Simultaneous dual use of audio equipment as a speaker and microphone

Abstract

An electrostatic acoustic device is operated simultaneously as a speaker and as a microphone. The electrostatic acoustic device includes a membrane and an electrode disposed proximate to the membrane. An input varying audio signal is input to the electrostatic acoustic device. The membrane is configured to mechanically respond to a varying electric field in response to the varying audio signal input. A portion of the input varying audio signal is tapped to generate a reference signal. A signal is detected in response to the membrane movement and converted to an output varying voltage signal. The output varying voltage signal is compared to the reference signal to generate a microphone signal. The microphone signal is responsive to the membrane movement induced by air pressure fluctuations of the ambient sound.

Description

[Technical field]

[0001] The present invention relates to electrostatic audio devices, including earphones and speakers. [Background technology]

[0002] In the art of high fidelity sound reproduction, electrostatic loudspeakers have garnered attention for their inherent superior sound quality and smooth response over a wide frequency range. In such devices, a flexible sound-generating membrane is positioned near an electrode or, in the case of a push-pull configuration, near a pair of electrodes, one on each side of the membrane. A polarization potential is applied between the membrane and the electrodes, and an audio signal is superimposed on the electrodes, thereby causing the membrane to move in response to the audio signal. The electrodes are acoustically transparent so that sound produced by the moving membrane radiates outward through the electrodes to a listening area.

[0003] Electrostatic devices are highly efficient, both electrically and mechanically. The electrical impedance is high and decreases with increasing acoustic frequency. The high electrical impedance results in very low operating currents and minimal electrical losses. Mechanically, there are no moving parts other than a very light moving membrane. Thus, electrostatic devices are inherently more energy efficient than the electrodynamic audio devices currently used in battery-powered electronics. Summary of the Invention

[0004] Disclosed herein are various methods and drivers for configuring an electrostatic acoustic device to operate simultaneously as a speaker and as a microphone. The electrostatic acoustic device includes a membrane and an electrode disposed proximate to the membrane. An input varying audio signal is input to the electrostatic acoustic device. The membrane is configured to mechanically respond to a varying electric field in response to the varying audio signal input. A portion of the input varying audio signal is tapped to generate a reference signal. A signal is detected in response to the movement of the membrane and the signal is converted to an output varying voltage signal. The output varying voltage signal is compared to the reference signal to generate a microphone signal. The microphone signal is responsive to the movement of the membrane induced by air pressure fluctuations of the ambient sound. The input varying audio signal is input to the membrane and the electrodes may be connected to a high voltage dual DC bias symmetric or asymmetric source. Alternatively, the input varying audio signal may be input to the electrodes and the membrane may be connected to a high voltage DC bias. The electrodes may include a first electrode disposed on a first side of the membrane and a second electrode disposed on a second side of the membrane opposite the first side. The input varying audio signal may include an inverted varying audio signal input to the first electrode and a non-inverted varying audio signal input to the second electrode. The reference signal may be responsive to the inverted varying audio signal input and the non-inverted varying audio signal input. The radio frequency varying probe signal may be injected into the input of the electrostatic acoustic device. The detection may be performed by converting the current signal output or the charge signal output to a modulated voltage signal. The current signal or the charge signal may include an audio frequency varying audio signal that modulates the radio frequency of the probe signal. The modulated voltage signal may be demodulated to generate an output varying voltage signal that varies at the audio frequency. The output varying voltage signal that varies at the audio frequency may be obtained by homodyne detection of the radio frequency modulated voltage signal. The homodyne detection of the modulated radio frequency carrier signal may be achieved via a lock-in amplifier detector having an output low pass filter bandwidth higher than the audio frequency range of interest. The radio frequency modulated voltage signal may be phase / frequency locked and the radio frequency carrier signal responsive to the probe signal may vary at the radio frequency. The oscillator signal may be generated synchronously with the radio frequency carrier of the modulated voltage signal.The probe signal may be output in response to a synchronized oscillator signal. Demodulation of the modulated voltage signal may be performed by low pass filtering or by rectification prior to low pass filtering.

