# US 2018/0027338 A1 · Digital microphone assembly with improved frequency response and noise characteristics > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2018/0027338 A1 - Document ID: US20180027338A1 - Status: Published application - Kind: A1 - Application number: 15/216,928 - Filed: 2016-07-22 - Published: 2018-01-25 - Earliest claimed: 2016-07-22 - Assignees: Knowles Electronics, LLC - Applicants: Knowles Electronics, LLC - Inventors: Mohammad SHAJAAN; Claus Erdmann FÜRST; Per Flemming HØVESTEN; Kim Spetzler BERTHELSEN; Henrik THOMSEN - Web page: https://patentgrep.com/patent/US20180027338A1 ## CPC - H04R 19/04: Microphones (H04R19/01 takes precedence) - G06F 3/162: {Interface to dedicated audio devices, e.g. audio drivers, interface to CODECs} - H04R 3/02: for preventing acoustic reaction {, i.e. acoustic oscillatory feedback (specially adapted for hearing aids H04R25/453)} - H04R 3/06: of electrostatic transducers - H04R 19/005: {using semiconductor materials} - H04R 2201/003: Mems transducers or their use - H04R: LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS (producing sounds with frequency not determined by supply frequency G10K) - G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N) ## Abstract A microphone assembly includes an acoustic transducer element configured to convert sound into a microphone signal in accordance with a transducer frequency response including a first highpass cut-off frequency. The microphone assembly additionally includes a processing circuit including a signal amplification path configured to receive, sample and digitize the microphone signal to provide a digital microphone signal. A frequency response of the signal amplification path includes a second highpass cut-off frequency which is higher than the first highpass cut-off frequency of the acoustic transducer element. ## Claims ### Claim 1 A microphone assembly comprising: an acoustic transducer element configured to convert sound into a microphone signal in accordance with a transducer frequency response comprising a first highpass cut-off frequency; and a processing circuit comprising a signal amplification path configured to receive, sample and digitize the microphone signal to provide a digital microphone signal; wherein a frequency response of the signal amplification path comprises a second highpass cut-off frequency; and wherein the first highpass cut-off frequency is lower than the second highpass cut-off frequency. ### Claim 2 (depends on 1) The microphone assembly of claim 1, wherein the acoustic transducer element comprises a capacitive transducer supplying the microphone signal across first and second mutually charged transducer plates. ### Claim 3 (depends on 2) The microphone assembly of claim 2, wherein the capacitive transducer exhibits a transducer capacitance between 0.5 pF and 10 pF. ### Claim 4 (depends on 1) The microphone assembly of claim 1, wherein the acoustic transducer element comprises a microelectromechanical (MEMS) element. ### Claim 5 (depends on 1) The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 20 Hz. ### Claim 6 (depends on 1) The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 10 Hz. ### Claim 7 (depends on 1) The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 5 Hz. ### Claim 8 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 20 Hz. ### Claim 9 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 40 Hz. ### Claim 10 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 100 Hz. ### Claim 11 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 4000 Hz. ### Claim 12 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is between 100 Hz and 1 kHz. ### Claim 13 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is at least two octaves higher than the first highpass cut-off frequency. ### Claim 14 (depends on 1) The microphone assembly of claim 1, wherein the second highpass cut-off frequency is one decade higher than the first highpass cut-off frequency. ### Claim 15 (depends on 1) The microphone assembly of claim 1, wherein the signal amplification path comprises: a forward signal path comprising a summing node for combining the microphone signal and an analog feedback signal; and a feedback signal path comprising: a digital loop filter configured to receive and filter the digital microphone signal and in response generate a lowpass filtered digital feedback signal in accordance with a lowpass transfer function of the digital loop filter; and a digital-to-analog converter (DAC) configured to convert the lowpass filtered digital feedback signal into the analog feedback signal. ### Claim 16 (depends on 15) The microphone assembly of claim 15, wherein the summing node is arranged at the transducer output. ### Claim 17 (depends on 16) The microphone assembly of claim 16, wherein the analog feedback signal is directly connected to the transducer output without any intervening active devices. ### Claim 18 (depends on 17) The microphone assembly of claim 17, wherein the analog feedback signal is directly connected at least one of first and second mutually charged plates of the acoustic transducer element. ### Claim 19 (depends on 17) The microphone assembly of claim 17, wherein the forward signal amplification path comprises: a microphone preamplifier comprising an input connected to the summing node for receipt of the microphone signal, the microphone preamplifier being configured to generate at least one of an amplified microphone signal and a buffered microphone signal; and an analog-to-digital converter (ADC) configured to receive, sample and quantize the amplified or buffered microphone signal to generate the digital microphone signal. ### Claim 20 (depends on 19) The microphone assembly of claim 19, wherein the analog-to-digital converter comprises a sigma-delta modulator generating a single-bit (PDM) or multibit digital microphone signal at a first sampling frequency. ### Claim 21 (depends on 20) The microphone assembly of claim 20, wherein the feedback path comprises a decimator in-front of the digital loop filter configured to convert the single-bit or multibit digital microphone signal into a