Microphones are widely used in a variety of applications, such as in smartphones, mobile phones, tablets, headsets, hearing aids, sensors, automobiles, etc. It is desirable to increase sound quality in such microphones. Present day microphones have limitations due to their configuration and the way they operate.
The present disclosure relates generally to a system and method for compensating for distortion in an output of a microphone assembly including an acoustic transducer and a processing circuit. Generally, distortion in the output of the microphone assembly is attributable at least in part to non-linearity in the acoustic transducer and the processing circuit. In condenser type MEMS microphones, the non-linearity may be due to the bending of a diaphragm, especially at higher sound pressure levels, and asymmetry in the deflection of the diaphragm among other factors. The non-linearity in the processing circuit may be due to receiving and processing an analog output signal from the acoustic transducer and/or charge sharing between the acoustic transducer and the processing circuit, among other factors. Non-linearity in other types of MEMS microphones (e.g., piezo-electric or optical transducers) may result from other sources.
As the sound pressure levels increase, the non-linearity of acoustic transducers tends to increase, which in turn increases distortion in the output of the microphone assembly. Distortion may include harmonic components, intermodulation components, or other distortion components. These distortion components impact the sound quality and are therefore undesirable. Distortion may be expressed as a percentage of deviation in the output of the microphone assembly relative to an acoustic input signal applied to the acoustic transducer.
The present disclosure provides systems and methods to identify the distortion in the output of the microphone assembly and compensate for that distortion. The distortion is determined using a known input tracking signal. In implementations that require a bias voltage, the input tracking signal is input to the acoustic transducer via the bias voltage. In condenser type acoustic transducers, for example, the bias voltage is applied by a charge pump and thus the input tracking signal may be combined with the charge pump signal. Other types of acoustic transducers may have other bias voltage sources through which the input tracking signal may be applied to the acoustic transducer. In other embodiments, the input tracking signal is input to the acoustic transducer as an acoustic signal. The output signal of the acoustic transducer includes a tracking signal component based on the input tracking signal and an audio signal component representative of the acoustic input signal applied to the acoustic transducer. The audio signal component may be distorted, particularly at higher sound pressure levels, as discussed above.
By looking at the changes in the tracking signal component, the distortion in the audio signal component may be identified and compensated. Specifically, the input tracking signal is a static signal, the frequency and amplitude of which is known. The non-linearity of the acoustic transducer and processing circuits causes distortion in the input tracking signal as well. The distortion in the input tracking signal may be used to detect and compensate for distortion in the audio signal component.
In some embodiments, the housing 125 includes external contacts on a surface thereof forming an external device interface, also called a physical interface, for integration with a host device in a reflow or wave soldering operation. In some embodiments, the external device interface includes power, ground, clock, data, and select contacts. The particular contacts constituting the external device interface, however, depend on the protocol with which data is communicated between the microphone assembly 100 and the host device. Such protocols include, but are not limited to, PDM, SoundWire, I2S, and I2C among others.
The processing circuit 110 (also referred to herein as an electrical circuit, an audio signal processing circuit, or audio signal electrical circuit) is configured to receive an electrical signal (also referred to herein as a transducer output signal or an output signal) from the acoustic sensor 105. The acoustic sensor 105 may be operationally connected to the processing circuit 110 using one or more bond wires 150. In other embodiments, other connecting mechanisms such as, vias, traces, electrical connectors, etc. may be used to electronically connect the acoustic sensor 105 to the processing circuit 110. After processing the electrical signal from the acoustic sensor 105, the processing circuit 110 provides the processed electrical signal or microphone signal at an output or interface of the microphone assembly for use by a computing or host device (e.g., a smartphone).
Only certain components of the microphone assembly 100 are discussed herein. Other components, such as motors, charge pumps, power sources, filters, resistors, etc. that may be used to implement functions described herein and/or other functions of the discussed devices, are not discussed in detail but are contemplated and considered within the scope of the present disclosure.
