Abstract:
The present invention is a microphone that applies the principle of negative feedback directly to the diaphragm, greatly reducing the non-linearity of the diaphragm. In a further embodiment, digital negative feedback is used, incorporating the diaphragm into the digitization loop of a sigma-delta converter, creating a direct sound pressure to digital electrical output converter. In one embodiment, positive feedback is used in an analog circuit, causing a negative feedback response on the diaphragm.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention is related to the distributed digital conversion system, filed Apr. 12, 1996 application serial number 08/630,691. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a conversion of sound to electrical signals in either analog or digital form. Sound as detected by microphones is caused by pressure changes in a medium. 
     2. Description of the Related Art 
     Sound is a propagating change or series of changes in the local pressure of a medium. The device which converts these pressure changes into electrical signals is the microphone. The microphone generally consists of some sort of transduction element, which physically moves in response to the pressure changes, and a mechanism which converts this physical motion into an electrical output signal. In most cases the transduction element is a diaphragm, but this is not always the case. In the current art, there are several commonly used detection mechanisms. In what follows, the description will focus on microphones designed for use in picking up audible sound in air, however the general principal of the present invention may be applied to all other microphone types. 
     There are disadvantages to this type of system. Firstly, the diaphragm (or other transduction element) is supported by a spring composed of the tension of the diaphragm itself, among other supports which depend upon microphone type. This spring will be imperfect, and will be acceptably linear only up to a maximum sound pressure level. Secondly, the diaphragm will have mass, which will limit the frequencies to which the microphone may respond, and which will introduce one or more resonant frequencies, which will result in non-linearity of frequency response at and near a given resonant frequency. Further, there is always an analog electronic system interfacing the diaphragm with the balance of system. This interface will itself be frequency sensitive and subject to non-linearity. 
     Realized microphone designs strive to minimize the effect of these various distortions in the intensity and frequency range of interest, through suitable selection of diaphragm type and electronic components. However, highly linear microphones are expensive and complex, and as the present invention will show, highly linear microphone systems can be constructed with more accurate output as well as lower cost. Presently, this is not available to the art. 
     Feedback is the principal of returning the output of an amplified system to the input, so as to reduce the gain of the system. While a reduction in gain is often considered a detriment, the benefits are that the linearity of the amplification system becomes dominated by the linearity of the feedback device. 
     While it is quite difficult to produce a linear amplifier, it is quite easy to produce an extremely high gain but non-linear amplifier. Coupled to highly linear resistive feedback, such an amplifier can be used to construct a highly linear, moderate gain system. 
     The analog to digital converter is a device which converts an input analog signal into a digital representation. This digital representation consists of a series of numbers which represent the amplitude of the analog input at specific moments in time. The series of numbers is a discretely sampled, quantized representation of the input. 
     Of particular interest are differential coding techniques, in which the output of an initial conversion step is subtracted from the input for the next conversion step, creating an output which corresponds to the difference between one sample and the next. With the addition of an integration filter, sigma-differential coding can be achieved, wherein the output of the converter is again a direct representation of the analog input, however the spectrum of the quantization noise is shifted to higher frequencies. 
     The limit of this design paradigm is the one bit sigma-differential pulse code modulator, or “Sigma-Delta” converter. With reference to FIG. 4, the quantized output of the one bit digital to analog converter is subtracted from the input analog signal, with the difference being integrated. The output of the integrator is fed to a comparator which acts as a single bit analog to digital converter. The output of this converter both supplies the digital output to the rest of the system, as well as providing the quantized output to be subtracted from the input. 
     The operation of the sigma-delta converter is as follows. The output of the integrator is evaluated by the comparator. The comparator outputs a “1” if the integrator output is above the reference value, and a “0” if the integrator output is below the reference value. The analog values of “1” and “0” are such that, through the differential stage feedback, a “1” will tend to cause the integrator output to fall below the reference value, and a “0” will tend to cause the integrator output to climb above the reference value. Over time, the duty cycle of “1” and “0” will represent the value of the input signal. Often higher order feedback loops are used which will tend to decrease the low frequency quantization noise at the expense of high frequency noise. 
