Headphone ANC and pass-through with multiple reference microphones

Abstract

The present disclosure relates to headphone ANC and pass-through with multiple reference microphones. The present disclosure discloses an earcup enclosure with several reference microphones, an error microphone, and a loudspeaker. A processor drives the loudspeaker to implement acoustic noise cancellation and pass-through by processing microphone signals, and performs a supervisory process by adjusting the reference microphone signals in response to detecting wind noise events and scratch events. In another aspect, the earcup enclosure has an outer face joined to an inner face by a perimeter, and the reference microphones are located on the perimeter. Other aspects are also described and claimed.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to headphone audio systems, and more particularly to headphones with digital audio signal processing to implement acoustic noise cancellation, ANC, and transparency using multiple reference microphones in a single earcup. BACKGROUND

[0002] Headphones enable their wearers to listen to audio programs (e.g., music, podcasts, movie soundtracks, and phone calls) without disturbing others nearby. Different headphone types include over-ear, on-ear, loose-fitting earbuds, and in-ear sealed. Headphones have different amounts of passive sound isolation from ambient noise depending on their materials and how well they fit the wearer’s head or ears. But in most cases, there is some ambient noise that leaks into the ear that the wearer can hear. A technology known as acoustic noise cancellation or active noise control, ANC, can be used to drive a speaker of the headphone to generate a sound field that is electronically designed to destructively interfere with the leaked ambient sound to create a quiet zone at the wearer’s eardrum. Another technology, referred to herein as (active) transparency, can be used to drive a speaker of the headphone to faithfully reproduce ambient sound. Transparency is useful in situations where passive sound isolation is particularly strong but the wearer sometimes also prefers to hear their surroundings without removing the headphone. SUMMARY

[0003] One aspect of the present disclosure is a headphone where an earcup housing has an outer face joined to an inner face by a periphery. Several reference microphones are located on the periphery of the earcup housing, while an error microphone and a speaker are located on the inner face of the earcup housing. A processor is configured to i) drive the speaker to implement acoustic noise cancellation, ANC, by processing reference microphone signals and error microphone signals from the reference microphones and the error microphone, and ii) drive the speaker to implement transparency (to reproduce ambient sound) by processing the reference microphone signals. The transparency function and the ANC function perform better due to the multiple reference microphones that pick up ambient sound, including sound from directional sources, especially with at least three and no more than four reference microphones. The reference microphones can all be located on the periphery. The processor can perform a supervision process to further ensure that the ANC function and the transparency function can take full advantage of the diversity of the reference microphones.

[0004] In another aspect, the headphone has several reference microphones, an error microphone, and a loudspeaker all located in its earcup enclosure. A processor i) drives the loudspeaker to implement acoustic noise cancellation by processing the reference microphone signals and the error microphone signal, ii) drives the loudspeaker to implement pass-through by processing the reference microphone signals, and iii) performs a supervision process by automatically adjusting the reference microphone signals in response to detecting wind noise events and scraping events that occur while the ANC function or the pass-through function is active. This helps the ANC function and the pass-through function to take full advantage of the diversity of the reference microphones.

[0005] The above summary does not include an exhaustive list of all aspects of the present disclosure. It is contemplated that the present disclosure includes all systems and methods that can be practiced with the various aspects disclosed above and in the detailed description below, and with the drawings attached hereto, and that such aspects can be combined with elements of the various aspects in any and all permutations. Such combinations constitute various embodiments and are within the scope of the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0006] Aspects of the present disclosure are illustrated by way of example, and not by way of limitation, in the accompanying drawings, in which like reference numerals indicate like elements. It should be noted that “a” or “one” aspect in the present disclosure is not necessarily a same aspect, and it means at least one. In addition, to be succinct and reduce the total number of drawings, a given drawing can be used to show features of more than one aspect of the present disclosure, and not all elements in such a drawing can be required for a given aspect.

[0007] Figure 1 An earcup enclosure is shown with three reference microphones.

[0008] Figure 2 An earcup enclosure is shown with four reference microphones.

[0009] Figure 3 A block diagram of a headphone audio system in which a supervision process is performed by a processor.

[0010] Figure 4 A table is shown that illustrates example decision logic used by the supervision process.

