Abstract:
Earphone systems and methods for automatically directing ambient sound to an earphone device are provided. An ambient microphone signal from an ambient microphone proximate a sound isolating earphone or head-set device is directed to a receiver within an earphone device according to mixing circuitry. The mixing circuitry is controlled by voice activity of the earphone device wearer. This enables hands-free operation of an earphone system to allow the earphone device wearer to maintain situation awareness with the surrounding environment. During detected voice activity, incoming audio content is attenuated while ambient sound is increased and provided to the earphone device. User voice activity is detected by analysis of at least one of an ear canal microphone signal or an ambient sound microphone signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to and claims the benefit of U.S. Provisional Application No. 61/677,049 entitled “AUTOMATIC SOUND PASS-THROUGH METHOD AND SYSTEM FOR EARPHONES” filed on Jul. 30, 2012, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to earphones and headphones and, more particularly, to earphone systems, headphone systems and methods for automatically directing ambient sound to a sound isolating earphone device or headset device used for voice communication and music listening, to maintain situation awareness with hands-free operation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Sound isolating (SI) earphones and headsets are becoming increasingly popular for music listening and voice communication. Existing SI earphones enable the user to hear an incoming audio content signal (such as speech or music audio) clearly in loud ambient noise environments, by attenuating the level of ambient sound in the user&#39;s ear canal. 
         [0004]    A disadvantage of SI earphones/headsets is that the user may be acoustically detached from their local sound environment. Thus, communication with people in the user&#39;s immediate environment may therefore impaired. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to a method for passing ambient sound to an earphone device configured to be inserted in an ear canal of a user. Ambient sound is captured from an ambient sound microphone (ASM) proximate to the earphone device to form an ASM signal. An audio content (AC) signal is received from a remote device. Voice activity of the user of the earphone device is detected. The ASM signal and the AC signal are mixed to form a mixed signal, such that, in the mixed signal, an ASM gain of the ASM signal is increased and an AC gain of the AC signal is decreased when the voice activity is detected. The mixed signal is directed to an ear canal receiver (ECR) of the earphone device. 
         [0006]    The present invention also relates to an earphone system. The earphone system includes at least one earphone device and a signal processing system. The at least one earphone device includes a sealing section configured to conform to an ear canal of a user of the earphone device, an ear canal receiver (ECR) and an ambient sound microphone (ASM) for capturing ambient sound proximate to the earphone device and to form an ASM signal. The signal processing system is configured to: receive an audio content (AC) signal from a remote device; detect voice activity of the user of the earphone device; mix the ASM signal and the AC signal to form a mixed signal, such that, in the mixed signal, an ASM gain of the ASM signal is increased and an AC gain of the AC signal is decreased when the voice activity is detected; and direct the mixed signal to the ECR. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention may be understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized, according to common practice, that various features of the drawings may not be drawn to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. Moreover, in the drawing, common numerical references are used to represent like features. Included in the drawing are the following figures: 
           [0008]      FIG. 1  is a cross-sectional view diagram of an exemplary earphone device inserted in an ear, illustrating various components which may be included in the earphone device, according to an embodiment of the present invention; 
           [0009]      FIG. 2  is functional block diagram of an exemplary earphone system in relation to other data communication systems, according to an embodiment of the present invention; 
           [0010]      FIG. 3  is a functional block diagram of an exemplary signal processing system for automatic sound pass-through of ambient sound to an ear canal receiver (ECR) of a sound isolating earphone device, according to an embodiment of the present invention; 
           [0011]      FIG. 4  is a flowchart of an exemplary method for determining user voice activity of a sound isolating earphone device, according to an embodiment of the present invention; 
           [0012]      FIG. 5  is flowchart of an exemplary method for determining user voice activity of a sound isolating earphone device, according to another embodiment of the present invention; 
           [0013]      FIGS. 6A and 6B  are flowcharts of an exemplary method for determining user voice activity of a sound isolating earphone device, according to another embodiment of the present invention; and 
           [0014]      FIG. 7  is a flowchart of an exemplary method for controlling input audio content (AC) gain and ambient sound microphone (ASM) gain of an exemplary earphone system, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The following description of exemplary embodiment(s) is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. Exemplary embodiments are directed to or can be operatively used on various wired or wireless earphone devices (also referred to herein as earpiece devices) (e.g., earbuds, headphones, ear terminals, behind the ear devices or other acoustic devices as known by one of ordinary skill, and equivalents). 