     In common audio use is a sigma-delta converter with a sampling rate equal to sixty-four times the desired decimated output, using 5th order feedback in the sigma-delta conversion stage. The one bit output of this converter is decimated using digital techniques, and produces a 16 bit output with performance of a 16 bit linear converter. 
     Sigma-Differential conversion techniques, and the sigma-delta converter may be considered examples of the digital use of negative feedback in the digital domain. The quantizer is a highly non-linear device, however through the use of feedback substantial linearity may be achieved. 
     The method of the present invention makes new use of the concept of sigma-differential conversion techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a microphone that applies the principle of negative feedback directly to the diaphragm, greatly reducing the non-linearity of the diaphragm. In a further embodiment, digital negative feedback is used, incorporating the diaphragm into the digitization loop of a sigma-delta converter, creating a direct sound pressure to digital electrical output converter. In a particularly simple embodiment, positive feedback is used in an analog circuit, causing a negative feedback response on the diaphragm without modification to the microphone itself. 
     Accordingly, besides the objects and advantages of the force balance microphone described in the present specification, several objects and advantages of the present invention are as follows. 
     It is an object of the present invention to provide a microphone which compensates for the non-linearities inherent in physical diaphragms and microphone transducers. 
     It is an advantage of the present invention that difficulties associated with coil mass, resonance, and coil velocity in moving coil microphones may be greatly reduced. 
     It is an advantage of the present invention that non-linearities associated with the diaphragm, diaphragm mass, and diaphragm suspension in condenser microphones may be greatly reduced. 
     It is an advantage of the present invention that electrical non-linearity in the piezoelectric element and mechanical damping may be greatly reduced. 
     It is an object of the present invention that novel microphone transducers characterized by high gain combined with extreme non-linearity may be profitably used in functional microphone systems. 
     It is an advantage of the present invention that such microphone systems may significantly outperform conventional microphones of equivalent cost. 
     It is an advantage of the present invention that the microphone will be able to faithfully reproduce sounds with an intensity range far greater than that of conventional microphones without volume induced distortion. 
     It is an object of the present invention to provide a microphone that functions as a direct analog pressure to digital output sigma- delta conversion device, resulting in a higher quality pressure wave measurement, and superior characteristics to those of conventional systems. 
     It is an advantage of the present invention that the microphone will be highly linear over its whole range. 
     It is an object of the present invention to provide a microphone that can use inexpensive elements to greatly enhance the output signal. 
     It is an advantage of the present invention that such a microphone would be inexpensive, and greatly enhance the quality of the sound output for a nominal cost. 
     It is an advantages of the present invention that such a microphone would be easily mass-producible. 
     Other objects and further advantages of the present invention will become apparent to those skilled in the art after a careful consideration of the following specification and accompanying drawings wherein: The force balance microphone comprises a transduction element, as found in a conventional microphone, combined with a mechanism which converts motion of the transduction element into an electrical output. Where the force balance microphone differs from the conventional microphone is that feedback means is added to prevent the motion of the transduction element. This feedback means is selected so as to be highly linear, thereby linearizing the operation of the microphone. 
     The force balance microphone thus comprises the following components: 1) the transduction element, 2) the motion detection system, 3) and the feedback system. All of these elements may be realized by a variety of means, many of these elements being used in one form or the other in the current art. 
     1) The transduction element: The most common transduction elements in the art are the diaphragm and the ribbon. Such elements may be beneficially used in the method of the present invention. Less common transduction elements which are integral with the motion detection system may also be used, for example piezoelectric films, metal-insulator-metal films, or variable refractive index materials. 
     2) The motion detection system: Again these are common in the art. Any of the aforementioned microphone devices use motion detection of one sort or another, e.g.: the condenser microphone detects capacitance changes between the diaphragm and the backplate. Motion detection systems not commonly used in the art may also be used, including tunneling current proximity detection, optical tunneling techniques, and optical interference techniques. Tunneling current detection warrants specific description because of its great simplicity and utility. 
