AUDIO SYSTEM AND SIGNAL PROCESSING METHOD FOR AN EAR MOUNTABLE PLAYBACK DEVICE

Abstract

An audio system for an ear mountable playback device includes a speaker, an error microphone, which senses sound being output from the speaker, and a sound control processor. The processor is configured for controlling and/or monitoring a playback of a detection signal or a filtered version of the detection signal via the speaker, recording an error signal from the error microphone, and determining whether the playback device is in a first state, where the playback device is worn by a user, or in a second state, where the playback device is not worn by a user, based on processing of the error signal.

Claims

1. An audio system for an ear mountable playback device comprising a speaker, a feedforward microphone that predominantly senses ambient sound and an error microphone that senses sound being output from the speaker, the audio system being configured to perform noise cancellation and comprising a sound control processor that is configured to recording a noise signal from the feedforward microphone and using the noise signal as a detection signal; filtering the detection signal with a feedforward filter; controlling a playback of the filtered detection signal via the speaker; recording an error signal from the error microphone; and determining whether the playback device is in a first state, where the playback device is worn by a user, or in a second state, where the playback device is not worn by a user, based on processing of the error signal.

2. The audio system according to claim 1, wherein the sound control processor is configured to determine the first state and/or the second state based on an evaluation of a performance of the noise cancellation as a function of the error signal and the detection signal.

3. The audio system according to claim 1, wherein the sound control processor is configured to determine the second state based on an evaluation of a performance of the noise cancellation as a function of the error signal and the noise signal or detection signal.

4. The audio system according to claim 2, which further comprises a voice activity detector for determining whether a voice signal is recorded with the error microphone and/or the feedforward microphone, wherein the sound control processor is configured to pause a determination of the first and/or the second state, if the voice signal is determined to be recorded.

5. The audio system according to claim 2, wherein the sound control processor is configured to evaluate the performance of the noise cancellation by determining an energy ratio between the error signal and the noise signal or detection signal.

6. The audio system according to claim 5, wherein the sound control processor is configured, if a music signal is additionally played via the speaker, to take an energy level of the music signal into account when determining the energy ratio.

7. The audio system according to claim 2, wherein a filter response of the feedforward filter is constant and/or is kept constant by the sound control processor at least during the determination of the state of the play back device.

8. The audio system according to claim 1, wherein the sound control processor is configured to adjust a filter response of the feedforward filter based on the error signal; and to determine the second state based on an evaluation of the filter response of the feedforward filter at at least one predetermined frequency.

9. The audio system according to claim 8, wherein the sound control processor is configured to determine the second state if the filter response of the feedforward filter at the at least one predetermined frequency exceeds a response threshold value.

10. The audio system according to claim 8, wherein the sound control processor is configured to determine the second state by determining a linear regression of the filter response of the feedforward filter in a predefined frequency range, the linear regression being defined by at least a filter gradient and a filter gain, and by evaluating the filter gradient and/or the filter gain.

11. The audio system according to claim 10, wherein the sound control processor is configured to determine the second state if at least one of the following applies: the filter gradient exceeds a threshold gradient value; the filter gain exceeds a threshold gain value.

12. (canceled)

13. The audio system according to claim 1, wherein the sound control processor is configured to determine the first state based on an evaluation of a phase difference between the detection signal and the error signal.

14. The audio system according to claim 13, wherein the sound control processor is configured to determine the first state, if the phase difference between the detection signal and the error signal exceeds a phase threshold value at one or more predefined frequencies.

15. The audio system according to claims 1, wherein the detection signal is an identification signal, and wherein the sound control processor is configured to control and/or monitor the playback of the identification signal via the speaker; to filter the identification signal with an adjustable filter; to adjust the adjustable filter based on a difference between the filtered identification signal and the error signal, in particular such that the adjustable filter approximates an acoustic transfer function between the speaker and the error microphone; and to determine the second state based on an evaluation of a filter response of the adjustable filter at at least one further predetermined frequency.

16. The audio system according to claim 15, wherein the identification signal is one of the following or a combination of one of the following: a music signal; a payload audio signal; a filtered version of a noise signal that is recorded from a microphone predominantly sensing ambient sound.

17. The audio system according to one of claim 15, wherein the sound control processor is configured to determine the second state by determining a linear regression of the filter response of the adjustable filter in a further predefined frequency range, the linear regression being defined by at least an identification filter gradient and an identification filter gain, and by evaluating the identification filter gradient and/or the identification filter gain.

