Abstract:
A voice-activated microphone and transceiver system includes an interrogator unit for transmitting a signal, receiving a modulated signal, and demodulating the modulated signal such that the difference between the transmitted signal and the modulated signal correspond to a unique sound wave signal. An acoustically driven microphone unit is also included for receiving the signal from the interrogator unit, modulating the signal with the sound wave signal, wherein the sound wave signal contains instructions for controlling an electronic device, and transmitting the modulated signal back to the interrogator unit for analysis by a signal processor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an integrated microphone and transceiver system that allows voice-activated control of computer driven devices using a passive and wireless interface. 
     2. Description of the Prior Art 
     As the number of systems that are computer controlled increases, so too increases the need for more sophisticated approaches to controlling such systems. In particular, there is a need for voice-activated control of computer systems. For example, in automobile control systems, a driver&#39;s voice could be used to activate or deactivate accessories including, but not limited to, radios, headlights, cabin lights, windshield wipers and cellular phones. And, by controlling such accessories using voice-activation, a driver&#39;s hands would be freed up to operate the steering wheel, thus allowing the driver to more easily focus on the conditions of the road. Additionally, voice activation could be used in homes or similar environments to unlock doors, turn on and off lights, turn on and off appliances, etc. Conventional techniques for controlling computer systems are generally less effective, since they require manual intervention on the part of the system user. And, in those cases where control is carried out by voice activation, problems related to recognizing a voice in the presence of ambient noise and problems related to providing power to the microphone unit still exist. Problems related to recognizing a voice in the presence of ambient noise typically exist when the source of an operator&#39;s voice is located distant from the computer or the operator is situated in a noisy environment. For example, in the noisy interior of a car, recognition of a driver&#39;s voice is difficult unless the microphone is located close the driver&#39;s mouth. And, while both wired and wireless microphones are currently available, each presents problems related to powering the microphone. For example, wired microphones require costly wires that typically run through a car&#39;s body to the seatbelt, and frequent retracting of the seatbelt can eventually sever the wires. On the other hand, wireless microphones require batteries, and consumers are reluctant to replace batteries regularly since generally the equipment in a car&#39;s interior requires no such similar maintenance over the life of the car. 
     Thus, an integrated microphone and transceiver system for providing voice activated control of a computer system using a passive and wireless interface that does not require battery power is highly desirable. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings of the prior art are addressed and overcome by the present invention that provides a voice-activated microphone and transceiver system for providing sound wave activated control of an electronic device system. The system includes an interrogator unit for transmitting a signal pulse, receiving a modulated signal pulse, and demodulating the modulated signal pulse such that the delay between the transmitted signal pulse and the modulated signal pulse corresponds to a unique sound wave signal that is used to control the electronic device. A acoustically driven microphone unit is also included for receiving the signal pulse from the interrogator unit, modulating the signal pulse with the sound wave signal, wherein the sound wave signal contains instructions for controlling an electronic device, and transmitting the modulated signal pulse back to the interrogator unit for analysis by a signal processor. 
