Digital microphone interface circuit for speech recognition

Abstract

This invention discloses a digital microphone interface circuit for speech recognition, comprising: a mode selection module for receiving a selection signal and generating an enable signal based on the selection signal, wherein the enable signal includes a first enable signal and / or a second enable signal; a PDM interface conversion circuit connected to the mode selection module, for converting the input signal into a PDM signal when the first enable signal is valid; and a PCM interface conversion circuit connected to the mode selection module, for converting the input signal into a PCM signal when the second enable signal is valid. Through the interface circuit of this invention, both low-power audio and high-quality audio signal output are achieved, meeting the diverse application scenarios.

Description

Technical Field

[0001] This invention belongs to the field of digital integrated circuit technology, and more specifically, relates to a digital microphone interface circuit for speech recognition. Background Technology

[0002] MEMS (Micro-Electro-Mechanical System) microphones are widely used in smartphones, automotive electronics, and medical electronics due to their advantages such as small size, low power consumption, good consistency, strong anti-interference ability, and surface mountability, gradually replacing traditional electret microphones as the mainstream. MEMS microphones not only significantly reduce circuit board space, have low power requirements, and improve immunity to electrical noise, but also offer greater design flexibility, allowing integration with corresponding interface circuits and lower manufacturing costs. Currently, various digital interface MEMS microphones are available on the market. Through built-in analog-to-digital converters, they convert analog signals into digital signals for output, improving audio signal quality, enhancing anti-interference capabilities, and reducing system power consumption. Especially for signals requiring long-distance transmission, they effectively prevent audio signal distortion and have broad application prospects.

[0003] Specifically, PDM and PCM interfaces are two typical digital interfaces for MEMS microphones. The PDM interface, due to its simple hardware circuitry, has a significant advantage in low power consumption, but its output audio signal quality is relatively low. The PCM interface outputs a standard PCM format audio signal, suitable for devices with higher sound quality, but its hardware circuitry is more complex. Currently, most MEMS digital microphones launched domestically and internationally use a single digital interface, failing to simultaneously leverage the significant advantages of low power consumption of the PDM interface and high sound quality of the PCM interface, thus not meeting the diverse needs of various application scenarios. Therefore, researching a digital microphone interface circuit for speech recognition that simultaneously achieves low power consumption and high-quality audio signals is particularly necessary. Summary of the Invention

[0004] In view of the shortcomings and improvement needs of the existing technology, the present invention provides a solution, the purpose of which is to

[0005] To achieve the above objectives, according to one aspect of the present invention, a digital microphone interface circuit for speech recognition is provided, characterized in that it comprises:

[0006] A mode selection module is used to receive a selection signal and generate an enable signal according to the selection signal, wherein the enable signal includes a first enable signal and / or a second enable signal;

[0007] A PDM interface conversion circuit, connected to the mode selection module, is used to convert the input signal into a PDM signal when the first enable signal is valid; and

[0008] The PCM interface conversion circuit is connected to the mode selection module. When the second enable signal is valid, it is used to convert the input signal into a PCM signal.

[0009] Preferably, the PDM interface conversion circuit includes a low-pass filter module for receiving the input signal, a phase compensation module connected to the low-pass filter module, and a digital modulator connected to the phase compensation module. The low-pass filter module converts the input signal into a first filtered signal and inputs the first filtered signal to the phase compensation module. The phase compensation module converts the first filtered signal into a first compensated signal and inputs the first compensated signal to the digital modulator. The digital modulator processes the first compensated signal into a PDM signal conforming to the PDM interface format.

[0010] Preferably, the low-pass filter module includes a second-order IIR filter and a first-order IIR filter, the second-order IIR filter and the first-order IIR filter are cascaded, the input terminal of the second-order IIR filter is connected to the input signal, and the first-order IIR filter outputs the first filtered signal.

[0011] Preferably, the phase compensation module is a second-order IIR filter.

[0012] Preferably, the digital modulator includes a noise shaping circuit and an amplitude discrimination circuit. The noise shaping circuit is used to shape the in-band noise in the input signal to out-of-band, and the amplitude discrimination circuit is used to convert the multi-bit signal into a single-bit signal conforming to the PDM interface format and output the PDM signal.