18. The audio system according to claim 1, wherein the sound control processor is configured to control the audio system to a low power mode of operation, if the second state is determined, and to regular mode of operation, if the first state is determined.

19. The audio system according to claim 1, wherein the sound control processor is configured to determine whether the playback device is in the first state, only if the playback device is in the second state, and to determine whether the playback device is in the second state, only if the playback device is in the first state.

20. The audio system according to claim 1, wherein the playback device is a headphone or an earphone or a mobile phone.

21. A signal processing method for an ear mountable playback device comprising a speaker, a feedforward microphone that predominantly senses ambient sound and an error microphone that senses sound being output from the speaker, the method comprising recording a noise signal from the feedforward microphone and using the noise signal as a detection signal; filtering the detection signal with a feedforward filter; controlling a playback of the filtered detection signal via the speaker; recording an error signal from the error microphone; performing noise cancellation based on at least one of the noise signal and the error signal; and determining whether the playback device is in a first state, where the playback device is worn by a user, or in a second state, where the playback device is not worn by a user, based on processing of the error signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The improved concept will be described in more detail in the following with the aid of drawings. Elements having the same or similar function bear the same reference numerals throughout the drawings. Hence their description is not necessarily repeated in following drawings.

[0067] In the drawings:

[0068] FIG. 1 shows a schematic view of a headphone;

[0069] FIG. 2 shows a block diagram of a generic adaptive ANC system;

[0070] FIG. 3 shows an example representation of a “leaky” type earphone;

[0071] FIG. 4 shows an example headphone worn by a user with several sound paths from an ambient sound source;

[0072] FIG. 5 shows an example representation of an ANC enabled handset;

[0073] FIG. 6 shows a phase diagram for different wearing or leakage states of a playback device;

[0074] FIG. 7 shows a block diagram of a system with an adjustable identification filter; and

[0075] FIG. 8 shows a block diagram of a further system with an adjustable identification filter.

DETAILED DESCRIPTION

[0076] FIG. 1 shows a schematic view of an ANC enabled playback device in form of a headphone HP that in this example is designed as an over-ear or circumaural headphone. Only a portion of the headphone HP is shown, corresponding to a single audio channel. However, extension to a stereo headphone will be apparent to the skilled reader for this and the following disclosure. The headphone HP comprises a housing HS carrying a speaker SP, a feedback noise microphone or error microphone FB_MIC and an ambient noise microphone or feedforward microphone FF_MIC. The error microphone FB_MIC is particularly directed or arranged such that it records both sound played over the speaker SP and ambient noise. Preferably the error microphone FB_MIC is arranged in close proximity to the speaker, for example close to an edge of the speaker SP or to the speaker's membrane, such that the speaker sound may be the predominant source for recording. The ambient noise/feedforward microphone FF_MIC is particularly directed or arranged such that it mainly records ambient noise from outside the headphone HP. Still, negligible portions of the speaker sound may reach the microphone FF_MIC.

[0077] Depending on the type of ANC to be performed, the ambient noise microphone FF_MIC may be omitted, if only feedback ANC is performed. The error microphone FB_MIC may be used according to the improved concept to provide an error signal being the basis for a determination of the wearing condition, respectively leakage condition, of the headphone HP, when the headphone HP is worn by a user.

[0078] In the embodiment of FIG. 1, a sound control processor SCP is located within the headphone HP for performing various kinds of signal processing operations, examples of which will be described within the disclosure below. The sound control processor SCP may also be placed outside the headphone HP, e.g. in an external device located in a mobile handset or phone or within a cable of the headphone HP.

[0079] FIG. 2 shows a block diagram of a generic adaptive ANC system. The system comprises the error microphone FB_MIC and the feedforward microphone FF_MIC, both providing their output signals to the sound control processor SCP. The noise signal recorded with the feedforward microphone FF_MIC is further provided to a feedforward filter for generating and anti-noise signal being output via the speaker SP. At the error microphone FB_MIC, the sound being output from the speaker SP combines with ambient noise and is recorded as an error signal that includes the remaining portion of the ambient noise after ANC. This error signal is used by the sound control processor SCP for adjusting a filter response of the feedforward filter.

[0080] FIG. 3 shows an example representation of a “leaky” type earphone, i.e. an earphone featuring some acoustic leakage between the ambient environment and the ear canal EC. In particular, a sound path between the ambient environment and the ear canal EC exists, denoted as “acoustic leakage” in the drawing.