         [0016]    Processes, techniques, apparatus, and materials as known by one of ordinary skill in the art may not be discussed in detail but are intended to be part of the enabling description where appropriate. 
         [0017]    Additionally exemplary embodiments are not limited to earpiece devices, for example some functionality can be implemented on other systems with speakers and/or microphones for example computer systems, PDAs, BlackBerry® smartphones, mobile phones, and any other device that emits or measures acoustic energy. Additionally, exemplary embodiments can be used with digital and non-digital acoustic systems. Additionally, various receivers and microphones can be used, for example micro-electro-mechanical systems (MEMs) transducers or diaphragm transducers. 
         [0018]    To enable an SI earphone user to hear their local ambient environment, conventional SI earphones often incorporate ambient sound microphones to pass through local ambient sound to a loudspeaker in the SI earphone. In existing systems, the earphone user must manually activate a switch to enable the ambient sound pass-through. Such a manual activation may be problematic. For example, if the user is wearing gloves or has their hands engaged holding another device (e.g., a radio or a weapon), it may be difficult to press an “ambient sound pass-through” button or switch. The user may miss important information in their local ambient sound field due to the delay in reaching for the ambient sound pass-through button or switch. Also, the user may have to press the button or switch a second time to revert back to a “non ambient sound pass-through” mode. A need exists for a “hands-free” mode of operation to provide ambient sound pass-through for an SI earphone. 
         [0019]    Embodiments of the invention relates to earphone devices and earphone systems (or headset systems) including at least one earphone device. An example earphone system (or headset system) of the subject invention may be connected to a remote device such as a voice communication device (e.g., a mobile phone, a radio device, a computer device) and/or an audio content delivery device (e.g., a portable media player, a computer device), as well as a further earphone device (which may be associated with the user or another use). The earphone device may include a sound isolating component for blocking a meatus of a user&#39;s ear (e.g., using an expandable element such as foam or an expandable balloon); an ear canal receiver (ECR) (i.e., a loudspeaker) for receiving an audio signal and generating a sound field in an ear canal of the user; and at least one ambient sound microphone (ASM) for capturing ambient sound proximate to the earphone device and for generating at least one ASM signal. A signal processing system may receive an audio content (AC) signal from the remote device (such as the voice communication device or the audio content delivery device); and may further receive the at least one ASM signal. The signal processing system mixes the at least one ASM signal and the AC signal and may transmit the resulting mixed signal to the ECR in the earphone device. The mixing of the at least one ASM signal and the AC signal may be controlled by voice activity of the user. 
         [0020]    The earphone device may also include an Ear Canal Microphone (ECM) for capturing sound in the user&#39;s occluded ear-canal and for generating an ECM signal. An example earphone device according to the subject invention detects the voice activity of the user by analysis of the ECM signal from the ECM (where the ECM detects sound in the occluded ear canal of the user), analysis of the at least one ASM signal or the combination thereof. 
         [0021]    According to an exemplary embodiment, when voice activity is detected, a level of the ASM signal provided to the ECR is increased and a level of the AC signal provided to the ECR is decreased. When voice activity is not detected, a level of the ASM signal provided to the ECR is decreased and a level of the AC signal provided to the ECR is increased. 
         [0022]    In an example earphone device, following cessation of the detected user voice activity, and following a “pre-fade delay,” the level of the ASM signal provided to the ECR is decreased and the level of the AC signal fed to the ECR is increased. In an exemplary embodiment, a time period of the “pre-fade delay” may be proportional to a time period of continuous user voice activity before cessation of the user voice activity. The “pre-fade delay” time period may be bound by an upper predetermined limit. 
         [0023]    Aspects of the present invention may include methods for detecting user voice activity of an earphone system (or headset system). In an exemplary embodiment, a microphone signal level value (e.g., from the ASM signal and/or the ECM signal) may be compared with a microphone threshold value. An AC signal level value (from the input AC signal (e.g. speech or music audio from a remote device such as a portable communications device or media player)) may be compared with an AC threshold value. In an exemplary embodiment, the AC threshold value may be generated by multiplying a linear AC threshold value with a current linear AC signal gain. It may be determined whether the microphone Level value is greater than the microphone threshold value. According to another example, it may be determined whether the microphone level value is greater than the microphone threshold value and whether the AC level value is less than the AC threshold value. If the conditions are met, then a voice activity detector (VAD) may be set to an on state. Otherwise the VAD may be set to an off state. 
