The present invention relates to electrostatic audio devices, including earphones and loudspeakers.
In the art of high fidelity sound reproduction, the electrostatic loudspeaker has received attention because of inherent excellent sound quality and smooth response over wide frequency ranges. In such devices, a flexible sound producing membrane is positioned near an electrode, or in the case of a push-pull arrangement, a pair of electrodes, one on either side of the membrane. A polarization potential is applied between the membrane and the electrodes, and an audio signal is superimposed on the electrodes, causing the membrane to move in response to the audio signal. Electrodes are acoustically transmissive so that sound produced by the moving membrane radiates outward through the electrode to the listening area.
Electrostatic devices are highly efficient both electrically and mechanically. Electrical impedance is high and decreases with increasing acoustic frequency. High electrical impedance results in very low operating currents and minimal electrical losses. Mechanically, there are no moving parts other than the moving membrane which is very light in weight. Electrostatic devices are therefore inherently more energy efficient than electrodynamic acoustic devices currently used in battery operated electronic devices.
Various methods and drivers are disclosed herein 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 respond mechanically to a varying electric field responsive to the varying audio signal input. A portion of the input varying audio signal is tapped to produce a reference signal. A signal is detected responsive to motion of the membrane, to convert the signal to an output varying voltage signal. The output varying voltage signal is compared to the reference signal to produce a microphone signal. The microphone signal is responsive to motion of the membrane induced by air pressure variations of ambient sound. The input varying audio signal may be input to the membrane and the electrodes may connect to a high voltage dual DC bias symmetric or asymmetric source. Alternatively, the input varying audio signal may be input to the electrode and the membrane may be connected to a high voltage DC bias. The electrode 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. A probe signal varying at radio frequency may be injected into an input of the electrostatic acoustic device. The detection may be performed by converting a current or charge signal output to a modulated voltage signal. The current or charge signal may include an audio signal varying at audio frequencies modulating the radio frequency of the probe signal. The modulated voltage signal may be demodulated to produce the output varying voltage signal varying at audio frequency. The output varying voltage signal varying at audio frequency may be obtained by homodyne detection of the modulated voltage signal at radio frequency. The homodyne detection of the modulated radio frequency carrier signal may be achieved via a lock-in amplifier detector having the output low pass filter bandwidth higher than the audio frequency range of interest. The modulated voltage signal at radio frequency may be phase and frequency locked and a radio frequency carrier signal responsive to the probe signal may vary at radio frequency. An oscillator signal may be generated synchronous with a radio frequency carrier of the modulated voltage signal. The probe signal may be output responsive to the synchronous oscillator signal. The demodulation of the modulated voltage signal may be performed by low pass filtering or by rectifying prior to low pass filtering.
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.
Reference will now be made in detail to features of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The features are described below to explain the present invention by referring to the figures.
By way of introduction, different aspects of the present invention may be directed to a circuit for in-ear and/or over-ear electrostatic acoustic device which may be used simultaneously as a headphone and microphone. Circuits may be designed for an electrostatic speaker of maximum dimension, e.g. diameter D of 50 millimetres or less, or in some embodiments an electrostatic speaker of dimension D of 25 millimetres or less, or in yet other embodiments an electrostatic speaker of dimension D of 10 millimetres or less. For an earphone application, an electrostatic speaker may have maximum dimension, e.g. diameter D of 5 millimetres or less.
Thus, in embodiments of the present invention including electrostatic acoustic device 10 being used as an earphone and sealed into the ear canal, the mechanical displacement of the ear drum may become coupled with the mechanical displacement of membrane 15. Voice of a user may be transmitted internally by bone conduction to the ear drum and by the internal coupling to membrane 15 enabling membrane 15 for use as a microphone.
Referring now to the drawings, reference is now made to
During operation of electrostatic acoustic device 10, a constant direct current (DC) bias voltage, e.g. +VDC+100 to +1000 volts, may be applied using a conductive contact to membrane 15. Audio input voltage signals ±Vi may be applied to electrodes 11. Alternatively, voltage signal Vi may be applied to membrane 15 and electrodes 11 may be biased at ±VDC. Voltage signals ±Vi may vary at audio frequencies, nominally between 20-20,000 Hertz. A non-inverted voltage signal +Vi may be applied to one of electrodes 11 and an identical but inverted voltage signal −Vi may be applied to the other electrode 11. Dotted lines illustrate schematically membrane 15 moving in response to a changing electric voltage due to voltage signals ±Vi.