     In an alternate embodiment of the present inventions, an optical signal is transmitted from an optical interrogator unit and is received and reflected by an optical microphone unit. The optical signal is modulated in amplitude in response to the air pressure of a voice sound wave signal in the area surrounding the microphone unit and reflected back toward the interrogator source where a voice signal processor unit eventually processes it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the following description and attached drawings, wherein: 
     FIG. 1 is a mechanical diagram of a surface acoustic wave (SAW) microphone unit in accordance with an embodiment of the present invention; 
     FIG. 2 is a block diagram of an embodiment of a SAW interrogator unit in accordance with an embodiment of the present invention; 
     FIG. 3 is a mechanical diagram of a SAW microphone unit including levers in accordance with an alternate embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a capacitor microphone unit in accordance with an alternate embodiment of the present invention; 
     FIG. 5 is schematic diagram of a crystal microphone unit in accordance with an alternate embodiment of the present invention; 
     FIG. 6 is a block diagram of a capacitor or crystal interrogator unit in accordance with an alternate embodiment of the present invention; 
     FIG. 7 is a mechanical diagram of an optical microphone unit in accordance with an alternate embodiment of the present invention; 
     FIG. 8 a  is a graphical illustration of an optical microphone grating mechanism having a pattern of alternating clear and opaque regions each having a width W in accordance with the FIG. 7 embodiment of the present invention; 
     FIG. 8 b  is a graphical illustration of a first optical microphone grating in an W/2 offset position above a stationary second optical microphone grating; 
     FIG. 8 c  is a graphical illustration of the first optical microphone grating in an W/2 offset position above the stationary second optical microphone grating, providing zero light transmission; 
     FIG. 8 d  is a graphical illustration of the first optical microphone grating in an W/2 offset position above the stationary second optical microphone grating, providing maximum light transmission; and 
     FIG. 9 is a block diagram of an optical interrogator unit in accordance with an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system for providing voice-activated control of an electronic device is illustrated. 
     Generally, a signal pulse, such as a radio frequency (RF) signal pulse, is transmitted from an interrogator unit to a microphone unit. The microphone unit receives the signal pulse and modulates the transmitted signal pulse with a sound wave corresponding to a voice sound wave signal. The modulated signal is produced as a RF echo where the sound pressure from a voice in the air surrounding the microphone unit modulates the RF echo&#39;s delay or ringing frequency. Afterwards, the microphone unit retransmits the modulated version of the signal to the interrogator unit, where the voice signal is detected and later processed by a voice signal processor unit. 
     Alternatively, an optical signal is transmitted from an optical interrogator unit and is received and reflected by an optical microphone unit. The optical signal is modulated in amplitude in response to the air pressure of a voice sound wave signal in the area surrounding the microphone unit and reflected back toward the interrogator source where a voice signal processor unit eventually processes it. 
     For purposes of describing the preferred embodiments of the present invention, the present invention is illustrated using voice activation to control automobile systems. However, it is important to note that the present invention is not limited to providing control for a particular computer system or electronic device. In fact, the present invention can be utilized to provide voice-activated control of any computer-based system, including, but not limited to automobile systems and home systems (e.g. unlocking doors, turning on and off lights, appliances etc.). The present invention can also be utilized to provide access to secured systems, for example, those systems that grant access to a user only upon recognition of a uniquely identifiable voice signal command. 
     Referring to FIG. 1, in a first embodiment of the present invention, a microphone unit  10 , herein further referenced as a surface acoustic wave (SAW) microphone unit, is illustrated including a housing  12 , a thin flexible SAW element  14  mounted within the housing  12 , an antenna (or alternatively multiple antennas)  16  attached to the SAW element  14  through the housing  12  and a diaphragm cover  18  that seals the opening of the housing  12 . The SAW microphone unit  10  is preferably mounted on the driver&#39;s seatbelt, or, alternatively, to increase the microphone unit&#39;s reception sensitivity, multiple microphone units can be mounted on the driver&#39;s seatbelt. The housing  12  is preferably an approximately 0.1 inch thick ceramic package that includes feedthroughs and printed RF traces (not shown). The SAW element  14  is preferably a single-transducer SAW delay line device formed from an approximately 4 mil (0.004 inch) thick lithium niobate (LiNbO 3 ) piezoelectric crystal, but may alternatively be a SAW resonator device. The antennas  16  are shown in FIG. 1 as wire dipole antennas, but the antennas  16  may alternatively include patch, loop, or other small antennas that are suitable for RF frequency use. 
