阅读说明：本技术 具有可选择采样率的主动噪声消除（anc）系统 (Active Noise Cancellation (ANC) system with selectable sample rate ) 是由 加布里埃尔·沃格尔 杰弗里·艾德森 瑞安·A·海尔曼 尼廷·克瓦特拉 于 2019-01-30 设计创作，主要内容包括：一种主动噪声消除(ANC)系统,包括：可选择抽取率的抽取器,其接收过采样数字输入,并且具有选择抽取率的输入；滤波器,其接收抽取器的输出；以及可选择内插率的内插器,其接收滤波器的输出,并且具有选择内插率的输入。可选择抽取率的抽取器以及可选择内插率的内插器操作用于基于选择的抽取率和内插率为滤波器提供可选择的采样率。滤波器可以是抗噪声滤波器、反馈滤波器和/或对所述ANC系统的声学传递函数建模的滤波器。率选择可以基于电池或周围噪声水平被静态地或动态地控制。抽取率和内插率之比是固定的,与动态控制的抽取率和内插率无关。(An Active Noise Cancellation (ANC) system, comprising: a decimator having a selectable decimation rate that receives an oversampled digital input and has an input for selecting a decimation rate; a filter receiving an output of the decimator; and a selectable interpolation rate interpolator receiving the output of the filter and having an input selecting the interpolation rate. The decimator for selectable decimation rate and the interpolator for selectable interpolation rate operate to provide a selectable sample rate for the filter based on the selected decimation rate and interpolation rate. The filter may be an anti-noise filter, a feedback filter, and/or a filter that models an acoustic transfer function of the ANC system. Rate selection may be statically or dynamically controlled based on battery or ambient noise levels. The ratio of the decimation rate to the interpolation rate is fixed, independent of the decimation rate and the interpolation rate of the dynamic control.)

1. An Active Noise Cancellation (ANC) system, comprising:

a decimator having a selectable decimation rate that receives an oversampled digital input and has an input for selecting a decimation rate;

a filter receiving an output of the decimator;

a selectable interpolation rate interpolator receiving the output of the filter and having an input selecting an interpolation rate; and is

Wherein the decimator of the selectable decimation rate and the interpolator of the selectable interpolation rate are operative to provide the filter with a selectable sample rate based on the selected decimation rate and interpolation rate.

2. The ANC system according to claim 1,

wherein the filter is an adaptive filter.

3. The ANC system according to claim 2,

wherein the ANC system further comprises additional delays in one or more adaptive update paths to compensate for the decimator of the selectable decimation rate and the interpolator of the selectable interpolation rate.

4. The ANC system according to claim 1,

wherein the filter is a fixed filter.

5. The ANC system according to claim 1,

wherein the ANC system is one of the following: feed-forward ANC systems, feedback ANC systems, and hybrid ANC systems.

6. The ANC system according to claim 1,

wherein the filter is one of: an anti-noise filter, a feedback filter, and a filter that models an acoustic transfer function of the ANC system.

7. The ANC system according to claim 1,

wherein, for the ANC system, the decimation rate and interpolation rate are statically selected.

8. The ANC system according to claim 1,

wherein the decimation rate and interpolation rate are dynamically controlled; and is

Wherein the ratio of the decimation rate and the interpolation rate is fixed independent of the selected decimation rate and interpolation rate.

9. The ANC system according to claim 8,

wherein the decimation rate and interpolation rate are dynamically controlled based on a battery level in a portable device including the ANC system.

10. The ANC system according to claim 8,

wherein the decimation rate and interpolation rate are dynamically controlled based on a level of ambient noise that the ANC system is attempting to cancel.

11. The ANC system according to claim 1,

wherein the filter consumes less power when the selected sampling rate is a first sampling rate that is less than a second sampling rate than when the selected sampling rate is the second sampling rate; and is

Wherein the filter performs better noise cancellation when the selected sampling rate is the second sampling rate than when the selected sampling rate is the first sampling rate.