Reference is now also made to
Detection (step 55) of a signal proportional to or responsive to mechanical motion of membrane 15 may be performed by various detection methods known in the art. Detection of a change in electrostatic current or change in capacitance between membrane 15 and electrodes 11 is further described hereinafter in reference to
For any detection method (step 55) responsive to membrane 15 motion, a microphone signal may be extracted (step 59). Subtraction may be performed in the time domain by digital signal processing with an appropriate level adjustment and/or time delay. Alternatively, subtraction may be performed in the frequency domain by transforming the signals, 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 a microphone signal (step 59).
Reference is now made to
In response to ambient sound, distance d (
A reference signal 21 is split or tapped (step 53) from one or more input audio signals ±Vi and input to a comparator 23. Voltage output signal Vo is a second input to comparator 23. Comparator 23 is configured to compare reference signal 21 to output voltage signal Vo, e.g. subtract reference signal 21 from output voltage signal Vo or otherwise extract a microphone signal 25 responsive to sound inducing vibrations of membrane 10.
Reference is now made to
A changing current i(t) due to ambient sound is now shown using a trans-impedance amplifier 40. Probe signal from local oscillator (LO) 51 may be combined with the voltage output of amplifier 40 at signal combiner/multiplier 32. Amplifier 40 may be configured to be inverting or non-inverting, centred out-of-band for audio frequencies, between 0.1-2 megahertz including the radio frequency of LO 51, and preferably far from any resonances of membrane 15. Signal combiner/multiplier 32 outputs to a low pass filter 34 which demodulates and transmits voltage output signal Vo, varying at audio frequencies. System 26A is a homodyne detection circuit which uses local oscillator 51 as a reference which is multiplied with the measured signal output of amplifier 40 at the same frequency. The base band or DC component of this multiplication includes the signal which is frequency converted from a narrow band around LO 52 frequency detected with a very high signal to noise ratio. Multiplier 32 may be implemented with analogue circuit AD835 from Analog Devices Inc (Norwood, MA, USA), by way of example.
Alternatively, a charge amplifier may be considered, instead of a transimpedance amplifier 40, which integrates current i(t) to sense charge Q(t) which varies with changing capacitance of electrostatic acoustic device 10, and the sensed charge is converted to an output voltage signal Vo. Amplifier 40 may be configured to be inverting or non-inverting, and may have a band-pass including audio frequencies, 20-20000 Hertz.
Reference is now also made to
Reference is now made to
Still referring to
The term “homodyne” as used herein refers to a method of detection/demodulation of a signal which is phase and/or frequency modulated onto an oscillating signal by combining with a reference oscillation.
The term “ambient” as used herein refers to vicinity of the membrane of the electrostatic acoustic device.
The term “driver” as used herein is an electronic circuit configured to electrically bias, input and/or output signals from an electrostatic acoustic device.
The term “phase sensitive detector circuit” as used herein is an electronic circuit including essentially a multiplier (or mixer) and a loop filter that produces a direct-current output signal that is proportional to the product of the amplitudes of two alternating-current input signals of the same frequency and to the cosine of the phase between them.
The term “transimpedance amplifier” as used herein converts current to voltage. Transimpedance amplifiers may be used to process current output of a sensor to a voltage signal output.
The term “charge amplifier” as used herein converts a time varying charge to a voltage output typically by integrated a time varying current signal.
The term “audio” or “audio frequency” refers to an oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range 0-20,000 Hertz
The term “audio signal”, “audio output”, “audio output signal” as used herein refer to an electrical signal varying essentially at audio frequency.
The term “radio frequency” (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second (20 kHz) to around three hundred billion times per second (300 GHz).
The transitional term “comprising” as used herein is synonymous with “including”, and is inclusive or open-ended and does not exclude additional element or method steps not explicitly recited. The articles “a”, “an” is used herein, such as “a circuit” or “an electrode” have the meaning of “one or more” that is “one or more circuits”, “one or more electrodes”.
All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another. Although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features.
Number | Date | Country | Kind |
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2116592.3 | Nov 2021 | GB | national |