[0013] Preferably, the PCM interface conversion circuit includes a CIC filtering module for receiving the input signal, a CIC passband compensation module connected to the CIC filtering module, and a CIC stopband compensation module connected to the CIC passband compensation module. The CIC filtering module performs low-pass filtering on the input signal and outputs a second filtered signal after reducing the sampling frequency. The CIC passband compensation module receives the second filtered signal and outputs a second compensation signal based on the second filtered signal. The CIC stopband compensation module receives the second compensation signal and processes it into a PCM signal conforming to the PCM interface format.

[0014] Preferably, the CIC filtering module includes multiple cascaded delay filters, a downsampling unit, and multiple cascaded comb filters, wherein the delay filters receive the input signal, the comb filters output the second filtered signal, and the downsampling unit is connected between the multiple cascaded delay filters and the multiple cascaded comb filters.

[0015] Preferably, the CIC passband compensation module is an ISOP filter or an amplitude-increasing FIR compensation filter.

[0016] Preferably, the CIC stopband compensation module includes a first-stage multiphase full-pass filter and a second-stage multiphase full-pass filter. The first-stage multiphase full-pass filter receives the second compensation signal, and the input terminal of the second-stage multiphase full-pass filter is connected to the output terminal of the first-stage multiphase full-pass filter to output the PCM signal. The first-stage multiphase full-pass filter and the second-stage multiphase full-pass filter include multiple downsampling units and multiple full-pass filters. The order of the full-pass filter in the second-stage multiphase full-pass filter is higher than the order of the full-pass filter in the first-stage multiphase full-pass filter. The full-pass filter is an IIR filter structure.

[0017] Preferably, the mode selection module includes a first selector and a second selector, the first selector and the second selector are connected in parallel, the first selector outputs a first enable signal to control the PDM interface conversion circuit, and the second selector outputs a second enable signal to control the PCM interface conversion circuit.

[0018] The digital microphone interface circuit for speech recognition disclosed in this invention provides high-quality audio output signals and features low power consumption. By performing layered noise reduction processing on the sound signals acquired by the microphone, the circuit improves both digital interfaces, outputting PDM format signals and PCM format audio signals to meet the needs of different application scenarios. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0020] Figure 2 This is a schematic diagram of a mode selection module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0021] Figure 3 This is a schematic diagram of a low-pass filter module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0022] Figure 4This is a schematic diagram of a phase compensation module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0023] Figure 5 This is a schematic diagram of a digital modulator in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0024] Figure 6 This is a schematic diagram of a delay integrator and a delay-free integrator in a digital modulator according to an embodiment of the present invention;

[0025] Figure 7 This is a schematic diagram of a CIC filtering module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention;

[0026] Figure 8 This is a schematic diagram of a combing filter module in a CIC filter module provided according to an embodiment of the present invention;

[0027] Figure 9 This is a schematic diagram of a CIC passband compensation module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention; and

[0028] Figure 10 This is a schematic diagram of a CIC stopband compensation module in a digital microphone interface circuit for speech recognition provided according to an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0030] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0031] Figure 1 This is a schematic diagram of a digital microphone interface circuit for speech recognition according to an embodiment of the present invention, such as... Figure 1As shown, the interface circuit 200 includes a mode selection module 100, which receives a selection signal and generates an enable signal based on the selection signal. The enable signal includes a first enable signal EN1 and / or a second enable signal EN2. A PDM interface conversion circuit 110, connected to the mode selection module 100, converts the input signal into a PDM signal output when the first enable signal EN1 is valid. A PCM interface conversion circuit 120, also connected to the mode selection module 100, converts the input signal into a PCM signal output when the second enable signal EN2 is valid. The input signal is an audio signal digitized by an ADC.

[0032] Specifically, the PDM interface conversion circuit 110 includes: a low-pass filter module 111 for receiving input signals, a phase compensation module 112 connected to the low-pass filter module 111, and a digital modulator 113 connected to the phase compensation module 112. The low-pass filter module 111 converts the input signal into a first filtered signal IPF-OUT and inputs the first filtered signal IPF-OUT to the phase compensation module 112. The phase compensation module 112 converts the first filtered signal IPF-OUT into a first compensated signal PH-CP-OUT and inputs the first compensated signal PH-CP-OUT to the digital modulator 113. The digital modulator 113 processes the first compensated signal PH-CP-OUT into a PDM signal conforming to the PDM interface format. The PCM interface conversion circuit 120 includes: a CIC filter module 121 for receiving input signals, a CIC passband compensation module 122 connected to the CIC filter module 121, and a CIC stopband compensation module 123 connected to the CIC passband compensation module 122. The CIC filtering module 121 performs low-pass filtering on the input signal and outputs a second filtered signal CIC-OUT after reducing the sampling frequency. The CIC passband compensation module 122 receives the second filtered signal CIC-OUT and outputs a second compensation signal ISOP-OUT based on the second filtered signal CIC-OUT. The CIC stopband compensation module 123 receives the second compensation signal ISOP-OUT and processes the second compensation signal ISOP-OUT into a PCM signal that conforms to the PCM interface format.