[0005] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Brief description of the drawings]

[0006] [Figure 1] 1 is a schematic cross-sectional view of an electrostatic device in accordance with an aspect of the present invention; [Diagram 2] FIG. 1 is a system diagram including an electrostatic acoustic device and its driver for dual use as a speaker and microphone. [Figure 3A] FIG. 1 is an electronic block diagram of an electrostatic acoustic device and its driver. [Figure 3B] FIG. 3B is a further detailed view of the embodiment of the invention shown in FIG. 3A. [Figure 4A] 1 is a schematic diagram of an alternative driver for an electrostatic acoustic device in accordance with an aspect of the present invention; [Figure 4B] FIG. 3B is a further detailed view of the embodiment of the invention shown in FIG. 3A. [Diagram 5] FIG. 1 is a flow diagram of a method illustrating features of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] The foregoing and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

[0008] Reference will now be made in detail to the features of the present invention, examples of which are illustrated in the accompanying drawings, in which like reference numerals refer to like elements, and in which the invention will now be described by way of a description of its features, with reference to the accompanying drawings, in which:

[0009] By way of preamble, different aspects of the invention may be directed to circuits for in-ear and / or over-ear electrostatic acoustic devices that may be used simultaneously as headphones and microphones. The circuits may be designed for electrostatic speakers of maximum dimensions (e.g., diameter D of 50 millimeters or less), or in some embodiments, dimension D of 25 millimeters or less, or in still other embodiments, dimension D of 10 millimeters or less. For earphone applications, the electrostatic speaker may have a maximum dimension (e.g., diameter D of 5 millimeters or less).

[0010] Thus, in an embodiment of the invention involving an electrostatic acoustic device 10 used as an earphone and enclosed in the ear canal, the mechanical displacement of the tympanic membrane may be coupled with the mechanical displacement of the membrane 15. The user's voice is transmitted internally to the tympanic membrane by bone conduction and by an internal coupling to the membrane 15 that allows the membrane 15 to be used as a microphone.

[0011] Referring now to the drawings, reference is made first to FIG. 1, which is a schematic diagram of an electrostatic acoustic device 10 in accordance with aspects of the present invention. A vertical axis Z is shown through the center of the acoustic device 10. A tensioned membrane 15 is supported by the edges of the electrodes 11 in a plane essentially perpendicular to the vertical axis Z. The membrane 15 may be impregnated with a conductive, resistive, and / or electrostatic material such that the membrane 15 is mechanically responsive to changes in an electric field. A central region of the electrodes 11 is mounted proximate (e.g., parallel) to and nominally equidistant from the membrane 15 at a distance D (e.g., 20-500 micrometers) from the membrane 15. The illustrated electrodes 11 may be perforated with an aperture 12 that is transparent to sound waves emanating from the membrane 15 when the electrostatic acoustic device 10 is operating. Alternatively or additionally, one or more side ports 13 may allow sound waves to pass from the air surrounding the membrane 15 to the outside of the device 10.

[0012] During operation of the electrostatic acoustic device 10, a constant direct current (DC) bias voltage (e.g., +V DCAn audio input voltage signal ±V = +100 to +1000 volts) can be applied to the membrane 15 using conductive contacts. i may be applied to the electrode 11. Alternatively, a voltage signal V i can be applied to the membrane 15 and the electrodes 11 can be connected to ±V DC The voltage signals ±Vi can be biased at audio frequencies nominally 20 to 20,000 Hertz. The non-inverted voltage signal +V i is applied to one of the electrodes 11, and an identical but inverted voltage signal -V i can be applied to the other electrode 11. The dotted lines represent the voltage signals ±V i 1 shows a schematic of a membrane 15 that moves in response to a change in voltage due to a

[0013] Reference is now made to Fig. 2, which is a simplified electronic system block diagram 20 including electrostatic acoustic device 10, and Fig. 5, which is a flow diagram 50 of a method in accordance with an aspect of the present invention for dual operation simultaneously as a speaker and as a microphone. Block 26 supplies a voltage signal V to drive electrostatic acoustic device 10 to generate sound from movable membrane 15. i The reference signal 21 represents a driver or electronic circuit that inputs (step 51) an input audio signal V i , is divided or tapped (step 53) and input to comparator 23. Block 26 detects a signal proportional to or responsive to the mechanical movement of membrane 15 (step 55) and generates a signal, e.g., a voltage V o (Step 57). The voltage output signal V o is the second input to comparator 23. Comparator 23 converts reference signal 21 into output voltage signal V o (e.g., comparing the reference signal 21 to the output voltage signal V o 1 and 2. The microphone 25 may be arranged to receive the signal (subtracted from the signal) and, using appropriate signal processing, extract a microphone signal 25 responsive to vibrations of the membrane 15 caused by an external sound pressure.

[0014] Detection of a signal proportional to or responsive to the mechanical movement of membrane 15 (step 55) may be performed by various detection methods known in the art. Detection of electrostatic current or capacitance changes between membrane 15 and electrode 11 is described in more detail below with reference to Figures 3-7. Other detection (step 55) methods for measuring the movement of membrane 15 may be used according to different embodiments of the invention, including, by way of example, external field gradient (force) detection, such as electrostatic or magnetic field gradients using optical sensors, Hall effect magnetic sensors.