     The SAW element  14 , illustrated in FIG. 1 as a SAW delay line, provides a delayed echo of an applied RF signal burst. In particular, the SAW delay line includes an interdigital metal film transducer (not shown) that consists of two groups of interdigital electrode fingers separated by a gap area (not shown). When activated by a burst of RF radiation near the center frequency of the SAW transducers, each group of transducer fingers sends surface acoustic waves both left and right along the surface of the delay line crystal  14 . Such activation occurs as a result of the dipole antenna  16  receiving a transmitted RF signal produced by an interrogator unit oscillator (described below) at the same center frequency as the SAW transducers. Absorbers (not shown) suppress the waves moving to the ends of the delay line crystal  14  and the waves moving to the center of the crystal  14  reach the opposite group of electrodes several microseconds after the initial RF tone burst. There, the waves are reconverted to a RF tone burst that is retransmitted from the microphone unit antenna  16  as a delayed echo of the RF burst signal received from the interrogator unit. 
     The SAW device&#39;s  14  delay is modified in proportion to the surface strain on the crystal, therefore, the transmitted pulse delay of the SAW delay line  14  can be modulated by a sound wave signal, here, a driver&#39;s voice. In particular, the surface strain results from a force applied through a push rod  20  from the diaphragm  18 , which is forced up and down by the air pressure of the ambient sound in the air surrounding the microphone  10 . The diaphragm  18  converts the pressure produced by the sound wave of the driver&#39;s voice into the force. The force is then transmitted via the push rod  20  to the free end  22  of the SAW delay line  14 , which is mounted as a cantilever beam at the base of the housing  12 . The beam flexes the SAW delay line  14 , which causes mechanical strain on the crystal surface. As a result, the delay of the SAW delay line  14  varies with the air pressure at the microphone unit  10  generated by the driver&#39;s voice. 
     Because the SAW delay line  14  is designed to create a delayed echo at the two interdigital electrodes in the single transducer, the SAW delay line  14  is able to retransmit the delayed version of the RF signal burst out the antenna  16 . The delayed signal, now modulated with the driver&#39;s voice, is received by a receive antenna located in the interrogator unit where, as described below, it is demodulated by the interrogator unit as a representation of the driver&#39;s voice. 
     Referring to FIG. 2, an interrogator unit  26 , herein further referenced as a SAW interrogator unit, includes a surface acoustic wave (SAW) oscillator  28 , a RF transmit switch  30 , a transmit antenna  32 , a receive antenna  36 , a RF receive switch  38 , and a voice signal processor (voice vocoder)  48 . The solid path lines in FIG. 2 represent electrical pathways. For purposes of illustrating the preferred embodiment, the SAW interrogator unit  26  is preferably mounted in the dashboard or sun visor of an automobile where it measures the air pressure at a SAW microphone unit (see, e.g., FIG. 1 at numeral  10 ) by sending the microphone unit an RF signal pulse burst and receiving back the burst&#39;s delayed echo. A sequence of transmitted signal pulse bursts and received signal echo bursts is repeated many times per second such that the air pressure generated by a driver&#39;s voice at the SAW microphone unit is measured often enough by the SAW interrogator unit  26 , for example, approximately 500,000 time per second, that the measurements provide an accurate representation of the sound of the driver&#39;s voice. 
     More particularly, the SAW oscillator  28  is provided having the same center frequency, here 915 MHz, as the SAW delay line device  14  located in the SAW microphone unit  10  shown in FIG.  1 . The SAW oscillator  28  generates a continuous RF signal  27  that is applied to the RF transmit switch  30 . Simultaneously, a digital count down divider  34  counts positive pulses of the SAW oscillator&#39;s RF signal  27  until the number of pulses reaches 915. Once the number of pulses reaches 915, the digital count down divider  34  actuates the RF switch  30 , at numeral  35 , to pass a time-gated burst  33  of the SAW oscillator&#39;s RF signal  27  to the transmit antenna  32 , and the count down divider  34  is reset to start counting again. One microsecond later, the receive RF switch  38  is actuated by a delayed signal  41  from the digital count down divider  34  to receive a time-gated signal echo burst  43  from the receive antenna  36 . The digital count down divider delay  31  is set at one microsecond so that the receive RF switch  38  receives the delayed, sound-modulated signal echo burst  43  transmitted from the SAW microphone unit  10  and not the earlier more powerful time-gated signal burst  33  transmitted to the SAW microphone unit  10 . The SAW microphone unit  10  returns the signal echo bursts  43  as modulated signals having delays that are proportional to the instantaneous pressure of the air surrounding the microphone unit, as created by the sound of the driver&#39;s voice. 