[0011] Figure 5 And Figure 6 Respective primary sound inlet openings for three reference microphones integrated into an earcup enclosure are shown by way of example.

[0012] Figure 7 Respective primary sound inlet openings for four reference microphones integrated into an earcup enclosure are shown by way of example. DETAILED DESCRIPTION

[0013] Aspects of the disclosure will now be explained, by reference to the accompanying drawings. Whenever the shapes, relative positions and other aspects of the described components are not defined explicitly, the scope of the disclosure is not limited only to the illustrated components, which are used for illustrative purposes only. Additionally, while numerous details are set forth, it is to be understood that some aspects of the disclosure can be practiced without these details. In other instances, well-known circuits, structures and techniques have not been shown in detail so as not to obscure the understanding of this description.

[0014] Figure 1 and Figure 2 An ear cup housing 6 (e.g., of a left or right ear cup of a headphone (also referred to as a head phone)) is shown. The ear cup housing 6 has an outer face that is joined to an inner face along a side perimeter of the ear cup housing 6. Note that the term “perimeter” is defined here to encompass not only the side wall of the housing 6, but also the corners or curves that are partly a part of the side wall and partly a part of the outer face. Integrated in the perimeter of the housing are two or more outer or reference microphones, in this case three reference microphones. For example, as shown in Figure 1 Reference microphone 1, reference microphone 2 and reference microphone 3 are positioned equidistant from each other, e.g., d13 = d32 = d12, while in the alternative 4-microphone arrangement shown in Figure 2 Reference microphone 4 is also present, e.g., d12 = d23 = d34 = d41. The equidistant positioning or equidistant spacing of the microphones can achieve a desired balance between sensitivity and coverage angle of the combined sound pickup response (of the microphones acting together), which can be particularly useful for picking up directional sound sources.

[0015] The ear cup is shown in Figure 1 being worn by its user, positioned against the user’s head. In this example, the ear cup is a circum-aural ear cup, e.g., as a circum-aural headphone, but alternatively it can be an on-ear ear cup. The acoustic port arrangement (or openings for the primary sound inlet) for picking up sound by the reference microphones is not configured centrally on the face of the ear cup housing, but rather in the perimeter of the ear cup housing 6, e.g., as shown in the side view of Figures 5 to 7 for various examples of ear cups. Placing the reference microphones in the perimeter in this way essentially hides their primary sound inlets from direct view of the outer surface of the ear cup, as shown in Figure 1 This provides an aesthetically clean or simple appearance for the outer face of the ear cup, while also enabling directional source sound pickup or sound pickup from different directions.

[0016] As Figures 5 to 7The earcup side view (or a direct view of its periphery) shown can see that a soft cushion 19 can be attached to the inner face of the earcup housing 6. This makes the headphones more comfortable to rest against the user's head, and it can provide greater passive acoustic isolation from ambient sound. A headband 8 can also be added, which physically attaches the right earcup to the left earcup (not shown), and enables the pair of earcups to be more easily held in position against the left and right ears.

[0017] The earcup housing 6 acts as a passive sound barrier when worn against the wearer's head or ear, which isolates the wearer from ambient sound heard. To further reduce any ambient noise (undesired sound) that leaks through this barrier, an acoustic noise cancelling ANC subsystem can be added. This ANC subsystem has a digital processor 9 configured to process the microphone signal as part of an ANC algorithm that generates anti-noise by driving an earpiece speaker 7 (one or more earpiece speakers 7) located in the inner face of the earcup housing 6, e.g. according to instructions stored in memory - not shown. Figure 3 This aspect is further described below. The ANC algorithm electronically designs the anti-noise to destructively interfere with or cancel any ambient noise that has leaked through the earcup housing into the wearer's ear. In some cases, a feedback signal from an internal or error microphone 5 can be used to improve the performance of the ANC subsystem (not shown in Figure 3 This aspect).