[0081] FIG. 4 shows an example configuration of a headphone HP worn by a user with several sound paths. The headphone HP shown in FIG. 4 stands as an example for any ear mountable playback device of a noise cancellation enabled audio system and can e.g. include in-ear headphones or earphones, on-ear headphones or over-ear headphones. Instead of a headphone, the ear mountable playback device could also be a mobile phone or a similar device.

[0082] The headphone HP in this example features a loudspeaker SP, a feedback noise microphone FB_MIC and, optionally, an ambient noise microphone FF_MIC, which e.g. is designed as a feedforward noise cancellation microphone. Internal processing details of the headphone HP are not shown here for reasons of a better overview.

[0083] In the configuration shown in FIG. 4, several sound paths exist, of which each can be represented by a respective acoustic response function or acoustic transfer function. For example, a first acoustic transfer function DFBM represents a sound path between the speaker SP and the feedback noise microphone FB_MIC, and may be called a driver-to-feedback response function. The first acoustic transfer function DFBM may include the response of the speaker SP itself. A second acoustic transfer function DE represents the acoustic sound path between the headphone's speaker SP, potentially including the response of the speaker SP itself, and a user's eardrum ED being exposed to the speaker SP, and may be called a driver-to-ear response function. A third acoustic transfer function AE represents the acoustic sound path between the ambient sound source and the eardrum ED through the user's ear canal EC, and may be called an ambient-to-ear response function. A fourth acoustic transfer function AFBM represents the acoustic sound path between the ambient sound source and the feedback noise microphone FB_MIC, and may be called an ambient-to-feedback response function.

[0084] If the ambient noise microphone FF_MIC is present, a fifth acoustic transfer function AFFM represents the acoustic sound path between the ambient sound source and the ambient noise microphone FF_MIC, and may be called an ambient-to-feedforward response function.

[0085] Response functions or transfer functions of the headphone HP, in particular between the microphones FB_MIC and FF_MIC and the speaker SP, can be used with a feedback filter function B and feedforward filter function F, which may be parameterized as noise cancellation filters during operation.

[0086] The headphone HP as an example of the ear-mountable playback device may be embodied with both the microphones FB_MIC and FF_MIC being active or enabled such that hybrid ANC can be performed, or as a FB ANC device, where only the feedback noise microphone FB_MIC is active and an ambient noise microphone FF_MIC is not present or at least not active. Hence, in the following, if signals or acoustic transfer functions are used that refer to the ambient noise microphone FF_MIC, this microphone is to be assumed as present, while it is otherwise assumed to be optional.

[0087] Any processing of the microphone signals or any signal transmission are left out in FIG. 4 for reasons of a better overview. However, processing of the microphone signals in order to perform ANC may be implemented in a processor located within the headphone or other ear-mountable playback device or externally from the headphone in a dedicated processing unit. The processor or processing unit may be called a sound control processor. If the processing unit is integrated into the playback device, the playback device itself may form a noise cancellation enabled audio system. If processing is performed externally, the external device or processor together with the playback device may form the noise cancellation enabled audio system. For example, processing may be performed in a mobile device like a mobile phone or a mobile audio player, to which the headphone is connected with or without wires.

[0088] In the various embodiments, the FB or error microphone FB_MIC may be located in a dedicated cavity, as for example detailed in ams application EP17208972.4.

[0089] Referring now to FIG. 5, another example of a noise cancellation enabled audio system is presented. In this example implementation, the system is formed by a mobile device like a mobile phone MP that includes the playback device with speaker SP, feedback or error microphone FB_MIC, ambient noise or feedforward microphone FF_MIC and a sound control processor SCP for performing inter alia ANC and/or other signal processing during operation.

[0090] In a further implementation, not shown, a headphone HP, e.g. like that shown in FIG. 1 or FIG. 4, can be connected to the mobile phone MP wherein signals from the microphones FB_MIC, FF_MIC are transmitted from the headphone to the mobile phone MP, in particular the mobile phone's processor PROC for generating the audio signal to be played over the headphone's speaker. For example, depending on whether the headphone is connected to the mobile phone or not, ANC is performed with the internal components, i.e. speaker and microphones, of the mobile phone or with the speaker and microphones of the headphone, thereby using different sets of filter parameters in each case. In the following, several implementations of the improved concept will be described in conjunction with specific use cases. It should however be apparent to the skilled person that details described for one implementation may still be applied to one or more of the other implementations.