[0015] For any detection method (step 55) that is responsive to movement of the membrane 15, the microphone signal may be extracted (step 59). The subtraction may be performed in the time domain by digital signal processing with appropriate level adjustments and / or time delays. Alternatively, the subtraction may be performed in the frequency domain by transforming the signal (e.g., short-time Fourier transform), performing the subtraction in the frequency domain, and performing an inverse Fourier transform back to the time domain to extract the microphone signal (step 59).

[0016] Reference is now made to Figure 3A, which is a more detailed schematic diagram of a circuit 26A that is an alternative to system 26 of Figure 2, in accordance with an aspect of the present invention. Driver 26A includes an electrostatic acoustic device 10 that receives a high voltage audio input +V at a first electrode 11 fluctuating at audio frequencies intended to be converted to sound by electrostatic acoustic device 10. i and an inverted high voltage audio input -V at the second electrode 11. i In addition, membrane 15 may be mechanically responsive such that device 10 may behave as a microphone to ambient sound waves.

[0017] In response to the ambient sound, the distance D (FIG. 1) between the membrane 15 and the electrode 11 changes, resulting in a change in the capacitance C of the electrostatic acoustic device 10. The change in current i(t) due to the ambient sound may be sensed using a detector 30, which may include a transimpedance amplifier. The change in current i(t) may be approximated by the following equation:

number

[0018] The reference signal 21 is a reference to one or more input audio signals ±V i , is divided or tapped (step 53) and input to the comparator 23. o is the second input to comparator 23. Comparator 23 converts reference signal 21 into output voltage signal V o (e.g., comparing the reference signal 21 to the output voltage signal V o 1 and 2. The microphone 25 is adapted to receive the signal (subtract from the signal) or to extract a microphone signal 25 responsive to a sound that induces vibration of the membrane 10 .

[0019] Reference is now made to Figure 3B, which is a more detailed schematic diagram of driver 26A of Figure 3A, in accordance with an aspect of the present invention. A probe signal from a local oscillator (LO) 51, for example at a radio frequency of 0.1 to 2 megahertz, may be coupled across a primary winding P of a transformer T. An audio signal +V i and the inverted audio signal -V i can be supplied to the electrodes 11 via series-connected secondary windings S1, S2 of a transformer T. Audio signals ±V i can be a high voltage signal. Alternatively, the audio signal ±V i can be a low voltage signal of about ±20V or less with a high DC voltage applied to the membrane 15 as shown in the device 10 (FIG. 1). The probe signal generates a current with a magnitude determined by the characteristic reactance of the electrical circuit formed by the membrane 15 and the electrode 11, essentially a variable capacitor. The advantage of using radio frequency is that it does not generate perceptible mechanical movement, but is modulated by the electrical change in capacitance associated with the mechanical movement generated when an audio signal is present. Furthermore, a radio frequency amplitude modulated signal may have a higher SNR with respect to the total capacitance change of the device when compared to the current induced by the direct capacitance change shown in relation (2).

[0020] The change in current i(t) due to ambient sound is made explicit using a transimpedance amplifier 40. A probe signal from a local oscillator (LO) 51 may be combined with the voltage output of amplifier 40 in a signal combiner / multiplier 32. Amplifier 40 may be configured to be inverting or non-inverting, centered outside the band of audio frequencies from 0.1 to 2 megahertz, which includes the radio frequency of LO 51, and preferably away from any resonance of membrane 15. Signal combiner / multiplier 32 outputs to a low pass filter 34, which generates a voltage output signal V that varies at audio frequencies. o The system 26A may be a homodyne detection circuit using a local oscillator 51 multiplied with the measurement signal output of the amplifier 40 as a reference. The baseband or DC component of this multiplication contains a frequency-converted signal from a narrow band around the LO 52 frequency that is detected with a very high signal-to-noise ratio. The multiplier 32 may be realized, for example, with an analog circuit AD835 manufactured by Analog Devices, Inc. (NorwooD, Mass., USA).

[0021] Alternatively, instead of the transimpedance amplifier 40, a charge amplifier can be considered, which integrates the current i(t) to sense the charge Q(t) that varies with the change in capacitance of the electrostatic acoustic device 10, and the sensed charge is fed to an output voltage signal V o Amplifier 40 may be configured to be inverting or non-inverting and may have a band pass that includes audio frequencies from 20 to 20,000 Hertz.

