Low-noise directional microphone system

• US 20030169891A1
• Filed: 03/06/2003
• Published: 09/11/2003
• Est. Priority Date: 03/08/2002
• Status: Active Grant

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: Sound Design Technologies Limited (ON Semiconductor Corporation)
• Inventors: Csermak, Brian D., Ryan, Jim G.
• Granted Patent: US 7,409,068 B2
• Time in Patent Office: Days
• US Class Current: 381/92
• CPC Class Codes: H04R 1/08 Mouthpieces; Microphones; Attachments therefor
• CPC Class Codes: H04R 1/38 in which sound waves act upon both sides of a diaphragm and incorporating acoustic phase-shifting means, e.g. pressure-gradient microphone
• CPC Class Codes: H04R 2410/01 Noise reduction using microphones having different directional characteristics
• CPC Class Codes: H04R 2430/03 Synergistic effects of band splitting and sub-band processing
• CPC Class Codes: H04R 2430/21 Direction finding using differential microphone array [DMA]
• CPC Class Codes: H04R 25/407 Circuits for combining signals of a plurality of transducers

Abstract

A low-noise directional microphone system includes a front microphone, a rear microphone, a low-noise phase-shifting circuit and a summation circuit. The front microphone generates a front microphone signal, and the rear microphone generates a rear microphone signal. The low-noise phase-shifting circuit implements a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and to maintain a maximum level of noise amplification over a pre-determined frequency band. The summation circuit combines the front and rear microphone signals to generate a directional microphone signal.

Claims

1. A directional microphone system for a hearing instrument, comprising:

a front microphone that generates a front microphone signal;

a rear microphone that generates a rear microphone signal;

a low-noise phase-shifting circuit that implements a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a maximum level of noise amplification over a pre-determined frequency band; and

a summation circuit that combines the front and rear microphone signals to generate a directional microphone signal.

2. The directional microphone system of claim 1, wherein the low-noise phase-shifting circuit implements a time-of-flight delay on the rear microphone signal to compensate for a distance between the front microphone and the rear microphone.

3. The directional microphone system of claim 1, further comprising:

a delay circuit coupled to the rear microphone that filters the rear microphone signal to implement a time-of-flight delay.

4. The directional microphone system of claim 1, wherein the low-noise phase-shifting circuit is coupled to the rear microphone and modifies the rear microphone signal to implement the frequency-dependent phase difference.

5. The directional microphone system of claim 1, wherein the low-noise phase shifting circuit is coupled to the front microphone and modifies the front microphone signal to implement the frequency-dependent phase difference.

6. The directional microphone system of claim 1, wherein the low-noise phase shifting circuit is coupled to the front microphone and the rear microphone and modifies both the front microphone signal and the rear microphone signal to implement the frequency-dependent phase difference.

7. The directional microphone system of claim 1, wherein the summation circuit subtracts the rear microphone signal from the front microphone signal to generate the directional microphone signal.

8. The directional microphone system of claim 1, wherein the low-noise phase shifting circuit comprises:

a front infinite impulse response (IIR) filter coupled to the front microphone that filters the front microphone signal to implement a first frequency-dependent phase shift; and

a rear IIR filter coupled to the rear microphone that filters the rear microphone signal to implement a second frequency-dependent phase shift;

wherein the frequency-dependent phase difference between the front microphone signal and the rear microphone signal is a function of the difference between the first frequency-dependent phase shift and the second frequency-dependent phase shift.

9. The directional microphone system of claim 8, wherein the front IIR filter generates a first filtered output and the rear IIR filter generates a second filtered output, and wherein the summation circuit subtracts the second filtered output from the first filtered output to generate the directional microphone signal.

10. The directional microphone system of claim 8, further comprising:

a delay circuit coupled to the rear microphone that filters the rear microphone signal to implement a time-of-flight delay.

11. The directional microphone system of claim 8, further comprising:

an equalization filter coupled to the summation circuit that filters the directional microphone signal to equalize the on-axis frequency response of the directional microphone signal.

12. The directional microphone system of claim 1, wherein the low-noise phase-shifting circuit implements an optimal sensor-weight vector.