         [0024]    In an example method, the microphone signal may be band-pass filtered, and a time-smoothed level of the filtered microphone signal may be generated (e.g., smoothed using a 100 ms Hanning window) to form the microphone signal level value. In addition, the AC signal may be band-pass filtered, and a time-smoothed level of the filtered AC signal may be generated (e.g., smoothed using a Hanning window) to form the AC signal level value. 
         [0025]    Referring to  FIG. 1 , a cross-sectional view diagram of an exemplary earphone device  100  is shown. Earphone device  100  is shown relative to ear  130  of a user.  FIG. 1  also illustrates a general physiology of ear  130 . An external portion of ear  130  includes pinna  128 . An internal portion of ear  130  includes ear canal  124  and eardrum  126  (i.e., a tympanic membrane). 
         [0026]    Pinna  128  is a cartilaginous region of ear  130  that focuses acoustic information from ambient environment  132  to ear canal  124 . In general, sound enters ear canal  124  and is subsequently received by eardrum  126 . Acoustic information resident in ear canal  124  vibrates eardrum  126 . The vibration is converted to a signal (corresponding to the acoustic information) that is provided to an auditory nerve (not shown). 
         [0027]    Earphone device  100  may include sealing. section  108 . Earphone device  100  may be configured to be inserted into ear canal  124 , such that sealing section  108  forms a sealed volume between sealing section  108  and eardrum  126 . Thus, ear canal  124  represents an occluded ear canal (i.e., occluded by sealing section  108 ). Sealing section  108  may be configured to seal ear canal  124  from sound (i.e., provide sound isolation from ambient environment  132  external to ear canal  124 ). In general, sealing section  108  may be configured to conform to ear canal  124  and to substantially isolate ear canal  124  from ambient environment  132 . 
         [0028]    Sealing section  108  may be operatively coupled to housing unit  101 . As shown in  FIG. 1 , housing unit  101  of earphone device  100  may include one or more components which may be included in earphone device  100 . Housing unit  101  may include battery  102 , memory  104 , ear canal microphone (ECM)  106 , ear canal receiver  114  (ECR) (i.e., a loudspeaker), processor  116 , ambient sound microphone (ASM)  120  and user interface  122 . Although one ASM  120  is shown, earphone device  100  may include one or more ambient sound microphones  120 . In an exemplary embodiment, ASM  120  may be located at the entrance to the ear meatus. ECM  106  and ECR  114  are acoustically coupled to (occluded) ear canal  124  via respective ECM acoustic tube  110  and ECR acoustic tube  112 . 
         [0029]    In  FIG. 1 , housing unit  101  is illustrated as being disposed in ear  130 . It is understood that various components of earphone device  100  may also be configured to be placed behind ear  130  or may be placed partially behind ear  130  and partially in ear  130 . Although a single earphone device  100  is shown in  FIG. 1 , an earphone device  100  may be included for both the left and right ears of the user, as part of a headphone system. 
         [0030]    Memory  104  may include, for example, a random access memory (RAM), a read only memory (ROM), static RAM (SRAM), dynamic RAM (DRAM), flash memory, a magnetic disk, an optical disk or a hard drive. 
         [0031]    Although not shown, housing unit  101  may also include a pumping mechanism for controlling inflation/deflation of sealing section  108 . For example, the pumping mechanism may provide a medium (such as a liquid, gas or gel capable of expanding and contracting sealing section  108 ) and that would maintain a comfortable level of pressure for a user of earphone device  100 . 
         [0032]    User interface  122  may include any suitable buttons and/or indicators (such as visible indicators) for controlling operation of earphone device  100 . User interface  122  may be configured to control one or more of memory  104 , ECM  106 , ECR  114 , processor  116  and ASM  120 . User interface  122  may also control operation of a pumping mechanism for controlling sealing section  108 . 