     The RF receive switch  38  gates the signal echo burst  43  and the gated signal  45  is applied by the RF receive switch  38  to a low noise amplifier  40  that amplifies the gated signal echo burst  45 . The amplified signal  47  is then passed through a SAW band pass filter  42  to remove out-of-band noise and interference that would otherwise produce undesired noise in the voice signal received from the SAW microphone unit  10  and later processed by the voice vocoder  48 . The center frequency of the SAW band pass filter  42  is preferably set to be the same as the frequency of the SAW oscillator  28 . Since the bandwidth of the SAW bandpass filter  42  must pass the spectrum of the modulated radio echo  43  from the microphone unit, the bandwidth is made as narrow as practically possible, but not less than 20 kHz. And, because of the narrow bandwidth of the SAW bandpass filter  42 , out-of-band noise and interference are largely eliminated so that the difference in phase between the RF signal  27  and a returned signal echo burst  43  can be accurately measured. 
     Referring still to FIG. 2, the phase of the amplified signal echo burst  47  is measured against the phase of the continuous RF signal  27  via a phase detecting multiplier  44 . The phase change that is measured at the multiplier  44  is a consequence of the change in delay of the SAW microphone unit&#39;s RF echo as a result of the voice that modulated the original signal burst  33 . The phase signal  49  at the output of the multiplier  44  is applied to a low pass filter  46 , preferably a 10 KHz filter, that removes unwanted high frequency components from the phase signal  49  and converts the phase signal  49  into a smoothly varying voltage signal  51  corresponding to the sound of the driver&#39;s voice. The voltage signal  51  is sent to the signal processor unit  48  where, using conventional voice recognition techniques, the signal processor  48  interprets the voltage signal  51  as the driver&#39;s voice command and uses the command to electrically control a particular device, for example, an automobile windshield wiper. 
     Referring to FIG. 3, in an alternate embodiment of the present invention, a microphone unit  50  utilizing a lever element  52  to mechanically increase the acoustic sensitivity of the SAW microphone unit  50  is shown. As previously described, the air pressure from a voice sound wave located in the area surrounding the microphone unit  50  creates an initial force against a diaphragm  54 . But here, the diaphragm  54 , via a first push rod  56 , applies the force to the free end  58  of the lever  52  while the opposite end  60  of the lever  52  is constrained from moving by a fulcrum  62  or similar device. Located at the underside of the lever  52 , at a point approximately ⅕th as far from the fulcrum  62  as the first push rod  56 , is a second push rod  64  which transfers 5 times more force to the free end of the SAW element  66 . This additional force increases the flex of the SAW element  66  which in turn proportionally changes the delay of the RF echo, thereby increasing the sensitivity of the microphone unit  50  to the driver&#39;s voice. And, as described in FIG. 1, sound waves representing the driver&#39;s voice are reconverted to a RF tone burst that is retransmitted from the microphone unit antenna  67  as a delayed echo of the RF burst signal received from the interrogator unit. It is important to note that the use of a single lever, as shown in FIG. 3, can be extended to the use of several levers. For example, by using two levers, a first lever can provide a force multiplication factor equal to five that is then applied to a second lever, which also provides a force multiplication factor equal to five. Thus, increasing the total force pressing against the surface of the SAW element  66  by a factor of twenty-five. 
     Referring to FIG. 4, in accordance with another embodiment of the present invention, a capacitor microphone unit  68  is shown. The capacitor microphone unit  68  includes a capacitor microphone  70 , an inductor  72  and an antenna  74  (shown as a wire dipole antenna). 