12. A method performed by an Active Noise Cancellation (ANC) system, comprising:

receiving, by a decimator, an oversampled digital input and a decimation rate selection;

decimating, by the decimator, the oversampled digital input at a selected decimation rate to generate an output at a selected sampling rate, the selected sampling rate being based on the selected decimation rate;

filtering, by a filter, an output of the decimator at the selected sampling rate to generate a filtered output;

receiving, by an interpolator, the filtered output and an interpolation rate selection; and

interpolating, by the interpolator, the filtered output at the selected interpolation rate.

13. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the filter is an adaptive filter.

14. The method of claim 13, further comprising:

adding a delay in one or more adaptive update paths of the ANC system to compensate for a decimator of a selectable decimation rate and an interpolator of a selectable interpolation rate.

15. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the filter is a fixed filter.

16. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the ANC system is one of the following: feed-forward ANC systems, feedback ANC systems, and hybrid ANC systems.

17. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the filter is one of: an anti-noise filter, a feedback filter, and a filter that models an acoustic transfer function of the ANC system.

18. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein, for the ANC system, the decimation rate and interpolation rate are statically selected.

19. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the decimation rate and interpolation rate are dynamically controlled; and is

Wherein the ratio of the decimation rate and the interpolation rate is fixed independent of the selected decimation rate and interpolation rate.

20. The method of claim 19, wherein the first and second portions are selected from the group consisting of,

wherein the decimation rate and interpolation rate are dynamically controlled based on a battery level in a portable device including the ANC system.

21. The method of claim 19, wherein the first and second portions are selected from the group consisting of,

wherein the decimation rate and interpolation rate are dynamically controlled based on a level of ambient noise that the ANC system is attempting to cancel.

22. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

wherein the filtering by the filter consumes less power when the selected sampling rate is a first sampling rate that is less than a second sampling rate than when the selected sampling rate is the second sampling rate; and is

Wherein the filtering by the filter performs better noise cancellation when the selected sampling rate is the second sampling rate than when the selected sampling rate is the first sampling rate.

Background

Portable audio devices such as wireless telephones (e.g., mobile/cellular telephones, cordless telephones) and other consumer audio devices (e.g., mp3 players) are widely used. The performance of portable audio devices in terms of low power consumption is desired. The performance of such devices in terms of clarity is also desired. Intelligibility may be improved by providing noise cancellation, such as Active Noise Cancellation (ANC), measuring ambient acoustic events using a microphone, and then inserting an anti-noise signal into the output of the device using signal processing to cancel the ambient acoustic events. ANC systems have stringent latency requirements. That is, the anti-noise signal must arrive in time to cancel the surrounding noise. Longer noise immunity delays degrade ANC performance.

Disclosure of Invention

In one embodiment, the present disclosure provides an Active Noise Cancellation (ANC) system, comprising: a decimator having a selectable decimation rate that receives an oversampled digital input and has an input for selecting a decimation rate; a filter receiving an output of the decimator; and a selectable interpolation rate interpolator receiving the output of the filter and having an input selecting the interpolation rate. A decimator for selectable decimation rate and an interpolator for selectable interpolation rate are used to provide a selectable sample rate for the filter based on the selected decimation rate and interpolation rate.

In another embodiment, the present disclosure provides a method performed by an Active Noise Cancellation (ANC) system, comprising: the decimator receives an oversampled digital input and a decimation rate selection; a decimator decimates the oversampled digital input at a selected decimation rate to generate an output at the selected sampling rate based on the selected decimation rate; a filter filters the output of the decimator at a selected sampling rate to generate a filtered output; the interpolator receives the filtered output and an interpolation rate selection; and an interpolator interpolates the filtered output at the selected interpolation rate.

Drawings

Fig. 1A is an illustration of an example wireless telephone, according to an embodiment of the present disclosure.

Fig. 1B is an illustration of an example wireless telephone having a headset assembly coupled thereto in accordance with an embodiment of the present disclosure.

Fig. 2 is a block diagram illustrating details of an example ANC system in accordance with an embodiment of the present disclosure.

Fig. 3 is a graph illustrating an example of the relationship between ANC system gain and phase shift, according to an embodiment of the present disclosure.