     The quantum mechanical description of a particle uses wave equations to describe the position of the particle. This wave description of the location has a finite extent even for point particles such as the electron. An electron in one conductor has a certain finite probability of moving to another conductor, even if the two conductors are separated by a high potential barrier. The probability of an electron moving to the other conductor is greatly dependent upon the spacing between the two conductors. Tunneling current is extremely nonlinear and measurable only when the two conductors are separated by nanometer spacings. A tunneling probe may be separated from a surface by a spacing known to sub-angstrom accuracy. 
     3) The feedback system: The feedback system is not known in the art of microphones. Owing to the dual nature of many motion detection systems in common use, such systems may find new use in the method of the present invention. The feedback system may be any means useable to restore the transduction element to its equilibrium or rest state. The feedback system includes those circuit elements necessary to generate the appropriate signals for operating the feedback element, as well as the feedback element itself. As the feedback element is novel to the present invention, further general description is necessary. 
     Electrostatic feedback: The diaphragm of a condenser microphone may be moved by applying a potential difference between the diaphragm and the backplate. As potential difference is applied, the charge on the diaphragm and backplate increases, and the two are attracted together. This attraction is balanced by the diaphragm tension. Thus if a fixed potential difference is applied to the diaphragm and backplate, the diaphragm will move to an equilibrium potential. As the diaphragm moves under the influence of external pressure changes, it may be restored to the equilibrium position by altering the applied potential difference. Thus external pressure may be balanced through the use of extremely linear electrostatic forces. 
     Magnetic feedback: If a current is passed through the coil of a moving coil microphone, the coil will move. This is the principal of operation of the conventional dynamic speaker. By passing current through a coil attached to the transduction element, the transduction element may be restored to its equilibrium position. By keeping the transduction element, coil, and permanent magnet in the same relative position, the linearity of the microphone is greatly enhanced. Similarly, by passing a current through the ribbon of the ribbon microphone, the ribbon may be held in constant position. 
     Piezoelectric feedback: When a voltage is applied to a piezoelectric element, it moves. This may be used to restore a transduction element to equilibrium position. Feedback control electronics: All the above feedback mechanisms require some form of voltage or current input. Conventional analog electronics may be used to provide the feedback signals. Far more attractive, however, is to supply the feedback signal from the digital output of a differential-sigma analog to digital converter. In such a device, the mass of the diaphragm will act as the integrator, while the action of pressure variation and feedback mechanism upon the diaphragm will act as the adder element. Such a device would constitute a direct pressure to digital converter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a preferred embodiment of an analog force-balanced microphone. 
     FIG. 2 is a schematic diagram showing a condenser microphone with tunneling detection and digital electrostatic feedback. 
     FIG. 3 is a schematic diagram showing a ribbon microphone with tunneling detection and magnetic feedback. 
     FIG. 4, discussed in prior art section, is a block diagram showing a first order Delta-Sigma A/D converter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To enable an individual skilled in the art to implement the above stated principals of the present invention, specific embodiments are herein provided. These should not be considered as limiting the scope of the invention, as the above description makes clear that numerous variations are possible. The below comprise specific best modes of operation. 
     FIG. 1 shows a schematic diagram of a circuit of a particularly inexpensive force balance microphone using a single coil. It represents one embodiment of the present invention. Clearly, other modifications, applications and embodiments will become apparent to those skilled in the art. 
     With reference to FIG. 1, sound  100  displaces a diaphragm  111  and microphone coil  101 , generating electrical signals in microphone coil  101 . One output of the microphone coil  101  is buffered by operational amplifier  102 , configured as a unity gain inverting amplifier. 
     The output of operational amplifier  102  supplies high impedance variable gain inverting operational amplifier  103  and output operational amplifier  104 . Operational amplifier  103  is configured as a variable gain inverting amplifier, with gain between −1 and −2.5. Output of operational amplifier  103  is connected to the second output of coil  101 . Operational amplifier  104  is configured as a unity gain inverting amplifier. 
     Power supplies for the active components are not shown, but will be understood by anyone skilled in the art. Likewise, operational amplifier implementation details are excluded from this discussion. 