[0022] Reference is now made to Figures 4A and 5, which are schematic diagrams of another alternative 26B of block 26 of Figure 2 in accordance with an aspect of the present invention. i A bias voltage V may be applied to the membrane 15 (step 51). DC is connected to the first electrode 11, DC / 2 is applied to the second electrode 11, and +V DCDetector 31 may be used with inputs capacitively coupled to electrodes 11 respectively. The voltage output V of detector 31 is o may vary with the capacitance of the device 10 (step 55). The reference signal 21 is i , is divided or tapped (step 53) and input to the comparator 23. o is the second input to comparator 23. Comparator 23 converts reference signal 21 into output voltage signal V o (step 59), for example, comparing the reference signal 21 with the output voltage signal V o 1 and 2. The microphone 25 is adapted to subtract from or extract a microphone signal 25 responsive to a sound that induces vibration of the membrane 10 .

[0023] Reference is now made to Figure 4B, which is a more detailed diagram of a driver 26B as an alternative to block 26 of Figure 2, in accordance with a feature of the present invention. i can be applied to the membrane 15. The probe signal from the local oscillator 51 is connected to the primary P in parallel with the local oscillator 51 and the audio voltage V i A probe signal may be induced on the membrane 15 using a transformer T having a secondary S connected in series between the electrode 11 and the membrane 15. Another way of injecting the probe signal into the membrane may use capacitive coupling via a dedicated high voltage ceramic capacitor. A differential amplifier 41 may be used with its inputs each capacitively coupled to the electrode 11. The voltage output of the differential amplifier 41 varies with the capacitance of the device 10. The probe signal from a local oscillator (LO) 51 may be combined with the voltage output of the differential amplifier 41 in a signal combiner / multiplier 32. The signal combiner / multiplier 32 outputs to a low pass filter 34 which produces a voltage output signal V varying at audio frequencies. o and transmits the signal. Differential amplifier 41 may be implemented with a Texas Instruments / Burr-Brown® INA105. Driver 26B has an advantage over driver 26A because, in accordance with a feature of the present invention, one high voltage input amplifier can be used instead of two.

[0024] 4A and 4B, an alternative embodiment of the present invention is to connect a transformer T to the audio voltage ±V i can be replaced by a capacitive coupling.

[0025] The term "homodyne" as used herein refers to a method of detection / demodulation of a signal that is phase and / or frequency modulated onto an oscillator signal by combining with a reference oscillation.

[0026] The term "surrounding" as used herein refers to the vicinity of the membrane of an electrostatic acoustic device.

[0027] As used herein, the term "driver" refers to an electronic circuit configured to electrically bias, input, and / or output a signal from an electrostatic acoustic device.

[0028] As the term is used herein, a "phase sensitive detector circuit" is essentially an electronic circuit containing a multiplier (or mixer) and a loop filter that produces a DC output signal proportional to the product of the amplitudes of two AC input signals of the same frequency and the cosine of the phase between them.

[0029] As used herein, the term "transimpedance amplifier" refers to a current to voltage converter. A transimpedance amplifier may be used to process a current output of a sensor into a voltage signal output.

[0030] A "charge amplifier," as the term is used herein, converts a time-varying charge into a voltage output, typically via an integrated time-varying current signal.

[0031] The terms "audio" or "audio frequency" refer to the rate of oscillation of an alternating current or voltage, or of a magnetic, electric or electromagnetic field, or a mechanical system, in the frequency range 0 to 20,000 Hertz.

[0032] As used herein, the terms "audio signal", "audio output" and "audio output signal" refer to electrical signals that essentially fluctuate at audio frequencies.

[0033] The term "radio frequency" (RF) refers to the rate of oscillation of an alternating current or voltage, or a magnetic, electric or electromagnetic field, or a mechanical system, in the frequency range of about 20,000 times per second (20 kHz) to about 300 billion times per second (300 GHz).

[0034] The transitional term "comprising" as used herein is synonymous with "including" and is inclusive or open-ended and does not exclude additional elements or method steps not expressly recited. The articles "a" and "an" as in "a circuit" or "an electrode" have the meaning of "one or more" as in "one or more circuits" and "one or more electrodes."

[0035] All optional and preferred features and modifications of the described embodiments and the dependent claims can be used in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, and all optional and preferred features and modifications of the described embodiments can be combined and substituted for each other.

[0036] Although selected features of the invention have been illustrated and described, it will be understood that the invention is not limited to the described features.