         [0033]    In general, ECM  106 , ASM  120  may each be any suitable transducer capable of converting a signal from the user into an audio signal. Although examples below describe diaphragm microphones, the transducers may include electromechanical, optical or piezoelectric transducers. The transducer may also include bone conduction microphone. In an example embodiment, the transducer may be capable of detecting vibrations from the user and converting the vibrations to an audio signal. Similarly, ECR  114  may be any suitable transducer capable of converting an electric signal (i.e., an audio signal) to an acoustic signal. 
         [0034]    All transducers (such as ECM  106 , ECR  114  and ASM  120 ) may respectively receive or transmit audio signals to processor  116  in housing unit  101 . Processor  116  may undertake at least a portion of the audio signal processing described herein. Processor  116  may include, for example, a logic circuit, a digital signal processor or a microprocessor. 
         [0035]    Earphone device  100  may be configured to communicate with a remote device (described further below with respect to  FIG. 2 ) via communication path  118 . In general, the remote device may include another earphone device, a computer device, an audio content delivery device, a communication device (such as a mobile phone), an external storage device, a processing device, etc. For example, earphone device  100  may include a communication system (such as data communication system  216  shown in  FIG. 2 ) coupled to processor  116 . In general, earphone device  100  may be configured to receive and/or transmit signals. Communication path  118  may include a wired or wireless connection. 
         [0036]    Sealing section  108  may include, without being limited to, foam, rubber or any suitable sealing material capable of conforming to ear canal  124  and for sealing ear canal  124  to provide sound isolation. 
         [0037]    According to an exemplary embodiment, sealing section  108  may include a balloon capable of being expanded. Sealing section  108  may include balloons of various shapes, sizes and materials, for example constant volume balloons (low elasticity&lt;=50% elongation under pressure or stress) and variable volume (high elastic&gt;50% elongation under pressure or stress) balloons. As described above, a pumping mechanism may be used to provide a medium to the balloon. The expandable balloon may seal ear canal  124  to provide sound isolation. 
         [0038]    If sealing section  108  includes an expandable balloon, sealing section  108  may be formed from any compliant material that has a low permeability to a medium within the balloon. Examples of materials of an expandable balloon include any suitable elastomeric material, such as, without being limited to, silicone, rubber (including synthetic rubber) and polyurethane elastomers (such as Pellethane® and Santoprene™). Materials of sealing section  108  may be used in combination with a barrier layer (for example, a barrier film such as SARANEX™), to reduce the permeability of sealing section  108 . In general, sealing section  108  may be formed from any suitable material having a range of Shore A hardness between about 5 A and about 30 A, with an elongation of about 500% or greater. 
         [0039]      FIG. 2  is a functional block diagram of exemplary earphone system  200  (also referred to herein as system  200 ), according to an exemplary embodiment of the present invention. System  200  may be configured to communicate with other electronic devices and network systems, such as earphone device  220  (e.g., another earphone device of the same subscriber), earphone device  222  (e.g., an earphone device of a different subscriber), and/or mobile phone  228  of the user (which may include communication system  224  and processor  226 ). 
         [0040]      FIG. 2  illustrates exemplary hardware of system  200  to support signal processing and communication. System  200  may include one or more components such as RAM  202 , ROM  204 , power supply  205 , signal processing system  206  (which may include a logic circuit, a microprocessor or a digital signal processor), ECM assembly  208 , ASM assembly  210 , ECR assembly  212 , user control interface  214 , data communication system  216 , and visual display  218 . 
         [0041]    RAM  202  and/or ROM  204  may be part of memory  104  ( FIG. 1 ) of earphone device  100 . Power supply  205  may include battery  102  of earphone device  100 . ECM assembly  208 , ASM assembly  210  and ECR assembly  212  may include respective ECM  106  ( FIG. 1 ), ASM  120  and ECR  114  of earphone device  100  (as well as additional electronic components). User control interface  214  and/or visual display  218  may be part of user interface  122  ( FIG. 1 ) of earphone device  100 . Signal processing system  206  (described further below) may be part of processor  116  ( FIG. 1 ) of earphone device  100   
         [0042]    Data communication system  216  may be configured, for example, to communicate (wired or wirelessly) with communication circuit  224  of mobile phone  228  as well as with earphone device  220  or earphone device  222 . In  FIG. 2 , communication paths between data communication system  216 , earphone device  220 , earphone device  222  and mobile phone  224  may represent wired and/or wireless communication paths. 