     In operation, current flow through coil  101 , caused by motion of said coil through the magnetic field of the microphone, is detected, amplified, and fed back to the microphone. The combination of microphone coil impedance and circuit negative impedance presents a relatively low impedance circuit for currents generated by the motion of coil  101  with currents being much higher than normally encountered in voltage gain microphone circuits. 
     As per Lens&#39; Law, a current produced in a conductor by changing the magnetic flux linked by the conductor will act to retard the change of flux linking. In the present embodiment, the currents generated by the microphone coil, and fed back to the coil by the amplifier circuit, act to retard the motion of the microphone coil. Thus acoustic forces are balanced by electromagnetic forces. 
     Because of the positive feedback nature of the present embodiment, if gain of operational amplifier  103  is too high, the circuit will lock up or oscillate. However at suitably tuned gain levels, it is noted that resonance peaks of the microphone system are substantially reduced, with noticeable flattening of the frequency response and an increase in frequency response range. 
     To summarize, in operation, current flow through microphone coil  101 , caused by motion of microphone coil  101  through the magnetic field of the microphone is detected, amplified, and fed back into the microphone. This amplified current flow acts against motion of the coil  101  and diaphragm  111 , thereby linearizing operation of the microphone. 
     In listening tests the above circuit had the following acoustic effects: it reduced the two resonant “humps” in the frequency response curve. This noticeably improved intelligibility and quality of speech. Secondly, there was a marked improvement in the phase response which also aided intelligibility. 
     FIG. 1 represents a preferred embodiment because its cost is extremely low, allowing it to be used in inexpensive and very common microphones, such as those in telephones, voicemail systems, and the like. 
     With reference to FIG. 2, a metalized diaphragm  60  is supported near to a backplate  61  in the conventional fashion for condenser microphones. Mounted in center of backplate  61 , and electrically isolated therefrom, is a tunneling probe  62 . Diaphragm  60  is electrically connected to a bias voltage source  63 . 
     Tunneling probe  62  is electrically connected to a transconductance amplifier  64 , the output of which is connected to a latching comparator  65 . Latching comparator  65  is supplied with a reference voltage  66  and a sampling clock  67 . Output of latching comparator  65  is fed to a one bit digital to analog converter (DAC)  68 . The one bit DAC  68  is further supplied with sampling clock  67 . Output of one bit DAC  68  is electrically connected to backplate  61 . 
     Power supplies for the active components are not shown, but will be understood by anyone skilled in the art. Likewise, operational amplifier implementation details are excluded from this discussion. 
     In operation any tunneling current between diaphragm  60  and probe  62  will cause an output voltage to be produced by amplifier  64 . This output voltage is compared to reference voltage  66  by comparator  65 . Once per clock cycle, the result of the comparison is updated, and an output voltage corresponding to “1” or “0” is produced. This output voltage is available to the one bit DAC  68 . Every clock cycle, one bit DAC  68  latches an output which corresponds to its input voltage of the preceding clock cycle. In this way, the output of comparator  65  is fed back with a delay of one clock cycle. Output of comparator  65  comprises a one bit digital data stream, and is the output of the microphone system. 
     Diaphragm  60  is biased by bias source  63  to a negative voltage relative to ground. 
     One bit DAC  68 , in response to input from comparator  65  generates pulses of positive or negative charge which are carried to backplate  61 . An input of “1” indicates that the diaphragm  60  is closer to the backplate  61  than equilibrium, and causes DAC  68  to produce a pulse of negative charge. This pulse of negative charge will reduce the attraction between backplate  61  and diaphragm  60 . An input of “0” indicates that the diaphragm  60  is more distant from the backplate  61  than equilibrium, and causes DAC  68  to produce a pulse of positive charge. This pulse of positive charge will increase the attraction between backplate  61  and diaphragm  60 . 
     The net result will be an output one bit datastream with variable duty cycle corresponding to the force necessary to maintain the diaphragm in a fixed location. 