[0033] Figure 2 This is a schematic diagram of a mode selection module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 2 Combining Figure 1 The mode selection module 100 includes two parallel-connected selectors, such as... Figure 2As shown, a first selector 101 and a second selector 102 can be selected by switching modules. In one embodiment, when the selection signal is high, the first selector 101 is turned on, the second selector 102 is turned off, the first enable signal EN1 is high, the PDM interface conversion circuit 110 is enabled, and the interface circuit 200 outputs a PDM signal. Conversely, when the selection signal is low, the first selector 101 is turned off, the second selector is turned on, the second enable signal EN2 is high, the PCM interface conversion circuit 120 is enabled, and the interface circuit 200 outputs a PCM signal. Those skilled in the art should understand that the above is one embodiment of the present invention, and the way the enable signal selects the selector is not limited to the above embodiment, and does not constitute a limitation of the present invention.

[0034] Figure 3 This is a schematic diagram of a low-pass filter module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 3 Combining Figure 1 The low-pass filter module 111 includes a second-order IIR filter and a first-order IIR filter, which are cascaded together. The input of the second-order IIR filter is connected to the input signal, and the first-order IIR filter outputs a first filtered signal, IPF-OUT. Both the first-order and second-order IIR filters include multiple delay units, multiple amplifiers, and multiple adders. Both the first-order and second-order IIR filters adopt a direct type I structure with a passband cutoff frequency of 20kHz, covering the upper limit of the audible frequency band and effectively filtering out out-of-band noise, thus improving the signal-to-noise ratio of the digital audio signal. In one embodiment, the low-pass filter module 111 performs low-pass filtering on the input audio signal to filter out out-of-band high-frequency noise and obtain the audio signal within the sensitive frequency band of the human ear.

[0035] Figure 4 This is a schematic diagram of a phase compensation module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 4 Combining Figure 1 Describe, such as Figure 4 The phase compensation module 112 shown is a second-order IIR filter, which includes multiple amplification units, multiple delay units, and multiple adders, wherein the coefficients of the amplification units are adjustable. Specifically, the phase compensation module 112 compensates for the system nonlinearity caused by the low-pass filter module 111 without changing the system's amplitude-frequency response. The phase compensation module will have different coefficient configurations for different low-pass filter modules 111.

[0036] Figure 5 This is a schematic diagram of a digital modulator in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 5 Combining Figure 1 Describe, such as Figure 5 As shown, the digital modulator 113 includes a noise shaping circuit and an amplitude discrimination circuit. The noise shaping circuit receives a first compensation signal PH-CP-OUT and performs noise shaping on it. Specifically, it shapes the in-band noise in the first compensation signal PH-CP-OUT to out-of-band. The amplitude discrimination circuit performs amplitude discrimination on the shaped first compensation signal PH-CP-OUT and outputs a PDM signal. Specifically, after converting a multi-bit input signal into a single-bit signal, the digital modulator 113 improves the signal-to-noise ratio of the input signal by shaping the noise in the input signal and outputs a single-bit PDM signal conforming to the PDM interface format.

[0037] In one embodiment, the noise shaping circuit includes: multiple amplifiers, multiple delay integrators, multiple delay-free integrators, and multiple adders. Specifically, taking a 5th-order noise shaping circuit as an example, its connection relationship is as follows: Figure 5 As shown, the circuit includes 13 amplification units, 3 delay integrators, 2 no-delay integrators, 3 two-port adders, and 1 six-port adder. The multiplication operation of the 13 amplification units is implemented using a shift-and-add method, which reduces the overall circuit area and power consumption. The delay integrators are positioned at specific locations in the circuit to integrate the data and then output a delayed result. The no-delay integrators are positioned at specific locations in the circuit to integrate the data and then output a direct result. The five-port adder sums the data from the five integrators after appropriate amplification, and the output of the six-port adder goes to the amplitude discrimination circuit. The amplitude discrimination circuit sets input signals with amplitudes greater than 0 to 1 and input signals with amplitudes less than or equal to 0 to 0, thereby converting multi-bit input audio signals into single-bit audio signal outputs. Figure 5 The noise shaping circuit described herein is one embodiment of the present invention. Those skilled in the art should understand that other alternative shaping circuits that can achieve out-of-band noise shaping are included in the concept of the present invention.