decimated multibit (PCM) digital signal at a lower sampling frequency and higher bit-width than the single-bit or multibit digital microphone signal. ### Claim 22 (depends on 15) The microphone assembly of claim 15, wherein the processing circuit further comprises: a command and control interface for receipt of filter configuration data from a host processor. ### Claim 23 (depends on 22) The microphone assembly of claim 22, wherein the command and control interface includes an I2C, USB, SoundWire, UART or SPI command and control interface. ### Claim 24 (depends on 22) The microphone assembly of claim 22, wherein the lowpass transfer function of the digital loop filter is adjustably or programmably controlled by the filter configuration data. ### Claim 25 (depends on 16) The microphone assembly of claim 16, wherein the digital-to-analog converter (DAC) comprises: a hybrid Pulse-Width and Pulse-Amplitude Modulator (PWAM) configured to generate the analog feedback signal by converting the first digital feedback signal into a corresponding pulse-width and pulse-amplitude modulated signal at a higher sampling frequency than a sampling frequency of the first digital feedback signal. ### Claim 26 (depends on 23) The microphone assembly of claim 23, wherein the hybrid Pulse-Width and Pulse-Amplitude Modulator (PWAM) comprises: a current output converter configured to convert the pulse-width and pulse-amplitude modulated signal into a corresponding sequence of variable width and amplitude current pulses; wherein the current output converter comprises a plurality of individually controllable current generators connected in parallel to the DAC output. ### Claim 27 (depends on 26) The microphone assembly of claim 26, wherein the hybrid Pulse-Width-Modulator and Pulse-Amplitude-Modulator comprises a noise-shaping quantizer configured to: receive samples, having a first bit-width, of the first digital feedback signal outputted by the digital loop filter; and quantize the samples of the first digital feedback signal to generate samples of a second digital feedback signal with a reduced bit-width. ### Claim 28 (depends on 27) The microphone assembly of claim 27, wherein the hybrid Pulse-Width and Pulse-Amplitude Modulator (PWAM) is configured to: increase a sampling frequency of the second digital feedback signal with a predetermined upsampling factor, N, to generate a third digital feedback signal; and convert the third digital feedback signal into the pulse-width and pulse-amplitude modulated signal. ### Claim 29 (depends on 28) The microphone assembly of claim 28, wherein generation of the pulse-width and pulse-amplitude modulated signal comprises: dividing samples of the third digital feedback signal with the predetermined upsampling factor, N, to compute respective modulus values and remainder values of the samples; converting the modulus value of a sample into a first pulse segment having a combined pulse width and pulse amplitude representative of the modulus value; converting the remainder value of a sample into a second pulse segment with a pulse width representing the remainder value; and combining the first and second pulse segments to form a variable width and amplitude pulse of the pulse-width and pulse-amplitude modulated signal. ### Claim 30 (depends on 15) The microphone assembly of claim 15, wherein the forward signal path of the signal amplification path is DC-coupled from the transducer output to the digital microphone signal. ### Claim 31 (depends on 1) The microphone assembly of claim 1, further comprising: a microphone housing enclosing and supporting the acoustic transducer element and the processing circuit. ### Claim 32 (depends on 1) The microphone assembly of claim 1, wherein the acoustic transducer element comprises a ventilation hole or aperture acoustically connecting a frontal side and a back side of a diaphragm of the acoustic transducer element for setting the first highpass cut-off frequency. ### Claim 33 (depends on 32) The microphone assembly of claim 32, wherein the ventilation hole or aperture extends through the diaphragm or a diaphragm support structure; the ventilation hole having a diameter between 5 and 10 μm for setting the first highpass cut-off frequency. ### Claim 34 (depends on 1) The microphone assembly of claim 1, wherein the acoustic transducer element and the processing circuit are integrally formed on a single semiconductor die. ### Claim 35 (depends on 1) A portable communication device comprising the microphone assembly of claim 1. ### Claim 36 A semiconductor die comprising: a processing circuit comprising a signal amplification path configured to receive, sample and digitize a microphone signal converted from sound by an acoustic transducer element in accordance with a transducer frequency response comprising a first highpass cut-off frequency to provide a digital microphone signal; wherein a frequency response of the signal amplification path comprises a second highpass cut-off frequency; and wherein the first highpass cut-off frequency is lower than the second highpass cut-off frequency. ### Claim 37 A method of setting a frequency response of a signal amplification path of a microphone, comprising: a) converting incoming sound into a corresponding microphone signal by an acoustic transducer element, wherein the acoustic transducer element has a frequency response comprising a first highpass cut-off frequency; b) sampling and quantizing the microphone signal to generate a corresponding digital microphone signal; c) lowpass filtering the digital microphone signal by a digital loop filter to generate a first digital feedback signal; d) converting the first digital feedback signal into a corresponding analog feedback signal by a digital-to-analog converter (DAC); and e) combining the analog feedback signal and the microphone signal to close a feedback loop of the signal amplification path. ### Claim 38 (depends on 37) The method of claim 37, further comprising: f) applying the analog feedback signal to at least one transducer plate of a capacitive microelectromechanical (MEMS) transducer element supplying the microphone signal.