         [0043]    In an example embodiment, earphone system  200  may include one earphone device  100  ( FIG. 1 ). In another example, system  200  may include two earphone devices  100  (such as in a headphone system). Accordingly, in a headphone system, system  200  may also include earphone device  220 . In a headphone system, each earpiece device  100  may include one or more components such as RAM  202 , ROM  204 , power supply  205 , signal processing system  206 , and data communication system  216 . In another example, one or more components of these components (e.g., RAM  202 , ROM  204 , power supply  205 , signal processing system  206  or data communication system  216 ) may be shared by both earpiece devices. 
         [0044]    Referring next to  FIG. 3 , a functional block diagram of an exemplary signal processing system  206  is shown. Signal processing system  206  may be part of processor  116  ( FIG. 1 ) of earphone device  100  and may be configured to provide automatic sound pass-through of ambient sound to ECR  114  of earphone device  100 . Signal processing system  206  may include voice activity detection (VAD) system  302 , AC gain stage  304 , ASM gain stage  306 . mixer unit  308  and optional VAD timer system  310 . 
         [0045]    Signal processing system  206  receives an audio content (AC) signal  320  from a remote device (such as a communication device (e.g. mobile phone, earphone device  220 , earphone device  222 , etc.) or an audio content delivery device (e.g. music player)). Signal processing system  206  further receives ASM signal  322  from ASM  120  ( FIG. 1 ). 
         [0046]    A linear gain may be applied to AC signal  320  by AC gain stage  304 , using gain coefficient Gain_AC, to generate a modified AC signal. In some embodiments, the gain (by gain stage  304 ) may be frequency dependent. A linear gain may also be applied to ASM signal  322  in gain stage  306 , using gain coefficient Gain_ASM, to generate a modified ASM signal. In some embodiments, the gain (in gain stage  306 ) may be frequency dependent. 
         [0047]    Gain coefficients Gain_AC and Gain_ASM may be generated according to VAD system  302 . Exemplary embodiments of VAD system  302  are provided in  FIGS. 4 ,  5 ,  6 A and  6 B and are described further below. In general, VAD  302  may include one or more filters  312 , smoothed level generator  314  and signal level comparator  316 . 
         [0048]    Filter  312  may include predetermined fixed band-pass and/or high-pass filters (described further below with respect to  FIGS. 4 ,  6 A and  6 B). Filter  312  may also include an adaptive filter (described further below with respect to  FIG. 5 ). Filter  312  may be applied to ASM signal  322 , AC signal  320  and/or an ECM signal generated by ECM  106  ( FIG. 1 ). Gain stages  304 ,  306  may include analog and/or digital components. 
         [0049]    Smoothed level generator  314  may receive at least one of a microphone signal (e.g., ASM signal  322  and/or an ECM signal) and AC signal  320  and may determine respective time-smoothed level value of the signal. In an example, generator  314  may use a 100 ms Hanning window to form a time-smoothed level value. 
         [0050]    Signal level comparator  316  may use at least the microphone level (value) to detect voice activity. In another example, comparator  316  may use the microphone level and the AC level to detect voice activity. If voice activity is detected, comparator  316  may set a VAD state to an on state. If voice activity is not detected, comparator  316  may set a VAD state to an off state. 
         [0051]    In general, VAD system  302  determines when the user of earphone device  100  ( FIG. 1 ) is speaking. VAD system  302  sets Gain_AC (gain stage  304 ) to a high value and Gain_ASM (gain stage  306 ) to a low value when no user voice activity is detected. VAD system  302  sets Gain_AC (gain stage  304 ) to a low value and Gain_ASM (gain stage  306 ) to a high value when user voice activity is detected. The gain coefficients of gain stages  304 ,  306  for the on and off states may be stored, for example, in memory  104  ( FIG. 1 ). 
         [0052]    The modified AC signal and the modified ASM signal from respective gain stages  306  and  310  may be summed together with mixer unit  308 . The resulting mixed signal may be directed towards ECR  114  ( FIG. 1 ) as ECR signal  324 . 
