Electroacoustic measurement device with non-linear distortion compensation, compensation method, corresponding computer program and storage means

Abstract

The present invention relates to a technique for compensating the non-linear distortion of an electroacoustic transducer T, based on post-processing applied to the electrical signal delivered by the transducer by means of a harmonic non-linearizer component M disposed downstream of it. Figure for the abstract: Figure 2

Description

Title of the invention: Electroacoustic measurement device with non-linear distortion compensation, compensation method, corresponding computer program product and storage means. Technical field

[0001] The invention falls within the field of electroacoustics, and more particularly electroacoustic transducers with capacitive detection.

[0002] The invention relates to a technique for reducing non-linear distortion occurring in capacitive electroacoustic transducers.

[0003] The invention is of increasing interest in the field of consumer audio equipment, studio equipment, and also in measurement applications where greater precision is required. It can be applied to all types of capacitive sensing transducers, and more specifically, but not exclusively, to single backplate MEMS capacitive microphones. Technological background

[0004] A microphone is an electroacoustic transducer capable of accurately measuring sound pressure in a given environment. Such a device converts an input sound pressure signal into an output electrical signal. In the case of very compact electroacoustic transducers, a MEMS (for "MicroElectroMechanical System") microsystem can be integrated onto a silicon chip.

[0005] MEMS capacitive microphones represent a rapidly expanding market, particularly due to the development of mobile devices such as tablets, smartphones, and other connected objects. They comprise two main components: an acoustic pressure-sensitive element (often called a "transducer") that delivers the electrical signal corresponding to the acoustic pressure, and an electronic device responsible for conditioning and transmitting the signals (often called a "signal conditioner").

[0006] MEMS capacitive microphones have the following advantages: low sensitivity to temperature variations, high sensitivity to sound pressure levels, and good noise performance. They also offer low power consumption, a lightweight construction, and a relatively low price.

[0007] However, the electrical signal delivered at the output of these microphones exhibits non-linear distortion compared to the incident acoustic pressure signal. In other words, the output electrical signal is not proportional to the input acoustic signal, thus degrading the audio signal quality. This distortion phenomenon is primarily inherent to the electrostatic transduction principle of the microphone; the components of the transmission chain introduce non-linear behavior to varying degrees depending on their intrinsic characteristics and the specific features of the transmitted signal. Therefore, achieving a transmission chain with linear behavior over a wide bandwidth and a broad signal power range remains a major challenge for manufacturers of capacitive microphones today.

[0008] In order to reduce the non-linearity affecting capacitive microphones, it is known to design a transmission chain by arranging the components so that their respective non-linearities cancel each other out, and to use a negative feedback loop on a portion of the chain's gain. However, both approaches remain complex to implement in practice and often require the use of additional electronic components, without guaranteeing a significant reduction in the distortion level.

[0009] There is therefore a real need to provide a solution which allows, in a simple and effective way, the correction of non-linear distortion in capacitive sensing microphones. Description of the invention

[0010] In a particular embodiment of the invention, an acoustic measurement device is proposed comprising: - an electroacoustic transducer configured to deliver an electrical signal from an acoustic pressure signal, - a non-linear distortion compensation module disposed at the output of the electroacoustic transducer, said compensation module receiving at input the electrical signal delivered by the electroacoustic transducer, called the signal to be compensated U(t), and returning at output an electrical signal called the compensated signal Ulin(t),

[0011] characterized in that said compensation module is configured to apply to the signal to be compensated u(t) a compensation function characterized by the following equation: [00121 W 0 = ^(0+-^1^)

[0013] with:

[0014] u( t ), the signal to be compensated delivered by the electroacoustic transducer;

[0015] Ujin ( t ), the compensated signal restored at the output of the compensation module;

[0016] 1 / a quadratic compensation coefficient dimensioned to operate a phase opposition on the quadratic component of the signal to be compensated.

[0017] The general principle of the invention therefore consists of post-processing the electrical signal generated at the output of the electroacoustic transducer, by means of a non-linearizing component disposed at its output and configured to compensate for the non-linear distortion induced by the transducer. The compensation function applied by this non-linearizing component is designed to reduce the altered quadratic component of the signal delivered at the output of the transducer, by applying a phase opposition to this quadratic component, so as to restore a compensated signal of better quality.

[0018] According to a first possible implementation, the quadratic compensation coefficient 1 / K® is a predetermined value established as a function of intrinsic physical parameters of the electroacoustic transducer.

[0019] According to a second possible implementation, the quadratic compensation coefficient 1 / Kq is a value estimated from spectral components of the electroacoustic transducer.