Fig. 4 is a three-dimensional graph illustrating an example of the relationship between ANC system gain, time delay, and frequency, according to an embodiment of the present disclosure.

Detailed Description

Embodiments of an ANC system having selectable sample rates for filtering processing are described. Before and after the filter of the ANC system are a decimator for selectable decimation rate and an interpolator for selectable interpolation rate, respectively. The decimator for the selectable decimation rate and the interpolator for the selectable interpolation rate operate to provide a selectable sample rate for the filter. Processing of the filter at a lower sampling rate may advantageously reduce power consumption in a portable device that includes the ANC system. However, a lower sampling rate may introduce additional latency in the ANC system. In one embodiment, for example, the decimation rate and the interpolation rate may be statically selected based on the type of portable audio device employing the ANC system. For example, in one product, a manufacturer may prefer lower power consumption rather than higher noise cancellation, in which case higher decimation and interpolation rates may be selected statically; however, in a different product, the manufacturer may prefer higher noise cancellation rather than lower power consumption, in which case lower decimation and interpolation rates may be selected. In other embodiments, the decimation rate and the interpolation rate may be dynamically controlled based on various factors (e.g., the current battery level of the portable audio device, the level of ambient noise that the ANC system is attempting to cancel, or a combination thereof). For example, if the battery power is low, the decimation rate and interpolation rate can be dynamically controlled to be higher to reduce the power consumption of the filter by lower sample rate processing; however, if the ambient noise is high, the decimation rate and interpolation rate can be dynamically controlled lower to increase the performance of the filter by reducing the delay and improving the sample rate processing. The ratio of decimation rate to interpolation rate is fixed regardless of the dynamically selected decimation rate and interpolation rate. The filter may be an adaptive filter or a fixed-type filter, and may be an anti-noise filter, a feedback filter, and/or a filter that models the acoustic transfer function of the ANC system. The ANC system may be a feed-forward, feedback, or hybrid ANC system. The ANC system may also include additional delay in the adaptive update path to compensate for the decimator/interpolator for the selectable decimation rate/interpolation rate.

Referring now to FIG. 1A, a radiotelephone 10 is shown in proximity to a human ear 5 in accordance with embodiments of the present invention. The radiotelephone 10 is an example of a portable audio device to which techniques according to embodiments of the present disclosure may be applied, but it should be understood that not all of the elements or configurations included in the illustrated radiotelephone 10 or in the circuitry depicted in subsequent illustrations are required to practice the invention recited in the claims. The wireless telephone 10 may include a transducer such as a speaker SPKR that reproduces long-range speech received by the wireless telephone 10, as well as other local audio events such as ringtones, stored audio programming material, injection of near-end speech (i.e., speech of the user of the wireless telephone 10) to provide balanced conversational perception, and other audio that needs to be reproduced by the wireless telephone 10, such as sources from web pages or other network communications received by the wireless telephone 10, as well as audio indications such as battery low indications and other system event notifications. A near-speech microphone NS may be provided to capture near-end speech that is transmitted from the wireless telephone 10 to the other conversation participant or participants.

Wireless telephone 10 may include ANC circuitry and features that inject an anti-noise signal into speaker SPKR to improve the intelligibility of distant speech and other audio reproduced by speaker SPKR. A reference microphone R may be provided to measure the surrounding acoustic environment and may be placed at a typical location away from the user's mouth so that near-end speech may be minimized in the signal produced by the reference microphone R. When the wireless telephone 10 is very close to the ear 5, a further microphone (error microphone E) may be provided to further improve ANC operation by providing a measure of the ambient audio combined with the audio reproduced by the speaker SPKR close to the ear 5. In other embodiments, additional reference microphones and/or error microphones may be employed. Circuitry 14 within wireless telephone 10 may include an audio CODEC Integrated Circuit (IC)20 that receives signals from reference microphone R, near speech microphone NS, and error microphone E, and interacts with other integrated circuits having a wireless telephone transceiver, such as a Radio Frequency (RF) integrated circuit 12. In some embodiments of the present disclosure, the circuits and techniques disclosed herein may be incorporated in a single integrated circuit that includes the control circuitry as well as other functionality for implementing the entirety of a portable audio device, such as an on-chip MP3 player integrated circuit. In these and other embodiments, the circuits and techniques disclosed herein may be implemented, in part or in whole, in software and/or firmware embodied in a computer-readable medium and executable by a controller or other processing device.