[0091] Generally, the following steps are performed, e.g. with the sound control processor SCP: [0092] controlling and/or monitoring a playback of a detection signal or a filtered version of the detection signal via the speaker SP; [0093] recording an error signal from the error microphone FB_MIC; and [0094] determining whether the headphone or other playback device HP is in a first state, where the playback device HP is worn by a user, or in a second state, where the playback device HP is not worn by a user, based on processing of the error signal.

[0095] 1. Adaptive Headphone with Ear Cushion

[0096] In one embodiment of this disclosure there is a headphone with a front volume which is directly acoustically coupled to the ear canal volume of a user, a driver SP which faces into the front volume and a rear volume which surrounds the rear face of the driver SP. The rear volume may have a vent with an acoustic resistor to allow some pressure relief from the rear of the driver. The front volume may also have a vent with an acoustic resistor to allow some pressure relief at the front of the driver. An error microphone FB_MIC is placed facing the front face of the driver such that it detects ambient noise and the signals from the front of the driver; and a feedforward microphone FF_MIC is placed facing out of the rear of the headphone such that it detects ambient noise, but detects negligible signals from the driver SP. An ear cushion surrounds the front face of the driver and makes up part of the front volume.

[0097] In normal operation the headphone is placed on a user's head such that a complete or partial seal is made between the ear cushion and the users head, thereby at least in part acoustically coupling the front volume to the ear canal volume.

[0098] The feedforward microphone FF_MIC, the error microphone FB_MIC and driver SP are connected to the sound control processor SCP acting as a noise cancellation processor. Referring to FIG. 2, a noise signal detected by the FF microphone FF_MIC is routed through a FF filter and ultimately the headphone speaker SP, producing an anti-noise signal such that FF noise cancellation occurs at the error microphone point, and consequently the ear drum reference point (DRP). The noise signal is used as the detection signal. The error signal from the error microphone FB_MIC is routed to an adaption engine in the sound control processor SCP that in some way changes the anti-noise signal that is output from the speaker by changing at least one property of the FF filter to optimise noise cancellation at the error microphone FB_MIC.

[0099] The sound control processor SCP periodically monitors the FF filter response at at least one frequency and compares this to a predefined set of acceptable filter responses which are stored in a memory of the sound control processor SCP. If the FF filter response is judged to be beyond the acceptable filter responses, an off ear state, i.e. second state, is triggered and the adaption engine ceases to change the FF filter in response to the error microphone signal. For instance, the FF filter is set to a low leak setting.

[0100] For example, the FF filter may in some part represent the inverse of the low frequency characteristics of the driver response. The resultant FF filter response may be analysed at three low frequencies: 80 Hz, 100 Hz and 130 Hz. A different selection of the number of frequencies and the frequency range selected from this is possible. For example, a lower limit of a predefined frequency range may be between 40 Hz and 100 Hz and an upper limit of the predefined frequency range may be between 100 Hz and 800 Hz.

[0101] Therefore a linear regression may determine the gradient and gain of this FF filter. In this example there is one acceptable filter response stored in memory as a gradient and gain scalar values which e.g. represent a linear regression of the inverse of the low frequency portion of the driver response when it is almost off the ear, that is with a high acoustic leakage between the ear cushion and the head. When the gradient of the linear regression of the FF filter becomes greater than the acceptable threshold filter gradient, or if the gain is greater than the acceptable threshold filter gain value, then an off ear state is triggered.

[0102] The FF filter may be a close match of the transfer function:

[00002]$\frac{AE}{AFFM.DE}$

[0103] where AE is the ambient to ear transfer function, AFFM is the ambient to FF microphone transfer function and DE is the driver to ear transfer function.

[0104] When the headphone is in the off ear state, i.e. second state, the sound control processor SCP stops running unnecessary processes such as music playback and Bluetooth connection and switches to a low power mode with may include clocking processes at a lower rate, and which may include clocking the microphone ADCs at a lower rate.

[0105] In this second state, the sound control processor SCP monitors the signals from the error and FF microphones and the sound control processor SCP calculates a phase difference of these two signals, i.e. the detection signal and the error signal.

[0106] The phase calculation may occur by taking the argument of an FFT of the two signals and dividing them, then analysing when e.g. the mean of several bins from the FFT division moves beyond a threshold.