Additionally, several variations in the microphone assembly 100 are contemplated. For example, although the processing circuit 110 and the acoustic sensor 105 are shown as separate components, in some embodiments, the processing circuit and the acoustic sensor may be integrated together into a single component. In some embodiments, either or both the acoustic sensor 105 and the processing circuit 110 may be constructed from a semiconductor die using, for example, mixed-signal complementary metal-oxide semiconductor devices. In other embodiments, other techniques may be used to construct the acoustic sensor 105 and the processing circuit 110. In some embodiments, the processing circuit 110 may be configured as an application specific integrated circuit (ASIC).
In
At higher SPL, however, the diaphragm 210 deflects more, as shown in exaggerated positions 230 and 235. In the positions 230 or 235, the distance between the back plate 205 and the diaphragm 210 at the center location 220 is unequal relative to the distance between the back plate and the diaphragm at the edge locations 225. The asymmetric deflection of the diaphragm 210 toward and away from the back plate 205, among other reasons, produces distortion in the output signal. Additional distortion may be introduced by the processing circuit. Thus, the microphone signal that is output from the microphone assembly having the acoustic transducer 200 is not a substantially accurate reproduction of the acoustic input signal.
In
In some embodiments, the distortion in the output signal and/or the microphone signal may be tracked or determined using an input tracking signal. Specifically, when the input tracking signal is input into an acoustic transducer (e.g., the acoustic transducer 200), the output signal from the acoustic transducer includes an audio signal component and a tracking signal component. As the output signal is processed by a processing circuit (e.g., the processing circuit 110) of the microphone assembly (e.g., the microphone assembly 100), the output signal may become further distorted by non-linearity introduced by the processing circuit. The distortion introduced by the acoustic transducer and the processing circuit is reflected in the audio signal component of the microphone signal. The tracking signal component is subject to the same (or substantially same) distortion as the audio signal component. By tracking the changes in the tracking signal component relative to the known input tracking signal, the distortion in the audio signal component may be identified and compensated.
For example, in some embodiments, the input tracking signal 505 may be a forty-eight kilohertz (48 kHz) signal, ninety-six (96) kHz signal, one hundred ninety-two (192) kHz signal, or a three hundred eighty-four (384) kHz signal. In other embodiments, other frequencies may be used for the input tracking signal 505. Likewise, in some embodiments, the input tracking signal 505 may be between twenty and one hundred SPL (20-100 dB SPL) and, in some implementations, between one hundred forty and one hundred sixty decibel SPL (140-160 dB SPL). In other embodiments, other SPL signals may be used for the input tracking signal 505 depending upon the capabilities of the microphone assembly 500. Additionally, the input tracking signal 505 is a static signal, the frequency and SPL level of which is not generally varied. However, when an input acoustic signal into the microphone assembly 500 is at a low SPL, the input tracking signal 505 may be disabled or the SPL/frequency of the input tracking signal may be adjusted.
The input tracking signal 505 is combined, in a combination circuit 520, with a charge pump signal 525 generated by a charge pump 530 to produce the input signal 510. In some embodiments, the combination circuit 520 is a summation circuit that sums up the charge pump signal 525 with the input tracking signal 505. Combination of the charge pump signal 525 and the input tracking signal 505 are input into the acoustic transducer 535 of the microphone assembly 500. The input tracking signal 505 is modulated by an electrical signal produced upon transduction of an acoustic input signal 536 applied to the acoustic transducer 535. In response to the acoustic input signal 536, the acoustic transducer 535 outputs an output signal 540, which includes an audio signal component representative of the acoustic input signal 536 and a tracking signal component based on the input tracking signal 505.
The processing circuit 545 includes an amplifier 550 configured to amplify the output signal 540 into an amplified signal 555. Although not shown, the amplifier 550 may be a single ended amplifier or a differential amplifier. Further, the amplifier 550 may be configured with a specified gain, or in other words, an amplifying ability that may be expressed as a ratio of the output of the amplifier to the input of the amplifier. Also, although only a single amplifier is shown, in some embodiments, multiple amplifiers connected in series or having other topologies may be used. Likewise, in some embodiments, the amplifier 550 may use multiple gain stages, filters, or other components that may be deemed necessary or desirable in obtaining the amplified signal to perform the functions described herein.