     With reference to FIG. 3 a ribbon  40  is supported between poles of a magnet  41 , schematically shown as the magnetic field lines produced by the magnet, such that the magnetic field of magnet  41  is generally perpendicular to the long direction of ribbon  40  and is further perpendicular to the direction of motion of ribbon  40  when ribbon  40  flexes. A probe  42  is oriented perpendicular to the plane of ribbon  40 , and is further positioned proximally to the center of ribbon  40 , separated by a small gap. The probe  42  is close enough to the ribbon that the ribbon could flex so as to contact the probe. 
     The ends of ribbon  40  are electrically connected to the center tapped secondary of an audio transformer  43 . The center tap of the secondary of transformer  43  is connected to a bias voltage supply  44 , floating the secondary and the ribbon  40  about -10 volts below ground. The probe  42  is held at a virtual ground by a transconductance amplifier  45 , which converts any current between ribbon  40  and probe  42  into a voltage. 
     The output of amplifier  45  is compared with a reference voltage  46  by a comparator  47 . Reference voltage  46  is connected to the positive acting pin of comparator  47 . The output of comparator  47  charges an integrating capacitor  48 . The voltage on the integrating capacitor  48  is buffered by a unity gain follower  49 , the output of which is fed to the primary of the audio transformer  43 . The voltage on the integrating capacitor  48  is additionally buffered by a follower  50 , the output of which is the analog electrical output of the microphone system. 
     Power supplies for the active components are not shown, but will be understood by anyone skilled in the art. Likewise, operational amplifier implementation details are excluded from this discussion. 
     In operation, any tunneling current between ribbon  40  and probe  42  is amplified by amplifier  45  and compared with voltage  46 . If the ribbon  40  is far from probe  42 , then the tunneling current will be small or zero, and the comparator  47  will output a positive voltage. This positive voltage will charge capacitor  48  with a positive charge relative to ground. The following amplifier  49  will pass the voltage on capacitor  48  to the primary of transformer  43 . Transformer  43  will impose this voltage on the ribbon  40 , causing a current to flow in the ribbon  40 . 
     Due to the interaction of the current flowing in the ribbon  40  and the magnetic field generated of magnet  41 , ribbon  40  will experience a bending force. This force will act to push ribbon  40  toward probe  42 , increasing the level of tunneling current. 
     When ribbon  40  is sufficiently close to probe  42 , substantial tunneling current will flow, and the output of amplifier  45  will increase. When the output of amplifier  45  is equal to reference voltage  46 , then the output of comparator  47  will be zero, and the voltage on capacitor  48  will become constant. The current through the ribbon will become constant, and the ribbon will be held stationary in equilibrium between ribbon tension and magnetic force. 
     In response to acoustic pressure moving the ribbon, tunneling current to probe  42  will change, thus changing the output voltage from amplifier  45 , finally changing the results of comparison with reference voltage  46 . The change of voltage on capacitor  48  will result in changing current in ribbon  40 , restoring ribbon  40  to equilibrium position. 
     The voltage on capacitor  48  is a measure of the force needed to maintain ribbon  40  at its equilibrium position. This voltage is buffered by follower  50  to provide the output of the microphone system. 
     This invention is a method for using feedback to more accurately capture sound. While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of some of the preferred embodiments thereof. Many other variations are possible. For example, in the embodiment which uses op-amps, a wide number of other elements could alternatively be used, such as discrete transistors, MOSFETS, FETs, valves, vacuum tubes, and unijunction transistors. In fact, it would be apparent to one skilled in the art that any form of inverting amplifier could conceivably be employed in this circuit. Additionally, with reference to FIG. 1, the output signal could be taken from a combination of the coil output signal and the feedback signal, or it could also be taken solely from the feedback signal. 
     Detection means are not limited to those described, but could include any technique which detects the motion of the transduction element. For example, a variable refractive index interferometric system could be used, differential variable transformer techniques, or strain gauge techniques could be used. 
     Feedback means are not limited to those described, but could include any linear technique for restoring the transduction element to equilibrium. Electrostatic and electromagnetic systems have been described, however thermal, thermal resistive, optical and other possible systems could be used. 
     Accordingly, the scope of the invention should be determined not by the embodiment illustrated, but by the appended claims and their legal equivalents.