[0038] Figure 6 This is a schematic diagram of a delay integrator and a delay-free integrator in a digital modulator according to an embodiment of the present invention. Figure 6 As shown, both the delay integrator and the non-delay integrator consist of one adder and one delay unit. The output of the adder in the delay integrator is then passed through the delay unit to become the output of the delay integrator, with a time difference of one clock cycle between the output and the input. The output of the non-delay integrator is added to the input after passing through one delay unit to obtain the output, with no time difference between the output and the input.

[0039] Figure 7This is a schematic diagram of a CIC filtering module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 5 The example shown is a 5th-order, 32x downsampling CIC filter module. Other orders and downsampling modes can also be used in this invention, where the order of the delay-free integrator is the same as the order of the comb filter. Figure 7 As shown, the CIC filtering module 121 includes five cascaded delay-free integrators, one 32x downsampling unit, and five cascaded comb filters, with the downsampling unit connected between the delay-free integrators and the comb filters. It is used to perform low-pass filtering on the input digital audio signal, filtering out out-of-band high-frequency noise and reducing the sampling frequency, outputting an audio signal within the human ear's sensitive frequency band. Specifically, the five cascaded delay-free integrators perform low-pass filtering on the input signal to suppress high-frequency noise and smooth the signal; the 32x downsampling unit reduces the signal's sampling rate by a 32x downsampling factor; and the five cascaded comb filters perform differential operation on the input signal to suppress high-frequency components. Figure 7 The order of the delay-free integrator, the order of the comb filter, and the multiplier of the downsampling unit are all adjustable. The above are merely embodiments of the present invention and are not intended to limit the present invention.

[0040] Figure 8 This is a schematic diagram of a combing filter module in a CIC filter module provided according to an embodiment of the present invention. Figure 8 As shown, the comb filter includes one adder and one delay unit. The input of the comb filter is subtracted from the input after passing through the delay unit to obtain the output of the comb filter. The input signal is differentially processed, which effectively suppresses the high-frequency components of the input signal.

[0041] Figure 9 This is a schematic diagram of a CIC passband compensation module in a digital microphone interface circuit for speech recognition according to an embodiment of the present invention. Figure 9 Combining Figure 1 The CIC passband compensation module 122 is described as an ISOP filter, including two amplification units, two delay units, and two adders. The input of the CIC passband compensation module 122 is connected to the output of the CIC filter module 121. To address the frequency response unevenness of the CIC filter module 121 within the passband, the attenuation of the CIC filter module 121 within the passband frequency range is reduced by adjusting the coefficients of the two amplification units. In embodiments of the present invention, the CIC passband compensation module 122 includes an ISOP filter and an amplitude-enhancing FIR compensation filter, but is not limited to these.

[0042] Figure 10This is a schematic diagram of a CIC stopband compensation module in a digital microphone interface circuit for speech recognition provided according to an embodiment of the present invention. Figure 10 Combining Figure 1 Describe, Figure 10 As shown, the CIC stopband compensation module 123 includes: a first-stage multiphase full-pass filter and a second-stage multiphase full-pass filter. The output of the first-stage multiphase full-pass filter is connected to the input of the second-stage multiphase full-pass filter, and the second-stage multiphase full-pass filter outputs a PCM signal. The CIC stopband compensation module 123 is used to compensate for the stopband attenuation of the second compensation signal ISOP-OUT, and outputs a PCM signal with a PCM interface format after stopband compensation.

[0043] Specifically, both the first-stage and second-stage polyphase full-pass filters include one delay unit, two downsampling units, two full-pass filters, and one adder. In one embodiment, the first-stage polyphase full-pass filter has a passband cutoff frequency of 20kHz and a stopband cutoff frequency of 76kHz, and includes: one delay unit, two 2x downsampling units, one adder, one third-order full-pass IIR filter, and one second-order full-pass IIR filter; the second-stage polyphase full-pass filter has a passband cutoff frequency of 20kHz and a stopband cutoff frequency of 28kHz, and includes: one delay unit, two 2x downsampling units, one adder, and two fourth-order full-pass IIR filters. The sub-filters of the polyphase decomposition of the polyphase full-pass filter are all full-pass filters. By adjusting the poles and zeros of the full-pass filters, the polyphase full-pass filters have approximately linear phase in the passband and passband attenuation close to zero, significantly improving the system's stopband attenuation performance. All the all-pass filters in this invention are IIR filter structures. By adjusting the phase response of the system, the non-uniformity in the stopband response of the CIC filter module 121 is compensated, thereby improving the flatness of the stopband attenuation. In this embodiment of the invention, the order of the all-pass filter in the second-stage polyphase all-pass filter is greater than the order of the all-pass filter in the first-stage polyphase all-pass filter. The orders of the all-pass filters listed above are only one embodiment of the invention and are not a limitation thereof. The orders of the all-pass filters in the first-stage and second-stage polyphase all-pass filters can be other types.