     The capacitor microphone  70  is a capacitor in which a first plate  76  moves toward and away from a second plate  78  in response to the pressure of sound in the surrounding air. In its most basic form, the first plate  76  is a passively mounted diaphragm that seals the opening of a microphone unit housing (not shown) and the second plate  78  is rigidly fixed in position relative to the back of the microphone housing. Since the first plate  76  moves with the sound wave, the capacitance of the microphone  70  likewise varies with that of the sound wave. Thus, the capacitor microphone  70  indicates changes in the instantaneous pressure of the air by corresponding changes in capacitance. 
     The inductor  72  and the capacitor microphone  70  are combined in a parallel resonant circuit  80 . Since the capacitance of the microphone  70  changes with the sound wave, as described above, the circuit&#39;s  80  resonant frequency also changes with the sound wave. The resonant circuit  80  is connected to the antenna  74 , such that when a short and broadband RF burst is received by the antenna  74  having a resonant frequency near that of the resonant circuit  80 , the RF burst is applied to the resonant circuit  80  where an alternating current at the received frequency builds up in the circuit  80 , thereby storing energy. Once the received burst stops transmitting, the alternating current continues to re-radiate (“ring”) from the antenna  74  until the stored energy is depleted. Since the re-radiated signal&#39;s frequency is set at the resonant frequency of the resonant circuit  80 , the frequency provides an indication of the instantaneous acoustic pressure on the capacitor microphone&#39;s  68  diaphragm as a result of a voice wave signal. Consequently, a capacitor/crystal interrogator unit, like that described below in FIG. 6, can measure the “ringing” frequency and convert the measurement to one associated with the instantaneous pressure caused by a voice creating a force on the microphone&#39;s  70  diaphragm. 
     Referring to FIG. 5, in accordance with another embodiment of the present invention, a crystal microphone unit  82  is shown. The crystal microphone unit  82  includes a varying capacitor  84 , an inductor  86 , an antenna  88 , a piezoelectric (“crystal”) microphone  90 , a blocking capacitor  92  and a RF choke  94 . Similar to the capacitor microphone unit  68  illustrated in FIG. 4, the crystal microphone unit  82  contains a parallel resonant circuit  96  containing the fixed inductor  86  and the varying capacitor  84 , here a varactor, that modulates the resonant frequency of the parallel resonant circuit  96 . Also, similar to the capacitor microphone unit  68  illustrated in FIG. 4, the parallel resonant circuit  96  is connected to the antenna  88 . 
     Like the capacitor resonant circuit  80  shown in FIG. 4, the crystal circuit&#39;s  96  resonant frequency varies with the changes in the surrounding air pressure due to the sound of a driver&#39;s voice. However, unlike the capacitor of the capacitor microphone unit&#39;s resonant circuit  80  (see FIG.  4 ), the capacitor of the crystal resonant circuit  96  is provided as a varactor  84 . The varactor  84  is a known semiconductor device having capacitance that is adjusted by applying a direct current (DC) or low frequency bias. Here, the microphone  90 , preferably a conventional crystal microphone, generates the bias voltage. A crystal microphone  90  is preferred because of its high output voltage and high impedance, which provides superior sensitivity when, used with the high-impedance varactor  84 . 
     The RF choke  94  is provided to prevent the crystal microphone&#39;s  90  capacitance from interfering with the ringing resonant frequency of the resonant circuit  96 , and the blocking capacitor  92  is provided to prevent the microphone unit&#39;s  82  output voltage from being shorted out by the inductor  86 . 
     Referring to FIG. 6, in accordance with another embodiment of the present invention, a capacitor/crystal interrogator unit  98 , having similar components and operation as the interrogator unit  26  shown in FIG. 2 except for the inclusion of a two micro second delay  100  within a digital count down divider  99  and a phased lock loop (PLL)  110  preferably having a 0.1 millisecond (ms) time constant, is shown. And, instead of measuring the sound pressure of a voice located at a SAW microphone unit like that shown in FIGS. 1 and 3, the wireless capacitor/crystal interrogator unit  98  measures the sound pressures at a capacitor or crystal microphone unit like those described in FIGS. 4 and 5 above. 