[0107] The phase detection may occur by filtering each time domain signal, the filter may be one or more DFTs or implementations of the Goertzel algorithm at at least one frequency. The division of phase response of these two filtered signals at each frequency can give the phase difference at each frequency. For instance, the mean of these phase differences can be compared to a threshold.

[0108] The phase detection may occur entirely in the time domain.

[0109] If the phase difference moves beyond the threshold, then the earphone is returned to an on ear state, i.e. the first state. The FF filter is reset to a known stable state and adaption is re-enabled, that is the error signal from the error microphone FB_MIC continues to have an effect on the FF filter.

[0110] Referring to FIG. 6, a signal diagram displaying the phase difference between the error signal and the detection signal for different wearing states of a headphone or playback device is shown. For example, one phase difference signal corresponds to a 0 mm leak, another phase difference signal corresponds to a 28 mm leak and a third phase difference signal corresponds to an off ear state with a leakage that is larger than an acceptable maximum leakage, for example. These leakages are derived from a customised leakage adaptor, and are equivalent to a minimum and maximum realistic acoustic leakage. As can be seen from the diagram, in a frequency range from above 30 Hz to around 400 Hz, the phase difference in the off ear state is around 180°, whereas in the two other wearing states the phase difference is significantly different, in particular lower. Hence, for example, evaluation of the phase difference in the mentioned frequency range, in particular by comparing it to a phase threshold value, can give a good indication that the playback device is in or going to the on ear state.

[0111] 2. Adaptive, Acoustically Leaky Earphone

[0112] Another embodiment features an earphone with a driver, a rear volume and a front volume, e.g. like shown in FIG. 3. The rear volume has a rear vent which is damped with an acoustic resistor. The front volume has a front vent which is damped with an acoustic resistor. The physical shape of the earphone dictates that when placed into an ear there is often an acoustic leakage between the ear canal and the earphone housing. This leakage may change depending on the shape of the ear, and how the earphone is sitting in the ear. A FF microphone FF_MIC is placed on the rear of the earphone such that it detects ambient noise but does not detect a significant signal from the driver. An error microphone FB_MIC is placed in close proximity to the front face of the driver such that it detects the drivers signal and the ambient noise signal.

[0113] The noise signal from the FF microphone is, controlled by the sound control processor SCP, passed through the FF filter which outputs an anti-noise signal via the driver SP such that the superposition of the anti-noise signal and the ambient noise creates at least some noise cancellation. The error signal from the error microphone FB_MIC is passed into the signal processor and controls the FF filter such that the anti-noise signal changes based on the acoustic leakage between the ear canal walls and the earphone body. In this embodiment, the resultant filter response is analysed at at least one frequency and compared with an acoustics response that is representative of the earphone being at an extremely high leak. If the resultant filter response exceeds this acoustics response, the earphone enters an off ear state. This off ear state may stop adaption and set a filter for a medium acoustic leakage. In this off ear state, the signals from both microphones are monitored again at at least one frequency and when the phase difference exceeds a pre-defined threshold the earphone is returned to an on ear state, as described before in section 1 in conjunction with FIG. 6.

[0114] In the case that voice is present, the off ear detection still runs. In the case that quiet music is played from the driver, the off ear detection can still run. In the case that the music is substantially louder than the ambient noise, an alternative off ear detection metric may run as described in section 5 below.

[0115] In this embodiment, the resultant FF filter may be arranged according to ams patent application EP17189001.5.

[0116] 3. Non-Adaptive Earphone

[0117] In another embodiment, the ANC headphones as previously described do not have an adaption means, i.e. feature a constant for the response of the feedforward filter. The FF filter is fixed. In this embodiment, an approximation to the ANC performance is made. If ANC performance is substantially worse than what is expected, the playback device is assumed to be off the ear. For example, the ANC performance is approximated by dividing the energy levels of the error microphone and the FF microphone.

[0118] The headphone can then enter an off ear state. The on ear state can be triggered in exactly the same way or at least similar as for an adaptive headphone by monitoring the phase difference between the two microphones, as described before e.g. in section 1 in conjunction with FIG. 6.

[0119] In the case that voice is present, a voice activity detector may pause the off ear detection algorithm to avoid false positives. In the case that music is present, the energy level of the music, offset by the driver response may be subtracted from the energy level of the signal at the error microphone FB_MIC.