         [0053]    Signal processing system  206  may include optional VAD timer system  310 . VAD timer system  310  may provide a time period of delay (i.e., a pre-fade delay), between cessation of detected voice activity and switching of gains by gain states  304 ,  306  associated with the VAD off state. In an exemplary embodiment, the time period may be proportional to a time period of continuous user voice activity (before the voice activity is ceased). The time period may be bound by a predetermined upper limit (such as 10 seconds). VAD timer system  310  is described further below with respect to  FIG. 7 . 
         [0054]    Referring next to  FIG. 4 , a flowchart of an exemplary method is shown for determining user voice activity by VAD system  302  ( FIG. 3 ), according to an embodiment of the present invention. 
         [0055]    According to an exemplary embodiment, voice activity of the user of earphone device  100  ( FIG. 1 ) (i.e., the earphone wearer) may be detected by analysis of a microphone signal captured from a microphone. According to one example, the voice activity may be detected by analysis of an ECM signal from ECM  106  ( FIG. 1 ), where ECM  106  detects sound in the occluded ear canal  124 . According to another exemplary embodiment, voice activity may be detected by analysis of an ASM signal from ASM  120 . In this case, the method described in  FIG. 4  is the same except that the ECM signal (from ECM  106  of  FIG. 1 ) is exchanged with the ASM signal from the ASM  120 . At step  402 , a microphone signal is captured. The microphone signal  402  may be captured by ECM  106  or by ASM  120 . 
         [0056]    At optional step  404  the microphone signal may be band-pass filtered, for example, by filter  312  ( FIG. 3 ). In an exemplary embodiment, the band-pass filter  312  ( FIG. 3 ) has a lower cut-off frequency of approximately 150 Hz and an upper cut-off frequency of approximately 200 Hz, using a 2nd or 4th order infinite impulse response (IIR) filter or 2 chain biquadratic filters (biquads). 
         [0057]    At step  406 , a time-smoothed level of the microphone signal (step  402 ) or the filtered microphone signal (step  404 ) is determined, to form a microphone signal level value (“mic level”). The microphone signal level may be determined, for example, by smoothed level generator  314  ( FIG. 3 ). For example, the microphone signal may be smoothed using a 100 ms Hanning window. 
         [0058]    At step  412 , input audio content (AC) signal  320  ( FIG. 3 ) (e.g., speech or music audio from a remote device) may be received. At optional step  414 , the AC signal  320  may be band-pass filtered, for example by filter  312  ( FIG. 3 ). In an exemplary embodiment, the band-pass filter is between about 150 and about 200 Hz, using a 2nd or 4th order IIR filter or 2 chain biquads. 
         [0059]    At step  416 , a time-smoothed level of AC signal (step  412 ) or the filtered AC signal (step  414 ) is determined (e.g., smoothed using a 100 ms Hanning window), such as by smoothed level generator  314  ( FIG. 3 ), to generate an AC signal level value (“AC level”). 
         [0060]    At step  408 , the microphone signal level value (determined at step  406 ) is compared with a microphone threshold  410  (also referred to herein as mic threshold  410 ), for example, by signal level comparator  316  ( FIG. 3 ). Microphone threshold  410  may be stored, for example, in memory  104  ( FIG. 1 ). 
         [0061]    At step  418 , the AC signal level value (determined at step  416 ) is compared with a modified AC threshold (determined at step  422 ), for example, by signal level comparator  316  ( FIG. 3 ). The modified AC threshold is generated at step  422  by multiplying a linear AC threshold  420  with a current linear AC signal gain  424 . AC threshold  420  may be stored, for example, in memory  104  ( FIG. 1 ). 
         [0062]    At step  426 , it is determined whether voice activity is detected. At step  426 , if it is determined (for example by comparator  316  of  FIG. 3 ) that the microphone level is greater than the microphone threshold  410  (mic level&gt;mic threshold) and the AC level is less than the modified AC threshold (AC level&lt;modified AC threshold), then the state of VAD system  302  ( FIG. 3 ) is set to an on state at step  430 . Otherwise VAD system  302  ( FIG. 3 ) is set to an off state at step  428 . 