The amplified signal 555 is then input into a low pass filter 560. The low pass filter 560, which is analog in nature, may be configured to pass signals below a specific cutoff frequency, and to attenuate signals above that cutoff frequency. In some embodiments, the cutoff frequency may be set to around six hundred kilo hertz (˜600 kHz). By virtue of using the low pass filter 560, aliasing in the amplified signal 555 may be avoided. Filtered signal 565 from the low pass filter 560 is input into an analog to digital converter (“ADC”) 570.
The ADC 570 is configured to receive, sample, and quantize the filtered signal 565 and generate a corresponding digital signal 575, which is then input into a post compensation circuit 580. Thus, the ADC 570 receives an analog signal (e.g., the filtered signal 565) and converts that analog signal into a digital signal (e.g., the digital signal 575). The digital signal 575 also includes the audio signal component and the tracking signal component described above, albeit in digital form.
The ADC 570 may also be configured in a variety of ways. In some embodiments, the ADC 570 may be adapted to output the digital signal in a multibit format. In other embodiments, the ADC 570 may be configured to generate the digital signal 575 in a single bit format. In some embodiments, the ADC 570 may be based on a sigma-delta converter (IA), while in other embodiments, the ADC may be based on any other type of a converter, such as a flash ADC, a data-encoded ADC, a Wilkinson ADC, a pipeline ADC, etc. The ADC 570 may be also be configured to generate the digital signal 575 at a specific sampling frequency or sampling rate.
The digital signal 575 is input into the post compensation circuit 580, which identifies and compensates for the distortion in the audio signal component of the digital signal to obtain a compensated microphone signal 585. Although not shown, in some embodiments, the compensated microphone signal 585 may be transmitted as input to other components (e.g., an interpolator, a digital-to-digital converter, etc.) for further processing by a digital signal processing circuit of the microphone assembly or by a processor of a host device (e.g., smartphone). The post compensation circuit 580 is described in greater detail in
In
After the application of the input tracking signal in the input tracking signal portion 725, the acoustic transducer may be subject to the acoustic input signal 656 to obtain the acoustic input signal portion 730. The acoustic input signal portion 730 is solely an acoustic signal without having any component of the input tracking signal. Thus, the input plot 715 includes the input tracking signal portion 725 representative of the input tracking signal 505 and the acoustic input signal portion 730 representative of the acoustic input signal 656.
In response to the signal of the input plot 715, the acoustic transducer outputs an output signal, which is represented by the output plot 720. Like the input plot 715, the output plot 720 includes an output tracking signal portion 735 and an output audio signal portion 740. The output tracking signal portion 735 corresponds to the input tracking signal portion 725 when no acoustic input signal has been applied. The output audio signal portion 740 is obtained in response to the input tracking signal portion 725 and includes a tracking signal component and an audio signal component. The tracking signal component is the output representative of the input tracking signal 505 applied at the input of the acoustic transducer and the audio signal component is the output representative of the acoustic input signal 656 applied at the input of the acoustic transducer.
Due to distortion, the output plot 720 does not accurately track (i.e., follow the shape of) the input plot 715. As also seen from
Distortion may be compensated in a post compensation circuit.
The ADC 905 generates a digital signal 910. The digital signal 910 includes an audio signal component and a tracking signal component. The digital signal 910 is input into an extraction circuit 915. The extraction circuit 915 separates the audio signal component from the tracking signal component. Specifically, the extraction circuit 915 includes a low pass filter 920, which receives the digital signal 910 and extracts the audio signal component from the digital signal to obtain a filtered audio signal component 925, which is input into a signal correction circuit 930.