[0020] According to a first embodiment, the signal to be compensated u(t) is an analog signal and said compensation module includes an analog circuit configured to perform said compensation function.

[0021] According to a second embodiment, the signal to be compensated u( t ) by the delivering transducer is a digital signal and the compensation module includes a programmable or reprogrammable digital component configured to perform said compensation function.

[0022] In another embodiment of the invention, a method for compensating the non-linear distortion of an electroacoustic transducer is proposed, said electroacoustic transducer being configured to deliver an electrical signal from an acoustic pressure signal, the method being characterized in that it comprises the following steps: - obtaining a quadratic compensation coefficient value of 1 / K®; - apply a compensation function to the electrical signal delivered by the transducer, called the signal to be compensated U(t), so as to reproduce a compensated electrical signal t) according to the following equation: 100231 W 0 =

[0024] with:

[0025] u( t ), the signal to be compensated delivered by the electroacoustic transducer;

[0026] Ujin ( t ), the compensated signal;

[0027] 1 / Xq, the quadratic compensation coefficient dimensioned to operate a phase opposition on the quadratic component of the signal to be compensated.

[0028] The general principle of the invention is based on post-processing the electrical signal generated at the output of the electroacoustic transducer, making it possible to compensate for the non-linear distortion induced by the transducer, so as to restore a compensated electrical signal over a wide frequency band and power range. The compensation function is designed to reduce the altered quadratic component of the signal delivered at the output of the electroacoustic transducer by applying a phase opposition to this quadratic component.

[0029] According to a first possible implementation, the value of the quadratic compensation coefficient obtained is a predetermined value established as a function of the bias voltage, the capacitance of the acoustic transducer.

[0030] According to a second possible embodiment, the value of the quadratic compensation coefficient obtained is a value estimated by spectral analysis of the harmonic structure of an electrical signal delivered by the acoustic transducer from a reference acoustic pressure signal.

[0031] In another embodiment of the invention, a computer program product is proposed, comprising program code instructions for implementing the aforementioned process (in any of its various embodiments), when said program is executed on a computer.

[0032] In another embodiment of the invention, a computer-readable and non-transient storage medium is proposed, storing the aforementioned computer program product. List of figures

[0033] Other features and advantages of the invention will become apparent from the following description, given by way of illustrative and non-limiting example, and the accompanying drawings, in which:

[0034] [Fig-1] represents an acoustic measurement device according to an embodiment particular of the invention;

[0035] [Fig.2] is a functional diagram of a non-linear distortion compensation module according to the invention;

[0036] [Fig.3] presents a flowchart of a particular embodiment of the process according to the invention;

[0037] [Fig.4] represents the simplified structure of a unit implementing the process according to a particular embodiment of the invention. Detailed description of the invention

[0038] In all figures in this document, identical elements and steps are designated by the same numerical reference.

[0039] The present invention relates to a post-processing of the electrical signal delivered at the output of an electroacoustic transducer allowing, by means of a non-linearizing component disposed downstream of it, to correct the non-linear distortion manifesting itself in the electroacoustic transducer.

[0040] In the following description, an example of an implementation of the invention applied to MEMS capacitive sensing microphones is considered. The invention is of course not limited to this particular field of application, but applies to any electroacoustic transducer with capacitive sensing.

[0041] Figure 1 shows an acoustic measurement device (DMA) according to a particular embodiment of the invention. In this particular embodiment, the DMA is an acoustic measurement device integrated on a support P. It comprises a capacitive sensing microphone T and a non-linear distortion compensation module M, electrically connected to the microphone T downstream of it.

[0042] The capacitive sensing microphone T typically incorporates a MEMS microsystem and is configured to deliver an electrical signal, denoted U(t), from an incident acoustic pressure signal (represented by the sinusoid S). The compensation module M is a non-linearizing component located at the output of the microphone T and configured to compensate for the non-linear distortion occurring in the microphone T (the principle of which is described below with reference to Figures 2 and 3).

[0043] In this embodiment, the capacitive sensing microphone T and the compensation module M are both integrated on the same support (for example a Silicon chip), thus offering a compact, industrializable and low power consumption capacitive acoustic measurement solution.

[0044] The signal to be compensated is therefore the electrical signal U ( t ) delivered by the capacitive detection microphone T.

[0045] The principle of the nonlinear distortion compensation method, according to a particular embodiment, applied to the capacitive detection microphone T described above, is presented below in relation to Figures 2 and 3. It is based on the implementation of a post-processing stage aimed at artificially adding nonlinearity to the microphone's output signal, which is in opposite phase to the microphone's physical nonlinearity. The steps described below are implemented by the nonlinear distortion compensation module M.