[0018] The digital processor 9 can also process the reference microphone signal as part of an ambient sound enhancement subsystem that reproduces (ambient sound detected by the microphone signal) by driving the earpiece speaker 7. This is also referred to herein as a pass-through function or pass-through subsystem, which enables the wearer of the earcup to better hear their surroundings (and thus not be completely isolated from the sound environment around them while wearing the headphones). A feedback signal from the error microphone 5 can be used to improve the user experience during operation of the pass-through function. For example, the output of a feedback filter 10 operating on the audio signal from the error microphone 5 can be added, as Figure 3 shown, to drive the earpiece speaker 7 in a way that reduces the undesirable occlusion effect experienced by the wearer, especially in cases where the earcup is a closed back design or otherwise has a tendency to acoustically seal the ear (from the surrounding environment). This pass-through function is further described below in connection with Figure 3 .

[0019] The performance of an ANC subsystem using a single reference microphone not located at the center of the outside of the earcup will suffer due to directional issues. For example, consider a directional environmental noise source (e.g., a closing door) located in front of the wearer. The picking up of such environmental noise by the microphone located at the back of the earcup is delayed or otherwise reduced in quality, which negatively impacts the performance of the ANC subsystem. To address such issues, one aspect of the present disclosure is a headphone audio system having Figure 1 the mechanical arrangement shown in which exactly three reference microphones are not centrally located, but are located in and along the peripheral portion of the earcup housing. The three microphones can be located at the three vertices of an equilateral triangle, respectively; another aspect is Figure 4 the arrangement depicted in which exactly four reference microphones are located in and along the peripheral portion of the square, and specifically at the four vertices, respectively. When reference is made to a microphone being located at a particular location, it will be understood that such reference also refers to the primary sound inlet opening or acoustic port of that microphone (formed in the earcup housing). Figure 5 is shown Figure 1 is shown Figure 6 is shown Figure 7 is shown Figure 2 of the 4 microphones of The variety of locations of these openings (or their respective microphones) produces early picking up of directional environmental sound by the microphone closest to such sound source, as compared to the delayed “downstream” picking up of one or more of the rest of the microphones. This improves the response of the overall sound pickup arrangement to directional sound sources.

[0020] In both cases (of Figure 1 and Figure 2 The reference microphone signals are summed into a single reference audio signal, which is input to a typical feed-forward ANC subsystem (which then drives the earpiece loudspeaker to produce anti-noise). Such an arrangement produces good ANC performance for both directional environmental noise sources and diffuse environmental noise.

[0021] Additionally, the variety of locations of the three or four reference microphones, such as in Figures 1 to 7The wind suppression algorithm (which is executed by the processor 9) is implemented in any of the examples depicted) achieves more robust audio signal processing. This wind suppression algorithm configures the processor 9 to detect which one or ones of the microphone signals are affected by wind noise (a wind detector), and in response, attenuates (e.g., deemphasizes) that microphone signal without attenuating the other microphone signals. In this way, the pass-through functionality of all of the reference microphone signals can still be used effectively even in windy environments, resulting in less wind noise being reproduced. Any suitable wind detector 12 (see Figure 3 which is also described below) can be used for this purpose, noting that in one specific example, the wind detector 12 performs digital signal processing on signals from the reference microphones 1-3 but not from the error microphone 5.

[0022] Another advantageous result associated with the three or four reference microphones being positioned diversely is that they enable a more robust audio signal processing scratch suppression algorithm. Such an algorithm (also executed by the processor 9) can detect whether any one or more of the microphone signals are subject to a scratch event (a scratch detector), e.g., due to the earcups moving against the wearer's hair, and then in response, attenuates (e.g., deemphasizes) the affected one or more microphone signals without attenuating the other microphone signals. Without the scratch suppression algorithm, the pass-through functionality can reproduce an unpleasant sound, and the ANC subsystem will be less effective at reducing environmental noise heard by the wearer. Any suitable scratch detector 13 (see Figure 3 which is also described below) can be used for this purpose, noting that in one specific example, the scratch detector 13 performs digital signal processing on signals from the reference microphones 1-3 but not from the error microphone 5.