         [0063]    At step  430 , when voice activity is detected (i.e. VAD=on state), the level of ASM signal  322  ( FIG. 3 ) provided to ECR  114  ( FIG. 1 ) is increased by increasing Gain_ASM (via gain stage  306 ), and the level of AC signal  320  provided to ECR  114  is decreased by decreasing Gain_AC (via gain stage  304 ). 
         [0064]    At step  428 , when voice activity is not detected (i.e. VAD=off state), the level of ASM signal  322  ( FIG. 3 ) provided to ECR  114  ( FIG. 1 ) is decreased by decreasing Gain_ASM, and the level of AC signal  320  provided to ECR  114  is increased by increasing Gain_AC. A maximum value of gain_AC and gain_ASM may be limited, e.g. to about unity gain, and in one exemplary embodiment a minimum value of gain_AC and gain_ASM may be limited, e.g. to about 0.0001 gain. 
         [0065]    In an exemplary embodiment, a rate of gain change (slew rate) of the gain_ASM and the gain_AC in mixer unit  308  ( FIG. 3 ) may be independently controlled and may be different for “gain increasing” and “gain decreasing” conditions. In one example, the slew rate for increasing and decreasing “AC gain” in the mixer unit  308  is about 30 dB per second and about −30 dB per second, respectively. In an exemplary embodiment, the slew rate for increasing and decreasing “ASM gain” in mixer unit  308  may be inversely proportional to the gain_AC (on a linear scale, the gain_ASM is equal to the gain_AC subtracted from unity). 
         [0066]    Referring next to  FIG. 5 , a flowchart of an exemplary method is shown for determining user voice activity by VAD system  302  ( FIG. 3 ), according to another embodiment of the present invention. 
         [0067]    At step  502 , a microphone signal is captured. The microphone signal may be captured by ECM  106  ( FIG. 1 ) or by ASM  120 . At step  504 , AC signal  320  ( FIG. 3 ) is received. 
         [0068]    At step  506 , the AC signal  320  is adaptively filtered by an adaptive filter, such as filter  312  ( FIG. 3 ). At step  508 , the filtered signal (step  506 ), is subtracted from the captured microphone signal (step  502 ), resulting in an error signal. At step  510 , the error signal (step  508 ) may be used to update adaptive filter coefficients (for the adaptive filtering at step  506 ). For example, the adaptive filter may include a normalized least mean squares (NLMS) adaptive filter. Steps  506 - 510  may be performed, for example, by filter  312  ( FIG. 3 ) 
         [0069]    At step  512 , an error signal level value (“error level”) is determined, for example, by smoothed level generator  314  ( FIG. 3 ). At step  516  the error level is compared with an error threshold  514 , for example, by signal level comparator  316  of  FIG. 3 . The error threshold  514  may be stored in memory  104  ( FIG. 1 ). 
         [0070]    At step  518  it is determined (for example, by signal level comparator  316  of  FIG. 3 ) whether the error level (step  512 ) is greater than the error threshold  514 . If it is determined, at step  518 , that the error level is greater than the error threshold  514 , step  518  proceeds to step  522 , and VAD system  302  ( FIG. 3 ) is set to an on state. Step  522  is similar to step  430  in  FIG. 4 . 
         [0071]    If it is determined, at step  518 , that the error level is less than or equal to error threshold  514 , step  518  proceeds to step  520 , and VAD system  302  ( FIG. 3 ) is set to an off state. Step  520  is similar to step  428  in  FIG. 4 . 
         [0072]    Referring next to  FIGS. 6A and 6B , flowcharts are shown of an exemplary method for determining user voice activity by VAD system  302  ( FIG. 3 ), according to another embodiment of the present invention.  FIGS. 6A and 6B  show modifications of the method of voice activity detection shown in  FIG. 4 . 
         [0073]    Referring  FIG. 6A , the exemplary method shown may be advantageous for band-limited input AC signals  320  ( FIG. 3 ), such as speech audio from a telephone system that is typically band-limited to between about 300 Hz and about 3 kHz. At step  602 , AC signal  320  is received. At optional step  614 , AC signal  320  may be filtered (e.g., high-pass filtered or band-pass filtered, such as by filter  312  of  FIG. 3 ), to attenuate or remove low frequency components, or a region of low-frequency components, in the input AC audio signal  612 . At step  606 , an ECR signal may be generated from the AC signal  320  (which may be optionally filtered at step  614 ) and may be directed to ECR  114  ( FIG. 1 ). 