[0046] The non-linear distortion compensation module M is configured to apply to the signal to be compensated u(t) a compensation function characterized by the following equation:

[0047] U hn ( È ) = U ( 0 +

[0048] with:

[0049] U(t), the signal to be compensated delivered by the microphone T and received at the input of the module compensation M;

[0050] Ujjn ( t ), the compensated signal restored at the output of the compensation module M;

[0051] i / xcun quadratic compensation coefficient dimensioned to operate a phase opposition on the quadratic component of the signal to be compensated.

[0052] In other words, the compensation module M receives as input the electrical signal to be compensated U(t) delivered by the microphone T and outputs an electrical signal compensated signal Ujin(t).

[0053] To implement this compensation function, the compensation module M has a non-linear signal processing circuit comprising: - a multiplier block 10 capable of multiplying the signal to be compensated u( t ) by itself; - a multiplier block 20 capable of multiplying the signal from the multiplier block 10 placed upstream, by the multiplying constant 1 / Kq (the aforementioned quadratic compensation coefficient); - an adder block 30 capable of adding the signal to be compensated U(t) coming directly from the microphone T with the signal 1 / Xn X directly derived from the multiplier block 20, to deliver the compensated electrical signal U (done).

[0054] Such a processing circuit can be analog or digital depending on the nature of the electrical signal delivered by the microphone T.

[0055] The compensation function applied by the compensation module M is defined to correct the quadratic component affected by the non-linear distortion induced by the microphone T, by applying a phase opposition to this quadratic component. Such a compensation function makes it possible to artificially add non-linearity to the output signal of the microphone T, by aligning itself in phase opposition with the intrinsic physical non-linearity of the microphone.

[0056] There are mainly two ways to obtain the value of the constant Ko, both of which can be determined "cold" during the design of the microphone. This step is represented by the SI block in [Fig.3] (labeled "OBT_CCQ").

[0057] According to a first approach, a predetermined value of the quadratic compensation coefficient 1 / is obtained. This predetermined value is calculated in function of intrinsic parameters of microphone T, using the following equation:

[0058] tv- _ fil y TT K0“ Cp+C0 x u0

[0059] with:

[0060] Uo, the microphone bias voltage T,

[0061] Co, the static capacitance of microphone T,

[0062] Cp, the parasitic capacitance of the microphone T.

[0063] Typically, for a microphone with a bias voltage of 10 V, a static capacitance of 10 pF and a parasitic capacitance of IpF, the constant Kq is 9.1V.

[0064] According to a second approach (alternative or complementary to the first approach), for example when some of the physical parameters of the microphone T are not known (such as the microphone bias voltage, for example) or when it is necessary to verify the accuracy of the value of the quadratic coefficient calculated according to the first approach, it is possible to estimate the value of the quadratic compensation coefficient 1 / K. This estimation is based on an analysis of the spectral behavior of the microphone T with respect to a reference acoustic pressure signal. To this end, the following steps are carried out.

[0065] First, a time-frequency transform (for example, using a Fourier transform) is performed on the electrical signal delivered by the microphone T, starting from a reference acoustic pressure signal (a sinusoidal signal of frequency F0), in order to establish an amplitude spectrum as a function of the frequency of the acoustic signal. The first and second components of the signal are then identified from the previously established amplitude spectrum, namely the fundamental component F0 (whose amplitude is denoted Vj hereafter) and the harmonic component 2F0 (the "second harmonic," whose amplitude is denoted V2 hereafter). The amplitudes of these first and second components are estimated. Finally, the value of the quadratic compensation coefficient 1 / is calculated using the following equation:

[0066] _ Vf K0 “ 2xV2

[0067] with:

[0068] V15 the amplitude associated with the first component of the signal (fundamental component),

[0069] V2, the amplitude associated with the second component of the signal (component 2nd harmonic).

[0070] Typically, for a sinusoidal reference acoustic pressure signal with a frequency of 1 kHz and an acoustic pressure level between 85 and 120 dB SPL and a microphone T with a sensitivity of 13.6 mV / Pa, a frequency range between 100 Hz and 10 kHz and an acoustic overload point of 130 dB SPL, the amplitude of the first and second components were estimated to be 186 mV and 1.95 mV respectively, giving a value of the constant Ko equal to 8.87 V.

[0071] Thus, it is also possible to obtain a value of the quadratic compensation coefficient by simple spectral analysis of the harmonic structure of the signal.

[0072] Once the value of the quadratic compensation coefficient has been obtained (predetermined value or estimated value according to the technical implementation chosen), it is possible to integrate a non-linearizing FPGA or ASIC component into the microphone T applying the aforementioned compensation function with the predetermined and / or estimated value of the constant Ko.