[0023] Methods similar to the scratch suppression algorithm and the wind suppression algorithm can also be used to suppress the effects of a reference microphone signal that has been corrupted due to an ultrasonic or out-of-band directional sound source. For example, a motion detector mounted high on a ceiling or wall of a room can produce a high enough level of ultrasonic sound that corrupts or even clips the signal from a reference microphone, especially one located at the top of an earcup. The presence of the ultrasonic sound can be detected by analyzing the corrupted reference microphone signal itself, e.g., looking for certain patterns in the frequency components that are above the human hearing range but still picked up by the reference microphone. In response to detecting the ultrasonic sound, the processor 9 can decide to attenuate (e.g., deemphasize) any one or more of the corrupted reference microphone signals without attenuating the other microphone signals.

[0024] Turning now to Figure 3 this is a block diagram of a portion of a headphone audio system in which a supervisory process is executed by the processor 9 (see Figure 1 orFigure 2 ) to improve the performance of both the ANC subsystem and the pass-through functionality. This supervisory process manages in real-time which one or ones of several reference microphone signals in the earcup are adjusted (e.g., broadband attenuated or spectrally shaped) and by how much in response to outputs from the wind detector 12 and the scratch detector 13 and optionally from an ultrasonic detector (not shown). This is done prior to summing the (adjusted) microphone signals into a single reference input of the ANC anti-noise generation filter (called the feed-forward filter 14). The sum of the microphone signals is also provided to the input of the pass-through filter A, which can reduce noise in the ambient sound to be reproduced. The outputs of the feed-forward filter 14 and the pass-through filter A are then converted into sound by driving the earphone speaker 7. As shown in this block diagram, each microphone signal path is processed through a respective variable gain block (called a "gain ramp" herein) that can be changed by the supervisory process. In Figure 4 An example of the decision logic used by the supervisory process to change the gain ramps is given in the table of

[0025] Referring now to the first two rows of the table in Figure 4 , these rows describe how the processor 9 can be configured to detect wind noise events and scratch noise events, e.g., using the given detection strategies listed in the second column, and to respond to those events individually by the exemplary suppression strategies given in the third column. The detection strategies used by the wind detector 12 can be to compute coherence values and energy ratios for the reference microphones and compare them to certain thresholds. For example, up to three coherence values can be computed, each between a separate pair of the three reference microphones 1-3; for the case of four reference microphones 1-4, up to six coherence measures can be computed (each between a separate pair of the four reference microphones). Similarly, up to three energy ratios can be computed, or six energy ratios for the case of 4 microphones. The coherence values and energy ratios can be computed on a per-subband (frequency domain) basis. If the relevant thresholds are satisfied by a given set of coherence values and energy ratios, indicating that a particular reference microphone signal is now being corrupted by wind noise, then the listed suppression strategies are executed by the processor 9, which include attenuating (immediately) the affected reference microphone signal.

[0026] The detection strategies used by the scratch detector 13 can be to compute energy ratios for the reference microphones (such as on a per-subband basis) and compare them to certain thresholds. For example, up to three energy ratios can be computed, or six energy ratios for the case of 4 microphones. If the relevant thresholds are satisfied by a given set of energy ratios, indicating that a particular reference microphone signal is now being corrupted by scratch noise, then the listed suppression strategies are executed by the processor 9, which include attenuating (immediately) the affected reference microphone signal.

[0027] In one aspect, the supervisory process compensates for any one or more individual microphone signal gain reductions so as not to unduly reduce the power of the sum of all microphone signals. For example, if the gain (wideband or sub-band) on a particular microphone signal is to be reduced (e.g., attenuated) in response to detecting a scratch event or a wind event, the supervisory process can respond by also increasing the corresponding gain (wideband or corresponding sub-band) on one or more of the other microphone signals. The amount of gain compensation can be related to or dependent on the amount of reduction. This helps to reduce, if not minimize, the impact of the supervisory process, especially on the pass-through function (when the environmental sound reproduced by the pass-through function is to remain consistent or uniform during the gain adjustment). In one aspect, the supervisory process can calculate a set of target gains for all microphone signals in response to each detected scratch event or wind event, the set of target gains satisfying a target for uniform environmental sound reproduction in a particular frequency band, e.g., if each of three reference microphones 1-3 produces a power of 1 due to a scratch or wind event and the signal from one of them is to be reduced to 0.5, the signals from the other two microphones are each increased to 1.25.