In general, the ANC techniques of this disclosure measure ambient acoustic events impinging on reference microphone R (as opposed to the output of speaker SPKR and/or near-end speech), and by also measuring the same ambient acoustic events impinging on error microphone E, ANC processing circuitry of wireless telephone 10 adapts the anti-noise signal generated from the output of reference microphone R to have characteristics that minimize the magnitude of the ambient acoustic events at error microphone E. Because acoustic path p (z) extends from reference microphone R to error microphone E, ANC circuitry effectively estimates acoustic path p (z) while canceling the effects of electro-acoustic path s (z), which represents the response of the audio output circuitry of CODEC IC 20 and the acoustic/electrical transfer function of speaker SPKR including the coupling of speaker SPKR to error microphone E in the particular acoustic environment, which may be affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10 when wireless telephone 10 is not firmly pressed against ear 5. Although the illustrated wireless telephone 10 includes a dual microphone ANC system with a third near-speech microphone NS, aspects of the present invention may be practiced in systems that do not include separate error and reference microphones, or in wireless telephones that use near-speech microphone NS to perform the function of reference microphone R.

Referring now to fig. 1B, a wireless telephone 10 is depicted having a headset assembly 13 coupled thereto via an audio port 15. Audio port 15 may be communicatively coupled to RF integrated circuit 12 and/or CODEC IC 20, allowing communication between components of headphone assembly 13 and one or more of RF integrated circuit 12 and/or CODEC IC 20 (e.g., fig. 1A). In other embodiments, the headset assembly 13 may be wirelessly connected to the wireless telephone 10, for example, via bluetooth or other short-range wireless technology. As shown in fig. 1B, the headphone assembly 13 may include a combox (combox)16, a left headphone 18A, and a right headphone 18B. As used in this disclosure, the term "headset" broadly includes any speaker and structure associated therewith that is intended to be mechanically held near the ear canal of a listener and includes, but is not limited to, earpieces (earphones), earplugs (earruds), and other similar devices. As more specific examples, the "headphone" may refer to an earphone (intra-concha earphone), a headphone (supra-concha earphone), and a headphone (supra-aural earphone).

The combiner box 16 or another portion of the headset assembly 13 may also have a near-speech microphone NS for capturing near-speech in addition to or in lieu of the near-speech microphone NS of the wireless telephone 10. In addition, each earpiece 18A, 18B may include a transducer, such as a speaker SPKR, that reproduces long-range speech received by the wireless telephone 10, as well as other local audio events, such as ringtones, stored audio programming material, injection of near-end speech (i.e., speech of the user of the wireless telephone 10) to provide balanced conversational perception, and other audio that needs to be reproduced by the wireless telephone 10, such as sources from web pages or other network communications received by the wireless telephone 10, as well as audio indications, such as battery low indications and other system event notifications. Each earpiece 18A, 18B may include a reference microphone R for measuring the ambient acoustic environment and an error microphone E for measuring the ambient audio that is combined with the audio reproduced by the speaker SPKR proximate the ear of the listener when such an earpiece 18A, 18B engages the ear of the listener. In some embodiments, CODEC IC 20 may receive signals from reference microphone R, near-speech microphone NS, and error microphone E of each headset and perform adaptive noise cancellation for each headset as described herein.

In other embodiments, a CODEC IC similar to CODEC IC 20 of fig. 1A or another circuit may be present within headset assembly 13 and communicatively coupled to reference microphone R, near-speech microphone NS, and error microphone E and configured to perform adaptive noise cancellation as described herein. In such an embodiment, there may also be an acoustic path for the headphone assembly 13 with a transfer function p (z) extending from the reference microphone R to the error microphone E similar to that described for fig. 1A. Furthermore, in such embodiments, there may also be an electro-acoustic path for the headphone assembly 13 similar to that described for fig. 1A with a transfer function s (z) representing the response of the audio output circuitry of the CODEC IC of the headphone assembly 13 and the acoustic/electrical transfer function of the speaker SPKR including the coupling between the speaker SPKR and the error microphone E.