Claims

1. 1. A method comprising configuring an electrostatic acoustic device to simultaneously operate as a speaker and as a microphone, the electrostatic acoustic device including a membrane and an electrode disposed proximate to the membrane, the operating comprising: applying an input varying audio signal input to the electrostatic acoustic device, the membrane being configured to mechanically respond to a varying electric field in response to the varying audio signal input; tapping a portion of the input varied audio signal to generate a reference signal; detecting movement of the membrane thereby receiving a signal responsive to the movement of the membrane and converting the signal into an output varying voltage signal; comparing the output fluctuating voltage signal with the reference signal to generate a microphone signal responsive to movement of the membrane induced by ambient sound air pressure fluctuations; The method is realized by enabling

2. applying the input varying audio signal to the membrane; connecting the electrodes to a high voltage DC bias; The method of claim 1 further comprising:

3. applying the input varying audio signal to the electrodes; connecting the membrane to a high voltage DC bias; The method of claim 1 further comprising:

4. 4. The method of claim 1, wherein the electrodes include a first electrode disposed on a first side of the membrane and a second electrode disposed on a second side of the membrane opposite the first side, the input varying audio signal includes an inverted varying audio signal input to the first electrode and a non-inverted varying audio signal input to the second electrode, and the reference signal is responsive to the inverted varying audio signal input and the non-inverted varying audio signal input.

5. injecting a radio frequency varying probe signal into the input of said electrostatic acoustic device; converting the current or charge signal output to a modulated voltage signal, the current or charge signal comprising an audio signal varying at an audio frequency that modulates the radio frequency of the probe signal; demodulating the modulated voltage signal to generate the output varying voltage signal varying at an audio frequency; The method of any one of claims 1 to 3, further comprising:

6. 6. The method of claim 5, wherein the output varying voltage signal varying at an audio frequency is obtained by homodyne detection of the modulated voltage signal at a radio frequency.

7. phase / frequency locking a radio frequency carrier signal responsive to said radio frequency modulated voltage signal and said radio frequency varying probe signal; The method of claim 5 further comprising:

8. generating an oscillator signal synchronized with a radio frequency carrier of the modulated voltage signal; outputting the probe signal in response to the synchronized oscillator signal; The method of claim 5 further comprising:

9. The method of claim 5 , wherein demodulating the modulated voltage signal is performed by low-pass filtering.

10. The method of claim 5 , further comprising performing the demodulation by rectification before low-pass filtering.

11. 1. A driver for an electrostatic acoustic device including a membrane and an electrode disposed proximate to the membrane, the driver configured to cause the electrostatic acoustic device to operate simultaneously as a speaker and as a microphone, the operating comprising: applying a varying audio signal input to the electrostatic acoustic device, the membrane being configured to mechanically respond to a varying electric field in response to the varying audio signal input; tapping a portion of the input varied audio signal to generate a reference signal; detecting movement of the membrane thereby receiving a signal responsive to the movement of the membrane and converting the signal into an output varying voltage signal; comparing the output fluctuating voltage signal with the reference signal to generate a microphone signal responsive to movement of the membrane induced by ambient sound air pressure fluctuations; This is achieved by the driver.

12. applying the input varying audio signal to the membrane; connecting the electrodes to a high voltage DC bias; The driver of claim 11 further configured to:

13. applying the input varying audio signal to the electrodes; connecting the membrane to a high voltage DC bias; The driver of claim 11 further configured to:

14. The electrostatic acoustic device includes a first electrode disposed on a first side of the membrane and a second electrode disposed on a second side of the membrane opposite the first side, and the driver includes: inputting an inverted varying audio signal to the first electrode and a non-inverted varying audio signal to the second electrode, the reference signal being responsive to the inverted varying audio signal input and the non-inverted varying audio signal input.

14. A driver according to claim 11 or 13, configured to:

15. injecting a radio frequency varying probe signal into the input of said electrostatic acoustic device; converting a current signal or a charge signal output from the electrostatic acoustic device, the current signal or the charge signal including an audio signal fluctuating at an audio frequency that modulates the radio frequency of the probe signal, into a modulated voltage signal; demodulating the modulated voltage signal to generate the output varying voltage signal varying at an audio frequency; The driver of claim 11 further configured to:

16. 16. The driver of claim 15, further configured to obtain the output varying voltage signal varying at an audio frequency by homodyne detection of the modulated voltage signal at a radio frequency.

17. phase / frequency locking a radio frequency carrier signal responsive to said radio frequency modulated voltage signal and said radio frequency probe signal; The driver of claim 15 further configured to:

18. generating an oscillator signal synchronized with a radio frequency carrier of the modulated voltage signal; outputting the probe signal in response to the synchronized oscillator signal; The driver of claim 15 further configured to:

19. 16. The driver of claim 15, further comprising a low pass filter for demodulating the modulated voltage signal.

20. 16. The driver of claim 15, further comprising a rectifier configured to demodulate by rectifying before low-pass filtering.

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