     Similar to the SAW interrogator unit shown in FIG. 2, the capacitor/crystal interrogator unit  98  transmits a short RF burst  113  (e.g., 1 microsecond burst) generated by gating the continuous signal output  111  of an oscillator  112 . The short RF burst  113  is transmitted to a capacitor or crystal microphone unit, via a transmit antenna  114 , where it may be modulated with a voice sound wave signal. The modulated RF burst is received by a receive antenna located in the microphone unit, which excites the microphone unit resonant circuit storing energy in an alternating current at the radio frequency. When the interrogator&#39;s transmitted burst  113  stops, the stored energy in the microphone unit&#39;s resonant circuit continues as an alternating current at the microphone unit&#39;s own resonant frequency, which retransmits a “ringing” radio signal out its antenna as it loses energy. 
     Referring still to FIG. 6, the ringing radio signal transmitted out the microphone unit&#39;s antenna is received at the interrogator unit receive antenna  116  as a plurality of RF echo burst signals  117 . The signals  117  are each time-gated and amplified by a RF receive switch  115  and a low noise amplifier  119 , respectively. And, unlike the interrogator unit illustrated in FIG. 2, the signals  117  are demodulated from the frequency modulated echo of the capacitor or varactor microphone. To accomplish this, a phase locked loop  110  creates a narrowband continuous signal  121  that represents the average frequency and phase of the sequence of frequency-modulated echoes  117  from the microphone. The phase of this average signal  121  will vary along with the frequency of the echoes  117 , since the echoes  117  are initially in phase with the transmitted signal  113 , but then shift in phase over time due to their different frequencies. Thus, the phase of the signal  121  at the output of the phase locked loop  110 , when compared to the continuous signal  111  of the SAW oscillator  112 , is a measure of the pressure at the microphone and the multiplier (phase detector)  129  creates a voltage signal  123  corresponding to this phase. The voltage signal  123 , after low pass filtering at the filter  111 , becomes the audio signal  125  representing the sound heard at the microphone that is analyzed at the voice vocoder  127 . 
     Alternatively, the interrogator unit  98 , instead of transmitting short RF bursts  113 , could transmit a continuous signal, and the receiving capacitor or crystal microphone unit could receive signals from the interrogator unit on one polarization and retransmit the modulated signal on another polarization. Thus, the microphone unit could differentiate between a signal received from the interrogator unit and its own transmitted signal. The amplitude of the received signal, as described in previous embodiments, would vary with the sound wave pressure in the air surrounding the microphone unit, depending on how close or far the microphone (capacitive or varactor) resonance was in frequency from the interrogator unit&#39;s transmitted frequency. 
     Referring to FIG. 7, in accordance with another embodiment of the present invention, an optical microphone unit  120 , is shown. The optical microphone unit  120  includes a sealed housing  122 , a transparent diaphragm  124  mounted in an opening of the housing  122 , a lower optical grating  128 , an upper optical grating  126 , and an array of small corner cubes  130 . 
     In the present embodiment, the air-pressure from the sound of the driver&#39;s voice pushes and pulls the diaphragm  124  in a vertical motion. The force from this pressure is then converted from vertical to horizontal pressure by a bent lever  132 , which pivots against a notched bracket  134 . The lever  132  is held in place by a tab  136  protruding from the bottom of the diaphragm  124 . Spring tension in the spring clip  138  applies a force to the optical grating  126 , tending to push the grating  126  to the left. Pushing the grating  126  in this manner insures that when the diaphragm  124  moves up and down, the bent lever  132  stays in contact with the diaphragm  124 , a fulcrum positioning notch  131  in the notched bracket  134 , and the upper optical grating  126 . When a top portion of the bent lever  132  is pushed downward, a lower portion of the lever  132  moves to the left, allowing the spring clip  138  to push the upper optical grating  126  to the left while maintaining contact between the upper optical grating  126  and the bent lever  132 . 