[0028] In one aspect, the gain adjustment by any one or more of the gain slope blocks in the reference microphone signal path is frequency-selective or per sub-band (rather than wideband or full-band). For example, the gain slope block can be a low shelf filter. Upon command, a low (frequency) shelf filter can cut or boost frequencies below its fc, cutoff frequency, but above fc, the filter will pass its input audio signal without gain adjustment. In such a case, the above-mentioned compensation aspect can apply as follows. Consider a case where the supervisory process decides to command a cut of the low shelf filter (in the gain slope block) for reference microphone 1; in this case, the compensation capability will also command a related boost of the low shelf filter for reference microphone 2 and reference microphone 3. This low shelf behavior is consistent with the fact that the reference microphones are positioned in a single earcup, and thus, despite the diversity in location of the reference microphones, will have a similar phase response for low frequency sounds whose wavelengths are large compared to the spacing between the reference microphones in a given earcup.

[0029] Still referring to Figure 3 and Figure 4the decision logic table in FIG. 6, which shows yet another optional aspect of the supervision process, namely the suppression of howling by adding an overall howling detector 11 and its associated gain ramp block, which acts on the audio signal feedback path from error microphone 5. This overall howling detector 11 adjusts the so-called feedback gain of the feedback signal, in this case at the output of feedback filter 10, in response to detecting a howling event when processing the audio signal from error microphone 5. As Figure 4 reflected in the fourth row of the table in FIG. 6, this gain adjustment is in most cases an attenuation applied (via a separate gain ramp block) to the feedback audio signal path on a wideband gain or on a per-subband (frequency-selective) basis in response to having detected a feedback howling event using the overall howling detection process.

[0030] Another optional aspect of the supervision process, illustrated using Figure 3 and Figure 4 is the suppression of pass-through howling. This pass-through howling suppression algorithm operates in the form of a howling detector 15 and a pass-through filter B. This pass-through filter B acts on the audio signal path through pass-through filter A (as described above). A filter generator process determines (e.g., computes) the digital filter coefficients that define the transfer function of pass-through filter B. These are determined in dependence on detected pass-through howling events. As Figure 4 seen in the third row of the table in FIG. 6, the suppression strategy for responding to a detected pass-through howling event is to add a notch (or deepen an existing frequency response in) to the frequency response of pass-through filter B (a notch filter). Howling detector 15 can use a so-called dual-monocanal howling detection strategy. In this regard, processor 9 is configured to detect howling conditions not only based on its processing of the audio signal from error microphone 5 in the same ear cover as loudspeaker 7 but also based on the audio signal from another error microphone (not shown) within the complementary ear cover. Such a dual-monocanal detection strategy can compare the spectral content of the error microphones located in both ear covers in order to more reliably detect the type of howling that can be suppressed by adding or deepening a notch filter (in pass-through filter B). Note that some processing of the audio signal from the error microphone located in the other ear cover, such as the detection of the spectral content of the howling, can be performed by the processor in the other ear cover (rather than by processor 9), and the results of such processing can be transmitted from the other ear cover to processor 9.

[0031] In yet another aspect of the supervision process, also as Figure 3 and Figure 4As shown, the pass-through filter B can be "shared" by the wind suppression algorithm and the pass-through howling suppression algorithm. This is shown by the two arrows pointing from the wind detector 12 and the howling detector 15 to the filter generator block. In this regard, the filter generator determines (e.g., computes) the digital filter coefficients defining the transfer function of the pass-through filter B in dependence on both detected wind noise events and detected pass-through howling events. Referring now to the first row of the table in Figure 4 in the event of a detected wind noise, the pass-through filter B is configured (by the filter generator) to have a low shelf cut-off filter. Referring now to the third row of the table in Figure 4 Figure 4 in the event of a detected pass-through howling, the pass-through filter B is configured (by the filter generator) to also have a notch filter (e.g., centered on the dominant frequency component of the detected howling).

[0032] While certain aspects have been described and shown with reference to the attached drawing figures, it will be understood by those skilled in the art that various changes in form and detail can be made therein without departing from the spirit and scope of the disclosure. Therefore, the described aspects are to be considered in all respects as illustrative and not restrictive, and the disclosure is not to be limited to the specific structures and arrangements shown and described, since various other modifications will occur to those skilled in the art upon the reading and understanding of the disclosure.