Referring now to FIG. 2, details of an example ANC system 201 are shown, in accordance with an embodiment of the present disclosure. In some embodiments, the ANC system 201 may be used to implement an ANC system in a portable audio device (e.g., the wireless telephone 10 of fig. 1A or the headset assembly 13 of fig. 1B). The ANC system 201 includes a reference microphone R (e.g., the reference microphone R of fig. 1A or 1B) that converts ambient audio into a reference microphone signal that is provided to an analog-to-digital converter (ADC)202, the ADC202 generating a digital representation of the reference microphone signal at a reference input sampling rate. The ANC system 201 also includes an error microphone E (e.g., the error microphone E of fig. 1A or 1B) that converts ambient audio combined with audio output by a speaker SPKR (e.g., the SPKR of fig. 1A or 1B) into an error microphone signal that is provided to a second ADC 228, the second ADC 228 generating a digital representation of the error microphone signal at an error input sampling rate. The first decimator 204 receives a digital representation of the reference microphone signal at a reference input sample rate and selectively reduces it to a reference output sample rate according to a decimation rate N indicated by a control input to the decimator 204. In general, a decimator receives a digital input having a first sample rate and provides a digital output at a second sample rate that is less than the first sample rate. For example, if N is 4, the output sample rate of the decimator is one quarter of its input sample rate. The second decimator 208 receives the digital representation of the error microphone signal at the error input sample rate and selectively reduces it to the error output sample rate according to a decimation rate N indicated by a control input to the decimator 208. A third decimator 212 receives the digital playback/downlink signal at an input sample rate and selectively reduces it to an output sample rate according to a decimation rate N indicated by a control input to decimator 212. The input signal to the third decimator 212 may also include, for example, a sidetone derived from a signal generated by a near-speech microphone (e.g., the near-speech microphone NS of fig. 1A or 1B). Preferably, the decimator 204/208/212 receives its digital input signal at a sampling rate that is higher than the nyquist rate, i.e., the digital input signal is oversampled. In one embodiment, the sample rate into each of the decimators 204, 208, and 212 is the same, and N is the same for all of the decimators 204, 208, and 212, such that the sample rate output of each of the decimators 204, 208, and 212 is the same. However, other embodiments are contemplated in which one or more of decimators 204, 208, and 212 have different input sample rates, and N is different for one or more of decimators 204, 208, and 212, such that the sample rate output of each of decimators 204, 208, and 212 is the same. For example, decimator 204 may have an input sample rate of 6MHz and a decimation rate N of 8 such that its output sample rate is 750kHz, decimator 208 may have an input sample rate of 3MHz and a decimation rate N of 4 such that its output sample rate is 750kHz, and decimator 212 may have an input sample rate of 1.5MHz and a decimation rate N of 2 such that its output sample rate is 750 kHz. As described in more detail below, in designs and/or scenarios where reduced noise cancellation may be acceptable (e.g., due to increased latency), the selectable decimation rate N (in conjunction with the selectable interpolation rate M, which will be described in more detail below) may advantageously enable digital filters (e.g., filters 232, 234, 235, 216, described below) of the ANC system 201 to process at a lower sampling rate, and thereby reduce power consumption in exchange for potentially reduced noise cancellation relative to processing at a higher sampling rate.

The anti-noise filter 232 receives and filters the reference microphone signal from the decimator 204 to generate an anti-noise signal that is provided to the combiner 215. The sample rate of the reference microphone signal received by the anti-noise filter 232 is determined by the sample rate output by the ADC202 and by the decimation rate N selected for the decimator 204. The filter 232 processes the reference microphone signal at a selectable sample rate output by the decimator 204. Thus, if a higher decimation rate N is selected for decimator 204, filter 232 may consume less power; however, a higher decimation rate N may introduce more delay than selecting a lower decimation rate N, which may result in lower noise cancellation performance of the ANC system 201.

