Digital microphone noise attenuation

• US 10,559,293 B2
• Filed: 09/07/2018
• Issued: 02/11/2020
• Est. Priority Date: 09/08/2017
• Status: Active Grant

• Current Assignee: Knowles Electronics Llc (Knowles Corporation)
• Original Assignee: Knowles Electronics Llc (Knowles Corporation)
• Inventors: Thomsen, Henrik, Du, Yu
• Primary Examiner(s): Ton, David L
• Application Number:
  US16/124,794
• Publication Number:
  US 20190080680A1
• Time in Patent Office: 522 Days
• Field of Search: 381 71, 381 14
• CPC Class Codes: B81B 2201/0257 Microphones or microspeakers
• CPC Class Codes: B81B 3/0021 Transducers for transforming electrical into mechanical energy or vice versa dynamo-electric machines H02K99/00; electrostatic machines H02N1/00; piezoelectric devices H10N30/00
• CPC Class Codes: G10K 11/17853 of the filter
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: H03M 7/3004 Digital delta-sigma modulation
• CPC Class Codes: H04R 17/02 Microphones
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: H04R 19/04 Microphones H04R19/01 takes precedence
• CPC Class Codes: H04R 3/005 for combining the signals of two or more microphones specially adapted for hearing aids H04R25/407

Abstract

A digital microphone device includes circuitry that can reduce the risk of noise caused due to an idle tone frequency component in a digital signal output by the digital microphone device. In stereo mode and other applications where interference occurs between two or more such microphones, each microphone device includes a digital output having a corresponding idle tone frequency, one of which is offset to shift noise components outside of a desired frequency range.

Claims

1. A method in a digital microphone assembly including an acoustic microelectromechanical systems (MEMS) transducer and an integrated circuit, the method comprising:

converting an analog signal output by the acoustic transducer to a digital signal;

removing a direct current (DC) signal component from the digital signal;

after removing the DC signal component, converting the digital signal to a pulse density modulated (PDM) signal having an idle tone frequency that is approximately one-half a frequency at which at least a portion of the integrated circuit is clocked; and

offsetting the idle tone frequency by applying a DC offset to the digital signal after removing the DC signal component and before converting the digital signal into a PDM signal.

2. The method of claim 1, further comprising offsetting the idle tone frequency in response to a signal indicating that the microphone assembly is being used in stereo mode, and not offsetting the idle tone frequency absent the signal indicating that the microphone assembly is being used in stereo mode.

3. The method of claim 2, wherein the acoustic transducer and the integrated circuit are disposed in a housing of the microphone assembly, wherein receiving the signal indicating that the microphone assembly is being used in stereo mode is receivable via an external device interface of the microphone assembly.

4. The method of claim 1, wherein converting the digital signal into a PDM signal includes converting the digital signal into a 1-bit PDM signal.

5. The method of claim 1, wherein a difference between the idle tone frequency with and without the DC offset is greater than a human audible range of frequencies.

6. The method of claim 1, wherein converting the analog signal output by the acoustic transducer to the digital signal includes converting the analog signal using a multi-bit converter.

7. The method of claim 1, wherein removing the direct current (DC) signal component from the digital signal includes filtering the digital signal with a high-pass filter.

8. A microphone device comprising:

an acoustic microelectromechanical systems (MEMS) transducer configured to generate an analog signal responsive to an acoustic signal; and

a signal processing circuit coupled to the transducer, the signal processing circuit including;

an analog-to-digital converter configured to convert the analog signal generated by the transducer to a digital signal,

a filter configured to remove a direct current (DC) signal component from the digital signal,

a DC offset circuit configured to apply a DC offset to the digital signal after the DC component is removed, and

a digital-to-digital converter configured to convert the digital signal having the DC offset to a pulse density modulated (PDM) signal having an idle tone frequency that is approximately one-half a clock frequency of at least a portion of the signal processing circuit,

wherein the idle tone frequency is offset by the DC offset applied to the digital signal.

9. The microphone device of claim 8, wherein the signal processing circuit is further configured to:

apply the DC offset in response to receiving a signal indicating that the microphone device is being used in stereo mode, and

not apply the DC offset absent the signal indicating that the microphone device is being used in stereo mode.

10. The microphone device of claim 9, wherein the transducer and the signal processing circuit are disposed in a housing including a surface mount external-device interface wherein the signal indicating that the microphone device is being used in stereo mode is receivable via the external device interface.

11. The microphone device of claim 10, wherein the PDM signal is a 1-bit PDM signal.

12. The microphone device of claim 11, wherein a difference between the offset idle tone frequency and an idle tone frequency that would be produced without the DC offset is greater than a bandwidth corresponding to a range of human audible frequencies.

13. The microphone device of claim 12, wherein the analog-to-digital converter is a multi-bit converter.

14. The microphone device of claim 8, wherein the filter is a high pass filter.

15. An audio signal processing circuit for a microphone device, the circuit comprising:

an analog to digital converter configured to convert an analog signal received from an acoustic transducer to a digital signal,

a filter configured to remove a direct current (DC) signal component from the digital signal;

a DC offset circuit configured to apply a DC offset to the digital signal after the DC signal component is removed; and

a digital-to-digital converter configured to convert the digital signal to a 1-bit pulse density modulated (PDM) signal after the DC offset is applied to the digital signal,

wherein the PDM signal has an idle tone frequency that is offset by the DC offset and the offset idle tone frequency is approximately one-half a clock frequency of the digital-to-digital converter.

16. The circuit of claim 15, wherein the signal processing circuit is further configured to selectively apply the DC offset to the digital signal in response to receiving a selection signal.

17. The circuit of claim 16, wherein the PDM signal is a 1-bit PDM signal.

18. The circuit of claim 17, wherein a difference between the offset idle tone frequency and an idle tone frequency that would be produced without the DC offset is greater than a bandwidth corresponding to a range of human audible frequencies.

19. The circuit of claim 18, wherein the analog-to-digital converter is a multi-bit converter.

20. The circuit of claim 19, wherein the filter is a high pass filter.