         [0074]    Referring next to  FIG. 6B , at step  608 , a microphone signal is captured. The microphone signal may be captured by ECM  106  ( FIG. 1 ) or by ASM  120 . At optional step  610 , the microphone signal may be band-pass filtered, similarly to step  404  ( FIG. 4 ), for example, by filter  312  ( FIG. 3 ). At step  612 , a time-smoothed level of the microphone signal (captured at step  608 ) or the filtered microphone signal (step  610 ) may be determined, similarly to step  406  ( FIG. 4 ), to generate a microphone signal level value (“mic level”). 
         [0075]    At step  614 , the microphone signal level value is compared with a microphone threshold  616 , similarly to step  408  ( FIG. 4 ). At step  618  it is determined whether voice activity is detected. 
         [0076]    At step  618 , if it is determined (for example by signal level comparator  316  of  FIG. 3 ) that the microphone Level is greater than the microphone threshold, then VAD system  302  ( FIG. 3 ) is set to an on state at step  622 . Otherwise VAD system  302  is set to an off state at step  620 . Steps  620  and  622  are similar to respective steps  428  and  430  ( FIG. 4 ). 
         [0077]    Referring next to  FIG. 7 , a flowchart is shown of an exemplary method for controlling input AC gain and ASM gain by signal processing system  206  ( FIG. 3 ) including VAD timer system  310 , according to an embodiment of the present invention. In  FIG. 7 , following cessation of detected user voice activity by VAD system  302 , and following a “pre-fade delay,” the level of the ASM signal provided to ECR  114  ( FIG. 1 ) is decreased and the level of the AC signal provided to ECR  114  is increased. 
         [0078]    In an exemplary embodiment, the time period of the “pre-fade delay” (referred to herein as T initial ) may be proportional to a time period of continuous user voice activity (before cessation of the user voice activity), and the “pre-fade delay” time period T initial  may be bound by a predetermined upper limit value (T max ), which in an exemplary embodiment is between about 5 and 20 seconds. 
         [0079]    At step  702 , the VAD status (i.e., an on state or an off state) is received (at VAD timer system  310 ). At step  704  it is determined whether voice activity is detected by VAD system  302 , based on whether the VAD status is in an on state or an off state. 
         [0080]    If voice activity is detected at step  704  (i.e., the VAD status is an on state), then a VAD timer (of VAD timer system  310  ( FIG. 3 ) is incremented at step  706 . In an example embodiment, the VAD timer may be limited to a predetermined time T max  (for example, about 10 seconds). At step  708 , the gain_AC is decreased and the gain_ASM is increased (via gain stages  304  and  306  in  FIG. 3 ). 
         [0081]    If voice activity is not detected at step  704  (i.e., the VAD status is an off state), then the VAD timer is decremented at step  710 , from an initial value, T initial . The VAD timer may be limited at step  712  so that the VAD timer is not decremented to less than 0. As discussed above, T initial  may be determined from a last incremented value (step  706 ) of the VAD timer (prior to cessation of voice activity). The initial value T initial  may also be bound by the predetermined upper limit value T max . 
         [0082]    If it is determined, at step  712 , that the VAD timer is equal to 0, step  712  proceeds to step  714 . At step  714 , the AC gain value is increased and the ASM gain is decreased (via gain stages  304 ,  306  of  FIG. 3 ). 
         [0083]    If it is determined, at step  712 , that the VAD timer is greater than 0, step  712  proceeds to step  716 . At step  716 , the AC gain and ASM gain remain unchanged. Thus, the VAD timer system  310  ( FIG. 3 ) may provide a delay period between cessation of voice activity detection and changing of the gain stages for corresponding to the VAD off state. 
         [0084]    Although the invention has been described in terms of systems and methods for automatically passing ambient sound to an earphone device, it is contemplated that one or more steps and/or components may be implemented in software for use with microprocessors/general purpose computers (not shown). In this embodiment, one or more of the functions of the various components and/or steps described above may be implemented in software that controls a computer. The software may be embodied in non-transitory tangible computer readable media (such as, by way of non-limiting example, a magnetic disk, optical disk, flash memory, hard drive, etc.) for execution by the computer. 
         [0085]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.