[0120] 4. Headphone or Earphone with Hybrid ANC

[0121] In this embodiment, the headphone may be as described in previous embodiments, but also features FB ANC in addition to FF ANC. For FB ANC, the FB microphone FB_MIC is connected to the driver via a FB filter, which may or may not be adaptive.

[0122] The detection of reasons described previously still apply for such embodiments with hybrid ANC.

[0123] 5. Triggered by Music

[0124] Another embodiment may or may not feature noise cancellation, but adapts a filter in accordance with a response of the driver SP changing due to a varying acoustic leakage between the earphone and the ear canal. This filter may be used as all or part of a music compensation filter to compensate for music being attenuated by a feedback noise cancellation system, or may be used to compensate for the driver response changing due to the leakage.

[0125] Referring to FIG. 7, it shows an arrangement of this filter. In this case, the filter is adapted to match the acoustic “driver to error microphone” transfer function. In this embodiment, the headphone features at least the error microphone FB_MIC, wherein the presence of the feedforward microphone FF_MIC is not excluded. Here, a known identification signal WIS (e.g. a music signal or other payload audio signal) is output from the driver SP as a reference. The identification signal WIS is also filtered with the adaptive filter.

[0126] The off ear case may be triggered by monitoring the adapted filter and analysing it as previously described. In particular, a similar evaluation as done with an adaptive feedforward filter is performed with the adapted, adjustable filter, e.g. by comparing a gain and/or gradient to respective associated threshold values.

[0127] In this case, the on ear case may be triggered by monitoring the phase difference between the error signal from the error microphone FB_MIC and the known identification signal WIS driving the speaker SP.

[0128] 6. Quiet Ambient Noise and No Music

[0129] In this embodiment, an adaptive or non-adaptive noise cancelling earphone with a FF and a FB microphone is presented. In this case, the ambient noise may be extremely quiet, such that any useful signal from the microphones is in part masked by electronic noise from the microphones or other electronic means. That is, any signal from the microphones contain a significant portion of both useful ambient noise and random electronic noise. Furthermore, no music or only music with a low signal level is being played from the device. This case e.g. represents having the earphone in an ear but where there is negligible ambient noise and no useful sound is being played out of the driver.

[0130] In this case, the previously detailed on/off ear detection methods will not be able to run reliably because the microphones cannot detect a useable signal from ambient noise or music playback.

[0131] In this case, a similar approach as described above in section 5 may be used. For example, an identification signal WIS is generated by changing the filter between the FF microphone and the driver such that a small degree of noise boosting occurs at the FB microphone. Referring to FIG. 8, instead of changing the FF ANC filter, a dedicated boosting filter can be applied to the noise signal of the FF microphone FF_MIC in order to generate the identification signal WIS. This identification signal WIS can be used to adjust the adjustable filter to match the acoustic “driver to error microphone transfer function”, as described above.

[0132] With this process, the FB microphone can detect a useful signal from the driver, but because the filtered noise signal WIS from the FF microphone still contains a significant portion of quiet ambient noise the signal from the driver is largely coherent with the quiet ambient noise and is as such less perceivable to the user than playing an uncorrelated signal from the driver.

[0133] In this case, a useful identification signal WIS is played via the driver, which is barely detectable to the user, and can be used as in section 5, where a known identification signal WIS is played from the driver, to detect if the earphone is on or off the ear.

[0134] 7. Mobile Handset

[0135] Another embodiment implements a mobile handset with a FF microphone FF_MIC and an error microphone FB_MIC, e.g. as shown in FIG. 5. When the handset is placed on the ear, a partially closed air volume exists in the concha cavity with an acoustic leakage, and some ANC can take place. In this environment, the ANC would typically have some form of adaption as the acoustic leakage is liable to change significantly at each use. On and off ear detection can occur according to sections 1 or 2, for example.

[0136] Where applicable any combination of these embodiments as described in the previous sections is plausible. For example, an adaptive earphone may use off ear detection based on the FF filter and phase difference between the two microphones, but may switch to be triggered by music if the ambient noise level is quiet or the ratio of music to ambient noise is high.

[0137] In the following text, further aspects of the present disclosure are specified. The individual aspects are enumerated in order to facilitate the reference to features of other aspects.

[0138] 1. An audio system for an ear mountable playback device comprising a speaker and an error microphone that senses or predominantly senses sound being output from the speaker, the audio system comprising a sound control processor that is configured to [0139] controlling and/or monitoring a playback of a detection signal or a filtered version of the detection signal via the speaker; [0140] recording an error signal from the error microphone; and [0141] determining whether the playback device is in a first state, where the playback device is worn by a user, or in a second state, where the playback device is not worn by a user, based on processing of the error signal.