In the embodiment shown in FIG. 2, the anti-noise filter 232 is an adaptive filter; however, in other embodiments, the anti-noise filter 232 is a fixed-type filter. In the embodiment shown in fig. 2, the anti-noise filter 232 is a Finite Impulse Response (FIR) filter having a transfer function W1(z), and is referred to as a W1(z) FIR filter 232. The anti-noise filter 232 may adjust its transfer function W1(z) to P (z)/S (z), e.g., the transfer functions of the acoustic path P (z) and the electro-acoustic path S (z) of FIG. 1A or FIG. 1B, respectively. The coefficients of the anti-noise filter 232 may be controlled by a W1(z) coefficient adjustment block 231, which W1(z) coefficient adjustment block 231 uses the correlation of the signals from the reference microphone R and the error microphone E to determine the response W1(z) of the anti-noise filter 232, which typically minimizes the error between those components of the reference microphone signal present in the error microphone signal in a least mean square manner. The signals compared by the W1(z) coefficient adjustment block 231 may be a playback corrected error (PBCE) signal (which is based at least in part on the error microphone signal and is described more below) and a reference microphone signal shaped by a filter 235 (referred to as SE _ copy (z) FIR filter 235 in fig. 2). Filter 235 is a copy of filter 234 (referred to as se (z) FIR filter 234 in fig. 2), which is an estimate or model of the acoustic transfer function of path s (z).

The filter 234 filters the playback/downlink signal to generate a signal representative of the intended playback/downlink audio that is passed to the error microphone E. The sampling rate of the playback/downlink signal received by filter 234 is determined by the sampling rate of the playback/downlink signal and by the decimation rate N selected for decimator 212. The filter 234 processes the playback/downlink signal at a selectable sample rate output by the decimator 212. Thus, if a higher decimation rate N is selected for decimator 212, filter 234 may consume less power.

The combiner 236 generates the PBCE signal by subtracting the desired playback/downlink audio signal produced by the filter 234 from the error microphone signal (more precisely, the version of the error microphone signal whose sampling rate is selectively reduced by the decimator 208). The PBCE signal is provided to W1(z) coefficient adjustment block 231, se (z) coefficient adjustment block 233, and feedback filter 216. The filter 234 may have coefficients controlled by an se (z) coefficient adjustment block 233, and the se (z) coefficient adjustment block 233 may compare a version of the playback/downlink signal whose sampling rate is selectively reduced by the decimator 212 with the PBCE signal. The PBCE signal is equal to the error microphone signal after removal of the playback/downlink signal filtered by the filter 234, which filter 234 filtered playback/downlink signal represents the intended playback/downlink audio passed to the error microphone E. In other words, the PBCE signal includes the content of the error microphone signal that is not caused by the playback/downlink signal. Se (z) coefficient adjustment block 233 may correlate the playback/downlink signal with components of the playback/downlink signal present in the error microphone signal and responsively adjust the coefficients of filter 234. The filter 234 may thus be adapted to generate an estimation signal based on the replay/downlink signal, which is subtracted from the error microphone signal to generate the PBCE signal.

Feedback filter 216 provides a filtered version of the PBCE signal to combiner 215. The sampling rate of the PBCE signal received by the feedback filter 216 is determined by the sampling rate of the error microphone signal and by the decimation rate N selected for the decimator 208. Feedback filter 216 processes the PBCE signal at a selectable sample rate output by decimator 208. Thus, if a higher decimation rate N is selected for the decimator 208, the feedback filter 216 may consume less power; however, a higher decimation rate N may introduce more delay than selecting a lower decimation rate N, which may result in lower noise cancellation performance of the ANC system 201.