     Referring to FIG. 8, because the lower grating  128  is fixed, when the upper grating  126  is displaced by the air pressure and linkage of the driver&#39;s voice sound wave, the degree of light blockage by the combination of the two gratings ( 126 ,  128 ) changes accordingly. In particular, referring to FIG. 8 a , the pair of gratings ( 126 ,  128 ), each containing a pattern of alternating transparent and opaque lines, modulate the amplitude of transmitted light by changing the fraction of the combined pattern which is opaque. Depending on the position of the moving grating, the transmission ranges from approximately 0% to 50%. As shown in FIG. 8 b , the gratings ( 126 ,  128 ) are adjusted, for example, by shifting the moving grating  126  to the right, such that in the absence of sound, they are displaced by w/2 from each other with a transmission of approximately 25%, where w equals the width of an opaque line or transparent line in the grating (e.g., w=0.001 inches). As shown in FIG. 8 c , if the pressure of the driver&#39;s sound wave displaces the moving grating  126  one line width (w) farther to the right than the stationary grating  128 , the transmission is reduced gradually down to 0%. And, as shown in FIG. 8 d , if the pressure of the driver&#39;s voice shifts the moving grating  126  so that the opaque lines in the grating  126  are directly above the opaque lines in the grating  128 , the transmission is increased up to a maximum of 50%. Thus an advantageous rest position of the upper grating  126  in the absence of sound would be displaced w/2 left or right from the lower grating  128 , so that transmission was 25%. In this rest position, a sound wave would be able to continuously vary optical transmission with pressure changes in both directions up to a maximum of 50% and down to a minimum of 0%. 
     Referring again to FIG. 7, light from an optical microphone interrogator unit, in FIG. 9 described below, passes through a diaphragm  124  and shines on the pair of gratings ( 126 ,  128 ). The diaphragm  124  is preferably transparent, but may alternatively be mostly opaque except for a transparent window region. The instantaneous position of the upper grating  126  determines how much light passes through the grating pair ( 126 ,  128 ). Light that passes through the grating pair ( 126 ,  128 ) is reflected by the array of corner cubes  130  located at the base of the microphone unit housing  122 . The array of corner cubes  130  reflect the light in such a manner that the light reflects back through the gratings ( 126 ,  128 ) and into the interrogator unit. By converting the amplitude of the reflected light to a voltage using a photodetector, the optical microphone interrogator unit, as described in detail below, can recover an electrical audio signal corresponding to the sound detected at the microphone unit  120 . 
     Referring to FIG. 9, an optical microphone interrogator unit  150  is illustrated including an oscillator  152  or alternatively a pulse generator, a laser or modulated light emitting diode (LED)  154  in the near infrared (IR) range, a photodetector and amplifier element  156 , a multiplier  158 , a low pass filter (LPF)  160  and a signal processor  162 . The interrogator unit  150  is preferably mounted in the dashboard of a car where it is visible to the driver&#39;s seatbelt optical microphone unit. 
     The oscillator  152  produces a 20 kHz signal  153  that powers the near-infrared (IR) light emitting diode (LED)  154  so that the LED  154 , herein further referenced as a synchronous detector, transmits 20,000 pulses per second of light  155 . The 20 kHz signal  153  is also fed to the multiplier  158  as reference for detecting received light. A modulated version of the optical signal pulse  155  is later returned from the optical microphone unit where the light  163  is received and amplified by the photodetector and amplifier unit  156 . The amplified signal  157  is applied to the multiplier  158  where it is synchronously detected to improve its signal-to-noise ratio, thus eliminating all unwanted light signals not modulated at a frequency corresponding to the oscillator&#39;s  152  center frequency. The low pass filter  160 , preferably a 10 KHz filter, converts the amplitude modulated signal  159  to a smooth voltage signal  161  that is the electrical audio signal corresponding to the sound of the driver&#39;s voice. As in previous embodiments, the signal  161  is sent to the signal processor unit  162  where, using conventional voice recognition techniques, the signal processor  162  interprets the electrical audio signal as that corresponding to the driver&#39;s voice commands. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.