[0142] 2. The audio system according to aspect 1, wherein the sound control processor is configured to determine the first state based on an evaluation of a phase difference between the detection signal and the error signal.

[0143] 3. The audio system according to aspect 2, wherein the sound control processor is configured to determine the first state, if the phase difference between the detection signal and the error signal exceeds a phase threshold value at one or more predefined frequencies.

[0144] 4. The audio system according to aspect 2 or 3, wherein the evaluation of the phase difference is performed in the frequency domain.

[0145] 5. The audio system according to one of aspects 1 to 4, which is configured to perform noise cancellation.

[0146] 6. The audio system according to aspect 5, wherein the playback device further comprises a feedforward microphone that predominantly senses ambient sound and wherein the sound control processor is further configured to [0147] recording a noise signal from the feedforward microphone and using the noise signal as the detection signal; [0148] filtering the detection signal with a feedforward filter; and [0149] controlling the playback of the filtered detection signal via the speaker.

[0150] 7. The audio system according to aspect 6, wherein the sound control processor is configured to determine the first state based on an evaluation of a performance of the noise cancellation as a function of the error signal and the noise signal or detection signal.

[0151] 8. The audio system according to aspect 6 or 7, wherein the sound control processor is configured to determine the second state based on an evaluation of a performance of the noise cancellation as a function of the error signal and the noise signal or detection signal.

[0152] 9. The audio system according to aspect 7 or 8, which further comprises a voice activity detector for determining whether a voice signal is recorded with the error microphone and/or the feedforward microphone, wherein the sound control processor is configured to pause a determination of the first and/or the second state, if the voice signal is determined to be recorded.

[0153] 10. The audio system according to one of aspects 7 to 9, wherein the sound control processor is configured to evaluate the performance of the noise cancellation by determining an energy ratio between the error signal and the noise signal or detection signal.

[0154] 11. The audio system according to aspect 10, wherein the sound control processor is configured, if a music signal is additionally played via the speaker, to take an energy level of the music signal into account when determining the energy ratio.

[0155] 12. The audio system according to one of aspects 7 to 11, wherein a filter response of the feedforward filter is constant and/or is kept constant by the sound control processor at least during the determination of the state of the playback device.

[0156] 13. The audio system according to aspect 6, wherein the sound control processor is configured [0157] to adjust a filter response of the feedforward filter based on the error signal; and [0158] to determine the second state based on an evaluation of the filter response of the feedforward filter at at least one predetermined frequency.

[0159] 14. The audio system according to aspect 13, wherein the sound control processor is configured to determine the second state if the filter response of the feedforward filter at the at least one predetermined frequency exceeds a response threshold value.

[0160] 15. The audio system according to aspect 13 or 14, wherein the sound control processor is configured to determine the second state by determining a linear regression of the filter response of the feedforward filter in a predefined frequency range, the linear regression being defined by at least a filter gradient and a filter gain, and by evaluating the filter gradient and/or the filter gain.

[0161] 16. The audio system according to aspect 15, wherein the sound control processor is configured to determine the second state if at least one of the following applies: [0162] the filter gradient exceeds a threshold gradient value; [0163] the filter gain exceeds a threshold gain value.

[0164] 17. The audio system according to aspect 15 or 16, wherein a lower limit of the predefined frequency range is between 40 Hz and 100 Hz and an upper limit of the predefined frequency range is between 100 Hz and 800 Hz.

[0165] 18. The audio system according to one of aspects 6 to 17, wherein the feedforward microphone senses only a negligible portion of the sound being output from the speaker.

[0166] 19. The audio system according to one of aspects 1 to 18, wherein the detection signal is an identification signal, and wherein the sound control processor is configured [0167] to control and/or monitor the playback of the identification signal via the speaker; [0168] to filter the identification signal with an adjustable filter; [0169] to adjust the adjustable filter based on a difference between the filtered identification signal and the error signal, in particular such that the adjustable filter approximates an acoustic transfer function between the speaker and the error microphone; and [0170] to determine the second state based on an evaluation of a filter response of the adjustable filter at at least one further predetermined frequency.