13. The directional microphone system of claim 12, wherein the optimal sensor-weight vector implemented by the low-noise phase shifting circuit is calculated at each of a plurality of frequencies within the pre-determined frequency band using a set of closed form equations.

14. The directional microphone system of claim 12, wherein the optimal sensor-weight vector implemented by the low-noise phase-shifting circuit is calculated iteratively at each of a plurality of frequencies within the pre-determined frequency band.

15. The directional microphone system of claim 1, wherein the low-noise phase shifting circuit comprises:

a front finite impulse response (FIR) filter coupled to the front microphone that filters the front microphone signal to implement a first frequency response; and

a rear FIR filter coupled to the rear microphone that filters the rear microphone signal to implement a second frequency response;

wherein the frequency-dependent phase difference between the front microphone signal and the rear microphone signal is a function of the first and second frequency responses.

16. The directional microphone system of claim 15, wherein the front FIR filter generates a first filtered output and the rear FIR filter generates a second filtered output, and wherein the summation circuit sums the first filtered output with the second filtered output to generate the directional microphone signal.

17. The directional microphone system of claim 15, wherein the first and second frequency responses collectively equalize the on-axis frequency response of the directional microphone signal.

18. The directional microphone system of claim 1, wherein the front and rear microphones are omnidirectional microphones.

19. The directional microphone system of claim 1, wherein the front and rear microphones are directional microphones.

20. The directional microphone system of claim 1, wherein the directional microphone signal has a cardioid pattern.

21. The directional microphone system of claim 1, wherein the directional microphone signal has a super-cardioid pattern.

22. The directional microphone system of claim 1, wherein the directional microphone signal has a hyper-cardioid pattern.

23. The directional microphone system of claim 1, wherein the directional microphone signal has a bi-directional pattern.

24. A directional microphone system for a hearing instrument, comprising:

a front microphone that generates a front microphone signal;

a rear microphone that generates a rear microphone signal;

means for implementing a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a maximum level of noise amplification over a pre-determined frequency band; and

means for combining the front microphone signal and the rear microphone signal to generate a directional microphone signal.

25. The directional microphone system of claim 24, further comprising:

means for implementing a time-of-flight delay in the rear microphone signal.

26. The directional microphone system of claim 24, further comprising:

means for filtering the directional microphone signal to equalize the on-axis frequency response of the directional microphone signal.

27. A digital hearing instrument, comprising:

a front microphone that generates a front microphone signal;

a rear microphone that generates a rear microphone signal;

a directional processor coupled to the front and rear microphones that implements a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a maximum level of noise amplification over a pre-determined frequency band, and that combines the front and rear microphone signals to generate a directional microphone signal;

a sound processor coupled to the directional processor that selectively modifies the frequency response of the directional microphone signal to match pre-selected signal characteristics and generates a processed intended signal;

a digital-to-analog converter coupled to the sound processor that converts the processed intended signal into an analog hearing aid output signal; and

a speaker coupled to the digital-to-analog converter that converts the analog hearing aid output signal to an acoustical hearing aid output signal that is directed into the ear canal of the digital hearing aid user.

28. A method for reducing noise levels in a directional microphone system for a hearing instrument, comprising the steps of:

generating a front microphone signal from an acoustical signal;

generating a rear microphone signal form the acoustical signal;

causing a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a maximum level of noise amplification over a pre-determined frequency band; and

combining the front microphone signal and the rear microphone signal to generate a directional microphone signal.

29. The method of claim 28, comprising the further step of:

causing an additional phase difference between the front microphone signal and the rear microphone signal to compensate for a time-of-flight of the acoustical signal between a front microphone that generates the front microphone signal and a rear microphone that generates the rear microphone signal.

30. The method of claim 28, wherein the rear microphone signal is subtracted from the front microphone signal to generate the directional microphone signal.

31. The method of claim 28, wherein the rear microphone signal is summed with the front microphone signal to generate the directional microphone signal.

32. The method of claim 28, comprising the further step of:

equalizing the on-axis frequency response of the directional microphone signal.