The combiner 215 combines the filtered version of the PBCE signal with the anti-noise signal and provides a modified anti-noise signal to the interpolator 218. In general, an interpolator receives a digital input having a first sample rate and provides a digital output having a second sample rate that is greater than the first sample rate. The interpolator 218 increases the sampling rate of the modified anti-noise signal according to an interpolation rate M indicated by a control input to the interpolator 218. For example, if M is 8, the output sample rate of the interpolator 218 is eight times its input sample rate. The second combiner 221 subtracts the output of the interpolator 218 from the playback/downlink signal to generate a digital, noise-immune playback/downlink signal that is provided to a digital-to-analog converter (DAC)222, which DAC 222 generates an analog representation of the noise-canceled playback/downlink signal. The analog noise-canceled playback/downlink signal is amplified by an amplifier 224 to be supplied to the speaker SPKR.

In one embodiment, variable delay 206 is introduced to the reference output sample rate reference microphone signal provided by decimator 204 to filter 235. The delay introduced from the interpolator 218 and decimator 208 is a major contribution to the amount of variable delay 206 that can be configured. Since the ANC system 201 of fig. 2 includes both feedforward anti-noise (e.g., provided by the adaptive filter 232) and feedback anti-noise (e.g., provided by the filter 216), it may be characterized as a hybrid ANC system. However, in other embodiments, the ANC system may be only a feed-forward ANC system or a feedback ANC system.

Traditionally, the filters of an ANC system may consume a relatively large amount of power. Advantageously, the amount of power consumed by the filters of the embodiments of the ANC system described herein may be influenced by the selection of the decimation rate N of the decimator and the interpolation rate M of the interpolator, respectively, with one or more filters interposed between the decimator and the interpolator, and with the decimator and interpolator operating to provide selectable input sample rates to the filters. As mentioned above, the decimation rate N and the interpolation rate M are selectable rates, e.g. 1, 2, 4, 8. For example, if N is 4, the output sample rate of the decimator is one-fourth of its input sample rate, and the filters of the ANC system that receive the output of the decimator (e.g., the anti-noise filter 232, the feedback filter 216, and/or the acoustic transfer function estimation filters 234 and 235 of fig. 2) are processed at one-fourth of the output sample rate, and therefore consume less power than filters that are processed at higher input sample rates. In one embodiment, the values of N and M need not be the same. In embodiments where the values of N and M are dynamically selected, the ratio of N and M remains the same each time a new value is selected. For lower sample rates and corresponding lower power consumption of the filters of the ANC system 201 due to lower sample rate processing, a larger value of N may be selectedThis may result in lower resolution due to increased latency in the ANC system 201; however, for lower latency and higher definition performance, a smaller value of N may be chosen, which may result in higher power consumption of the filter due to higher sample rate processing. The filters 232, 234, 235 and 216 each include an input (not shown) specifying an input sample rate that is a function of their respective selectable decimation rate N. In one embodiment, one or more of filters 232, 234, 235 and 216 is z^-NFilters, which can automatically adjust their structure based on a specified sampling rate so that their filter response remains constant regardless of the selected sampling rate.

Referring now to fig. 3, a graph illustrating an example of the relationship between ANC system gain and phase shift is shown, in accordance with an embodiment of the present disclosure. The phase shift measured in degrees is represented on the horizontal axis in the figure. In the figure, the phase shift values range between 0 and 30 degrees. The maximum ANC gain, measured in decibels (dB), is represented on the vertical axis in the graph. In the figure, the range of maximum ANC gain values is between 0dB and infinity. The phase shift, which is a measure of time delay, represents a phase difference between ambient noise received at a reference microphone (e.g., reference microphone R of fig. 2) and a component of audio generated by a speaker (e.g., speaker SPKR of fig. 2) that is attributable to an anti-noise signal generated by an anti-noise filter (e.g., anti-noise filter 232 of fig. 2). The phase shift may be caused, at least in part, by decimation and interpolation performed by a decimator (e.g., decimator 204/208/212 of fig. 2) and an interpolator (e.g., interpolator 218 of fig. 2) whose decimation/interpolation rates are selectable in accordance with the described embodiments. The maximum ANC gain represents the maximum level of ambient noise that an ANC system (e.g., ANC system 201 of fig. 2) is capable of canceling at a given phase shift, measured at a reference microphone (e.g., reference microphone R of fig. 2). At zero degree phase shift, the maximum achievable ANC gain is infinite; however, as shown, as the phase shift approaches zero degrees (e.g., at about 0.1 degrees), the maximum achievable ANC gain is about 55dB, and at 30 degrees phase shift, the maximum achievable ANC gain is about 6 dB. From 0 degrees to 30 degrees, the maximum gain value decreases in an approximately exponential manner. It can be seen from fig. 3 that in order to achieve a lower input sample rate to reduce the power consumption of the add-in filter, a selected greater decimation rate/interpolation rate of the decimator/interpolator will correspondingly reduce the amount of noise cancellation achievable by the ANC system.