[0073] Step S2 (labeled “APP_FC” in the figure) represents the compensation module M applying the compensation function described above to the electrical signal to be compensated u(t), so as to restore a compensated electrical signal and therefore of better quality.

[0074] Such a technique, carried out entirely in post-processing, is to be implemented simple and inexpensive. In practice, it reduces the total harmonic distortion rate according to the level of acoustic excitation, and therefore produces a better quality signal.

[0075] The compensation module can be arranged upstream or downstream or within the microphone signal conditioner itself.

[0076] Figure 4 schematically and in a simplified manner represents a compensation unit in a particular embodiment of the invention. For example, the compensation unit corresponds to the compensation module M shown in Figure 2, implementing the non-linear distortion compensation method according to the invention.

[0077] The compensation unit 10 more particularly comprises a random access memory 130 (for example, RAM), a processing unit 110 (denoted CPU), equipped for example with a processor or microprocessor, and controlled by a computer program stored in a read-only memory 120 (for example, ROM or a hard drive). At initialization, the code instructions of the computer program are, for example, loaded into the random access memory 130 before being executed by the processor of processing unit 110. Such a computer program allows the execution of the steps of the algorithm of [Fig.3] described above.

[0078] The processing unit 110 receives as input the electrical signal to be compensated (E). The processor of the processing unit 110 executes the process from the received electrical signal to be compensated and delivers as output a compensated electrical signal (S), according to the instructions of the program stored in memory 120.

[0079] This [Fig. 4] illustrates only one particular way, among several possible ways, of carrying out the process described above. Indeed, the compensation process according to the invention can be carried out interchangeably: - on a reprogrammable computing machine (a DSP processor or a microcontroller) executing a program comprising a sequence of instructions; or - on a dedicated computing machine (for example a set of logic gates such as an FPGA or an ASIC, or any other hardware module).

[0080] In the case where the invention is implemented on a reprogrammable computing machine, the corresponding program (i.e. the sequence of instructions) may be stored in a removable storage medium (such as, for example, a floppy disk, a CD-ROM or a DVD-ROM) or not, this storage medium being readable partially or totally by a computer or a processor.

Claims

Demands

1. An electroacoustic measurement device comprising: - an acoustic transducer (T) configured to deliver an electrical signal from an acoustic pressure signal, - a non-linear distortion compensation module (M) disposed at the output of the electroacoustic transducer, said compensation module receiving at input the electrical signal delivered by the electroacoustic transducer, said signal to be compensated U(t), and returning at output an electrical signal said compensated signal Ujin(t), characterized in that said compensation module is configured to apply to the signal to be compensated 11(t) a compensation function characterized by the following equation: with: U(t), the signal to be compensated delivered by the electroacoustic transducer; U^n(t) the compensated signal returned at the output of the compensation module;1 / Kq, a quadratic compensation coefficient sized to operate a phase opposition on the quadratic component of the signal to be compensated, Kq being defined according to the following equation: Ko = ^;xUo with: Uo, the bias voltage of the acoustic transducer T, Co, the static capacitance of the acoustic transducer T, Cp, the parasitic capacitance of the acoustic transducer T.;

2. Device according to claim 1, wherein the signal to be compensated U(t) is an analog signal and said compensation module comprises an analog circuit configured to perform said compensation function.

3. Device according to claim 1, wherein the signal to be compensated U(t) by the delivering transducer is a digital signal and the compensation module includes a programmable or reprogrammable digital component configured to perform said compensation function.

4. A method for compensating the nonlinear distortion of an electroacoustic transducer (T), said electroacoustic transducer being configured to deliver an electrical signal from an acoustic pressure signal, the method being characterized in that it comprises the following steps: - obtaining (S1) a quadratic compensation coefficient value 1 / ; U - applying (S2) a compensation function to the electrical signal delivered by the transducer, said signal to be compensated U(t), so as to restore a compensated electrical signal unn(t) according to the following equation: with: U(t), the signal to be compensated delivered by the electroacoustic transducer; (t) 3rd compensated signal;1 / KQ, the quadratic compensation coefficient sized to operate a phase opposition on the quadratic component of the signal to be compensated, Kq being defined according to the following equation: K0 = AxU0 with: Uq, the bias voltage of the acoustic transducer T, Co, the static capacitance of the acoustic transducer T, Cp, the parasitic capacitance of the acoustic transducer T.;

5. Product computer program, comprising program code instructions for implementing the method according to claim 4, when said program is executed on a computer.

6. A computer-readable, non-transient storage medium storing a computer program product according to claim 5.

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