[0044] In summary, the digital microphone interface circuit for speech recognition disclosed in this invention provides high-quality audio output signals and features low power consumption. By performing layered noise reduction processing on the sound signals acquired by the microphone, the two digital interfaces—outputting PDM format signals and PCM format audio signals—are improved to meet the needs of different application scenarios.

[0045] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A digital microphone interface circuit for speech recognition, characterized in that, include: A mode selection module is used to receive a selection signal and generate an enable signal according to the selection signal, wherein the enable signal includes a first enable signal and / or a second enable signal; A PDM interface conversion circuit, connected to the mode selection module, is used to convert the input signal into a PDM signal when the first enable signal is valid. The PDM interface conversion circuit includes a low-pass filter module that receives the input signal, a phase compensation module connected to the low-pass filter module, and a digital modulator connected to the phase compensation module. The low-pass filter module converts the input signal into a first filtered signal and inputs the first filtered signal to the phase compensation module. The phase compensation module converts the first filtered signal into a first compensated signal and inputs the first compensated signal to the digital modulator. The digital modulator processes the first compensated signal into a PDM signal that conforms to the PDM interface format. as well as A PCM interface conversion circuit, connected to the mode selection module, converts the input signal into a PCM signal when the second enable signal is valid. The PCM interface conversion circuit includes a CIC filtering module that receives the input signal, a CIC passband compensation module connected to the CIC filtering module, and a CIC stopband compensation module connected to the CIC passband compensation module. The CIC filtering module performs low-pass filtering on the input signal and outputs a second filtered signal after reducing the sampling frequency. The CIC passband compensation module receives the second filtered signal and outputs a second compensation signal based on it. The CIC stopband compensation module receives the second compensation signal and processes it into a PCM signal conforming to the PCM interface format.

2. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The low-pass filter module includes a second-order IIR filter and a first-order IIR filter. The second-order IIR filter is cascaded with the first-order IIR filter. The input terminal of the second-order IIR filter is connected to the input signal, and the first-order IIR filter outputs the first filtered signal.

3. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The phase compensation module is a second-order IIR filter.

4. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The digital modulator includes a noise shaping circuit and an amplitude discrimination circuit. The noise shaping circuit is used to shape the in-band noise in the input signal to out-of-band. The amplitude discrimination circuit is used to convert the multi-bit signal into a single-bit signal that conforms to the PDM interface format and output the PDM signal.

5. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The CIC filtering module includes multiple cascaded delay filters, downsampling units, and multiple cascaded comb filters. The delay filters receive the input signal, the comb filters output the second filtered signal, and the downsampling units are connected between the multiple cascaded delay filters and the multiple cascaded comb filters.

6. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The CIC passband compensation module is an ISOP filter or an amplitude-increasing FIR compensation filter.

7. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The CIC stopband compensation module includes a first-stage multiphase full-pass filter and a second-stage multiphase full-pass filter. The first-stage multiphase full-pass filter receives the second compensation signal. The input terminal of the second-stage multiphase full-pass filter is connected to the output terminal of the first-stage multiphase full-pass filter to output the PCM signal. The first-stage multiphase full-pass filter and the second-stage multiphase full-pass filter include multiple downsampling units and multiple full-pass filters. The order of the full-pass filter in the second-stage multiphase full-pass filter is higher than the order of the full-pass filter in the first-stage multiphase full-pass filter. The full-pass filter is an IIR filter structure.

8. The digital microphone interface circuit for speech recognition according to claim 1, characterized in that, The mode selection module includes a first selector and a second selector, which are connected in parallel. The first selector outputs a first enable signal to control the PDM interface conversion circuit, and the second selector outputs a second enable signal to control the PCM interface conversion circuit.

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