[0171] 20. The audio system according to aspect 19, wherein the identification signal is one of the following or a combination of one of the following: [0172] a music signal; [0173] a payload audio signal; [0174] a filtered version of a noise signal that is recorded from a microphone predominantly sensing ambient sound.

[0175] 21. The audio system according to aspect 19 or 20, wherein the sound control processor is configured to determine the second state if the filter response of the adjustable filter at the at least one further predetermined frequency exceeds an identification response threshold value.

[0176] 22. The audio system according to one of aspects 19 to 21, wherein the sound control processor is configured to determine the second state by determining a linear regression of the filter response of the adjustable filter in a further predefined frequency range, the linear regression being defined by at least an identification filter gradient and an identification filter gain, and by evaluating the identification filter gradient and/or the identification filter gain.

[0177] 23. The audio system according to aspect 22, wherein the sound control processor is configured to determine the second state if at least one of the following applies: [0178] the identification filter gradient exceeds an identification threshold gradient value; [0179] the identification filter gain exceeds an identification threshold gain value.

[0180] 24. The audio system according to aspect 22 or 23, wherein a lower limit of the further predefined frequency range is between 40 Hz and 100 Hz and an upper limit of the further 2 0 predefined frequency range is between 100 Hz and 800 Hz.

[0181] 25. The audio system according to one of the preceding aspects, wherein the sound control processor is configured to control the audio system to a low power mode of operation, if the second state is determined, and to a regular mode of operation, if the first state is determined.

[0182] 26. The audio system according to one of the preceding aspects, wherein the sound control processor is configured to determine whether the playback device is in the first state, only if the playback device is in the second state, and to determine whether the playback device is in the second state, only if the playback device is in the first state.

[0183] 27. The audio system according to one of the preceding aspects, which includes the playback device.

[0184] 28. The audio system according to the preceding aspect, wherein the sound control processor is included in a housing of the playback device.

[0185] 29. The audio system according to one of the preceding aspects, wherein the playback device is a headphone or an earphone.

[0186] 30. The audio system according to aspect 29, wherein the headphone or earphone is designed to be worn with a variable acoustic leakage between a body of the headphone or earphone and a head of a user.

[0187] 31. The audio system according to one of aspects 1 to 27, wherein the playback device is a mobile phone.

[0188] 32. A signal processing method for an ear mountable playback device comprising a speaker and an error microphone that senses or predominantly senses sound being output from the speaker, the method comprising [0189] controlling and/or monitoring a playback of a detection signal or a filtered version of the detection signal via the speaker; [0190] recording an error signal from the error microphone; and [0191] determining whether the playback device is in a first state, where the playback device is worn by a user, or in a second state, where the playback device is not worn by a user, based on processing of the error signal.

[0192] 33. The method according to aspect 32, wherein the first state is determined based on an evaluation of a phase difference between the detection signal and the error signal.

[0193] 34. The method according to aspect 33, wherein the first state is determined, if the phase difference between the detection signal and the error signal exceeds a phase threshold value at one or more predefined frequencies.

[0194] 35. The method according to aspect 33 or 34, wherein the evaluation of the phase difference is performed in the frequency domain.

[0195] 36. The method according to one of aspects 32 to 35, further comprising performing noise cancellation.

[0196] 37. The method according to aspect 36, wherein the playback device further comprises a feedforward microphone that predominantly senses ambient sound and wherein the method further comprises [0197] recording a noise signal from the feedforward microphone and using the noise signal as the detection signal; [0198] filtering the detection signal with a feedforward filter; and [0199] controlling the playback of the filtered detection signal via the speaker.

[0200] 38. The method according to aspect 37, further comprising [0201] determining the first state and/or the second state based on an evaluation of a performance of the noise cancellation as a function of the error signal and the noise signal or detection signal.

[0202] 39. The method according to aspect 37, further comprising [0203] adjusting a filter response of the feedforward filter based on the error signal; and [0204] determining the second state based on an evaluation of the filter response of the feedforward filter at at least one predetermined frequency.

[0205] 40. The method according to one of aspects 32 to 39, wherein the detection signal is an identification signal, the method further comprising [0206] controlling and/or monitoring the playback of the identification signal via the speaker; [0207] filtering the identification signal with an adjustable filter; [0208] adjusting the adjustable filter based on a difference between the filtered identification signal and the error signal, in particular such that the adjustable filter approximates an acoustic transfer function between the speaker and the error microphone; and [0209] determining the second state based on an evaluation of a filter response of the adjustable filter at at least one further predetermined frequency.