Referring now to fig. 4, a three-dimensional graph of an example of the relationship between ANC system gain, time delay, and frequency is shown, in accordance with an embodiment of the present disclosure. As shown in fig. 3, the maximum ANC gain, measured in decibels (dB), is represented on the vertical axis, and the maximum ANC gain value ranges between 0dB and infinity. The time delay, measured in microseconds (mus), is shown on one horizontal axis in the graph, with values ranging between 0 and 40 mus. On the other horizontal axis, the frequency measured in hertz (Hz) is represented, with values ranging between 0Hz and 1000 Hz. Typically, the maximum achievable ANC gain decreases approximately exponentially with increasing time delay, and the maximum achievable ANC gain decreases approximately exponentially with increasing frequency. Thus, it can be observed that the time delay becomes more critical at higher frequencies. At zero delay, the maximum achievable ANC gain is infinite. However, as the time delay approaches zero microseconds at 0Hz (e.g., at about 0.1 degrees), the maximum achievable ANC gain is about 60 dB; at a time delay of 40 microseconds and at 1000Hz, the ANC system is limited to about 12dB of cancellation. The limitations include both silicon time delay and phase response of the speaker. A longer delay in the acoustic path p (z) extending from the reference microphone to the error microphone may help offset the increase in delay caused by the higher decimation rate. It can be seen from fig. 4 that, in order to achieve a lower input sampling rate to reduce the power consumption of the add-in filter, a selected greater decimation rate/interpolation rate of the decimator/interpolator will correspondingly reduce the amount of noise cancellation achievable by the ANC system, especially at higher frequencies when the ambient noise level is greater.

From the foregoing description, it can be seen that the advantages of inserting one or more filters of an ANC system between a decimator having a selectable decimation rate and an interpolator having a selectable interpolation rate can be obtained. First, a single product may be configured as a high performance or low power product. For example, a headset manufacturer may select a selected configuration based on the power/performance goals of the headset. Second, the system may change dynamically. For example, when the ambient noise level is low, the performance of the ANC system may be reduced by dynamically reducing the decimation rate and interpolation rate, since noise cancellation (if any) is not required urgently. For another example, when the battery level of the portable audio device becomes low, the battery time may be extended by reducing the ANC system performance by dynamically reducing the decimation rate and interpolation rate.

It should be understood that, particularly those of ordinary skill in the art having the benefit of the present disclosure, that various operations described herein, and in particular those described in conjunction with the figures, may be implemented by other circuits or other hardware components. The order of performing each operation of a particular method can be changed, and various elements of the systems illustrated herein can be added, reordered, combined, omitted, modified, etc., unless otherwise noted. The disclosure is intended to embrace all such modifications and variations, and therefore the above description should be taken as illustrative and not restrictive.

Similarly, while the present disclosure is directed to particular embodiments, certain modifications and changes may be made to those embodiments without departing from the scope and coverage of the present disclosure. Furthermore, any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element.

Likewise, other embodiments having the benefit of the present disclosure will be apparent to those of ordinary skill in the art and such embodiments are to be considered as included herein. All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the present disclosure and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

The present disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Furthermore, it is intended that an apparatus, system or component of an apparatus or system of the appended claims be adapted, arranged, capable, configured, enabled, operable or operative to perform a particular function encompasses that apparatus, system or component, whether or not that apparatus, system or component or that particular function is activated, enabled or unlocked, so long as that apparatus, system or component is so adapted, arranged, capable, configured, enabled, operable or operative.