More specifically, the low pass filter 920, which extracts the audio signal component, is configured with a cutoff frequency to allow the low pass filter to pass through signals below the cutoff frequency and cut off signals above the cutoff frequency. Thus, the low pass filter 920 may be set with a cutoff frequency that allows the audio signal component to pass through while blocking the tracking signal component. In some embodiments, the low pass filter 920 may be configured with a cutoff frequency of about forty-eight (48) kHz. In other embodiments, other cutoff frequencies may be used in the low pass filter 920 depending upon the frequency of the tracking signal component that is to be filtered out. Further, in some embodiments, the low pass filter 920 may be configured as a Sinc filter with a first notch placed at a frequency of the input tracking signal (e.g., the input tracking signal 505, 655) from which the digital signal 910 is obtained. In other embodiments, a cascaded integrator-comb (CIC) filter or any other low pass filter that is suitable to separate the audio signal component from the tracking signal component may be used. An example configuration of the low pass filter 920 is shown in
In addition to inputting the digital signal 910 into the low pass filter 920, the digital signal is also input into a peak filter 935 of the extraction circuit 915. The peak filter 935 is configured to extract the tracking signal component from the digital signal 910. In some embodiments, the peak filter 935 may be configured with a center frequency that corresponds to the frequency of the tracking signal component. An example configuration of the peak filter 935 is shown in
The envelope estimation circuit 945 estimates an envelope from the filtered tracking signal component 940 and normalizes the estimated envelope to obtain a tracking signal envelope 950, which is input into the signal correction circuit 930. To estimate the envelope of the filtered tracking signal component 940, the envelope estimation circuit 945 identifies a maximum value between two zero cross values of the filtered tracking signal component. This maximum value is called a current maximum value and may be classified in terms of a root mean square value, an absolute value, or another type of value. Several current maximum values make up the envelope. The envelope is shown in
Specifically, in the calibration process, the calibration value identified from the first portion 820 of
The signal correction circuit 930 thus receives two inputs—a first input of the filtered audio signal component 925 and a second input of the normalized envelope (e.g., the tracking signal envelope 950). The signal correction circuit 930 is configured to apply a Trapezoidal integration method to compensate for distortion in the filtered audio signal component 925 using the tracking signal envelope 950. Specifically, the signal correction circuit 930 may be configured to apply the Trapezoidal integration method for approximating the tracking signal envelope 950 and the filtered audio signal component 925 to obtain the compensated filtered audio signal component, which has been compensated for distortion. The Trapezoidal integration may be applied using the following formula:
out=∫dYenvelope*dYAudio
In terms of a MATLAB implementation, the Trapezoidal integration method may be implemented as follows:
out(n)=out(n−1)+dmdi
The Trapezoidal integration method alters the filtered audio signal component 925 using the tracking signal envelope 950 to compensate for the distortion in the filtered audio signal component 925. Thus, the signal correction circuit 930 adjusts (e.g., reduces) distortion in the filtered audio signal component 925. The output of the Trapezoidal integration method is a compensated microphone output signal 955. The compensated microphone output signal 955 is equivalent to the compensated microphone output signal 585 of
Additionally, the digital signal 1020 is input into a multiplier circuit 1040. The multiplier circuit 1040 multiplies the digital signal 1020 with input tracking signal 1045 to extract the tracking signal component from the digital signal 1020 to obtain a multiplied signal 1050. The input tracking signal 1045 is similar to the input tracking signal 505, 655. By multiplying the digital signal 1020 with the input tracking signal 1045, an amplitude of the tracking signal component in the digital signal 1020 may be modulated and the tracking signal component converted into a direct current signal. The multiplied signal 1050 is then input into a low pass filter 1055.
In some embodiments, instead of using the multiplier circuit 1040, a special ADC may be used. The special ADC may be configured with a low sampling frequency using a Nyquist algorithm. The output of the special ADC may be similar to the multiplied signal 1050, which may then be input into the low pass filter 1055.
The low pass filter 1055, in some embodiments, may be configured with a cutoff frequency of about ten kilo hertz (10 kHz), although other cutoff frequencies may be used in other embodiments. The multiplied signal 1050 is filtered through the low pass filter 1055. By filtering the multiplied signal 1050 through the low pass filter 1055, a filtered tracking signal component 1060 is obtained.
The filtered tracking signal component 1060 is then used to estimate an envelope in the envelope estimation circuit 1015. In contrast to the process described in
The digital signal 1120 is also input into a bandpass filter 1140 of the extraction circuit 1105 to generate a filtered tracking signal component 1145. The bandpass filter 1140 may be configured with specific frequencies such that the bandpass filter allows the tracking signal component to pass through, while blocking the audio signal component in the digital signal 1120. The filtered tracking signal component 1145 is then down sampled in a down sampling circuit 1150 such that a sampling frequency of the filtered tracking signal component is similar to the frequency of the tracking signal component in the digital signal 1120. Down sampled tracking signal component 1155 is input into the envelope estimation circuit 1110.
