined amount.

18. The method of claim 15, wherein providing the control parameter to the feedback element causes parallel modification of transfer functions of a feedback filter, a feedforward filter, and an audio input filter.

19. The method of claim 15, wherein providing the control parameter to the feedback element changes a bandwidth of the feedback element by a predetermined amount.

20. The method of claim 15, wherein providing the control parameter to the feedback element causes a low pass filter to be selectably applied to the feedback element based on the provided parameter.

21. The method of claim 15, wherein the ratio of the first feedback signal to the second feedback signal is representative of an acoustic impedance of the acoustic path.

22. The method of claim 15, wherein the first sensor comprises a pressure microphone and the second sensor comprises a velocity microphone.

23. The method of claim 15, wherein the first sensor comprises a pressure microphone and the second sensor comprises a pressure microphone.

24. The method of claim 15, wherein the plurality of sensors includes a third sensor for generating a third feedback signal, and detecting the instability condition includes determining the validity of the instability condition based on the third feedback signal.

25. The method of claim 15, further comprising:

an input signal is accepted at the feedback element,

wherein detecting the unstable condition further comprises

-receiving the input signal in a digital signal processing circuit,

receives the driving signal, and

determining a characteristic of the feedback element based on the input signal and the drive signal.

26. The method of claim 25, wherein the unstable condition comprises the presence of oscillation in a specified frequency range.

27. The method of claim 26, wherein the specified frequency range is mutually exclusive from a frequency range in which active noise reduction occurs.

28. The method of claim 26, wherein detecting the instability condition further comprises analyzing the input signal and the drive signal to determine whether the oscillation is present in the drive signal and not present in the input signal.