Claims

1. A headphone comprising: an earcup housing having an outer face joined to an inner face by and along a peripheral sidewall of the earcup housing, wherein the outer face is spaced apart from the inner face by the peripheral sidewall; a plurality of reference microphones integrated in the earcup housing and respectively associated with a plurality of primary sound inlets formed on the peripheral sidewall of the earcup housing; an error microphone and a loudspeaker located on the inner face of the earcup housing; and a processor configured to: i) drive the loudspeaker to implement acoustic noise cancellation by processing a plurality of reference microphone signals and an error microphone signal from the plurality of reference microphones and the error microphone, and ii) drive the loudspeaker to implement pass-through by processing the plurality of reference microphone signals.

2. The headphone of claim 1, wherein the plurality of reference microphones is three reference microphones.

3. The headphone of claim 2, wherein the outer face has no primary sound inlets for microphones, and a soft cushion is attached to the inner face.

4. The headphone of claim 2, wherein the three reference microphones are respectively positioned at the vertices of an equilateral triangle.

5. The headphone of claim 4, wherein the outer face has no primary sound inlets for microphones.

6. The headphone of claim 1, wherein the plurality of reference microphones is four reference microphones.

7. The headphone of claim 6, wherein the outer face has no primary sound inlets for microphones.

8. The headphone of claim 6, wherein the four reference microphones are respectively positioned at the vertices of a square.

9. The headphone of claim 8, wherein the outer face has no primary sound inlets for microphones.

10. The headphone of claim 1, wherein for acoustic noise cancellation, the processor is configured to sum the plurality of reference microphones into a single reference input to an anti-noise generation filter, an output of the anti-noise generation filter driving the loudspeaker.

11. The headphone of claim 10, wherein for pass-through, the processor is configured to sum the plurality of reference microphones into a single input to a first pass-through filter, an output of the first pass-through filter driving the loudspeaker.

12. The headphone of claim 11, wherein the processor is to perform supervision by adjusting the plurality of reference microphone signals in response to detecting a wind noise event and a scratch event.

13. The headphone of claim 11, wherein the processor is to perform supervision by detecting that one or more of the reference microphone signals are affected by wind noise, and in response, attenuating the affected reference microphone signal without attenuating the other microphone signals. ​ 14. The headphone of claim 13, further comprising a second pass-through filter cascaded with the first pass-through filter, wherein the processor adjusts the second pass-through filter when it detects that one or more of the reference microphone signals are affected by wind noise.

15. The headphone of claim 14, wherein the second pass-through filter comprises a low frequency shelf cutoff filter.

16. The headphone of claim 11, wherein the processor is to perform supervision by detecting that one or more of the reference microphone signals are affected by scratch noise, and in response, attenuating the affected reference microphone signal without attenuating the other microphone signals.

17. The headphone of claim 16, further comprising a second pass-through filter cascaded with the first pass-through filter, wherein the processor adjusts the second pass-through filter when it detects that one or more of the reference microphone signals are affected by scratch noise.

18. The headphone of claim 17, wherein the second pass-through filter further comprises a notch filter.

19. A headphone comprising: an earcup housing having an outer face joined to an inner face by and along a peripheral sidewall of the earcup housing, wherein the outer face is spaced apart from the inner face by the peripheral sidewall; at least three and no more than four reference microphones integrated in the earcup housing and respectively associated with at least three and no more than four primary sound inlets formed in the peripheral sidewall of the earcup housing to respectively produce three or four reference microphone signals; an error microphone and a loudspeaker, both located on the inner face of the earcup housing, the error microphone to produce an error microphone signal; and a processor configured to: i) drive the loudspeaker to implement acoustic noise cancellation by processing the three or four reference microphone signals and the error microphone signal, and ii) drive the loudspeaker to implement pass-through by processing the three or four reference microphone signals.

20. The headphone of claim 19, wherein the outer face has no primary sound inlets for microphones, and a soft cushion is attached to the inner face.

21. The headphone of claim 20, wherein the three reference microphones are respectively positioned at the vertices of an equilateral triangle.

22. The headphone of claim 20, wherein the four reference microphones are respectively positioned at the vertices of a square.

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