The envelope estimation circuit 1110 is similar to the envelope estimation circuit 1015 of
After estimating the envelope 1410, the envelope 1410 is normalized. As noted above, to normalize the envelope 1410, the tracking signal component 1405 is calibrated by dividing the calibration value by the current maximum values. A normalized envelope 1500 is shown in
At operation 1720, an acoustic input signal is input to, or detected, by the acoustic transducer. The output signal of the acoustic transducer includes an audio signal component and a tracking signal component. Since the input tracking signal is a known signal, variations in the tracking signal component, and thus the distortion in the audio signal component may be determined.
As discussed above, the output signal is converted into a digital signal using an analog-to-digital converter. From the digital signal, the tracking signal component and the audio signal component are separated (e.g., using any of the mechanisms discussed in
Using the tracking signal envelope, the signal correction circuit compensates for the distortion in the audio signal component at operation 1820. Specifically, the signal correction unit applies a Trapezoidal integration method, discussed above, to compensate for the distortion in the audio signal component. By compensating, the distortion in the audio signal component is reduced. A compensated microphone signal is output at operation 1825 and the process ends at operation 1830.
Thus, the system and method described herein advantageously reduces distortion in a microphone signal, thereby improving sound quality.
In accordance with some aspects of the present disclosure, an audio signal electrical circuit is disclosed. The audio signal electrical circuit includes an extraction circuit configured to receive a digital signal having an audio signal component and a tracking signal component and to extract the tracking signal component and the audio signal component from the digital signal, the audio signal component representative of an acoustic signal detected by an acoustic transducer. The audio signal electrical circuit also includes an envelope estimation circuit configured to estimate a tracking signal envelope from the tracking signal component and a signal correction circuit configured to reduce distortion in the audio signal component using the tracking signal envelope.
In accordance with other aspects of the present disclosure, a microphone assembly is disclosed. The microphone assembly includes an acoustic transducer and an audio signal electrical circuit configured to receive an output signal from the acoustic transducer. The output signal includes an audio signal component and a tracking signal component, and the audio signal component is representative of an acoustic signal detected by the acoustic transducer and the tracking signal component is based on an input tracking signal applied to the acoustic transducer. The audio signal electrical circuit includes an analog to digital converter configured to convert the output signal into a digital signal, an extraction circuit configured to separate the tracking signal component and the audio signal component from the digital signal, and an envelope estimation circuit configured to estimate a tracking signal envelope from the tracking signal component. The audio signal electrical circuit also includes a signal correction circuit configured to reduce distortion in the audio signal component using the tracking signal envelope.
In accordance with yet other aspects of the present disclosure, a method in an audio signal electrical circuit is disclosed. The method includes converting an amplified signal, by an analog to digital converter, to a digital signal. The digital signal includes an audio signal component representative of an acoustic signal and a tracking signal component based on an input tracking signal. The method also includes separating, by an extraction circuit, the audio signal component and the tracking signal component from the digital signal, estimating, by an envelope estimation circuit, a tracking signal envelope from the tracking signal component, and reducing, by a signal correction circuit, distortion in the audio signal component using the tracking signal envelope.
The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. While various embodiments and figures are described as including particular components, it should be understood that modifications to the embodiments described herein can be made without departing from the scope of the present disclosure. For example, in various implementations, an embodiment described as including a single component could include multiple components in place of the single component, or multiple components could be replaced with a single component. Similarly, embodiments described as including a particular component may be modified to replace that component with an alternative component or group of components designed to perform a similar function. In some embodiments, method steps described herein could be performed in a different order, additional steps than are shown may be performed, or one or more steps may be omitted.
This application is a National Stage Application of PCT/US2018/039617, filed Jun. 26, 2018, which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/525,640, filed Jun. 27, 2017, the entire contents of which are incorporated herein by reference
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PCT/US2018/039617 | 6/26/2018 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2019/005885 | 1/3/2019 | WO | A |
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