Noise reduction system

• US 4,953,217 A
• Filed: 04/07/1989
• Issued: 08/28/1990
• Est. Priority Date: 07/20/1987
• Status: Expired due to Term

• Current Assignee: Selex Communications Limited (Leonardo S.p.A.)
• Original Assignee: Plessey Overseas Limited (Marconi Holdings Company Limited)
• Inventors: Salloway, Anthony J., Twiney, Robert C.
• Primary Examiner(s): Isen, Forester W.
• Application Number:
  US07/340,809
• Time in Patent Office: 508 Days
• Field of Search: 381/71, 381/72, 381/96, 381/59, 381/94
• US Class Current: 381/71.6
• CPC Class Codes: A61F 11/145 electric, e.g. for active noise reduction
• CPC Class Codes: G10K 11/17825 Error signals
• CPC Class Codes: G10K 11/1785 Methods, e.g. algorithms; Devices G10K11/1781, G10K11/1783 take precedence
• CPC Class Codes: G10K 11/17853 of the filter
• CPC Class Codes: G10K 11/17875 using an error signal without a reference signal, e.g. pure feedback
• CPC Class Codes: G10K 11/17885 additionally using a desired external signal, e.g. pass-through audio such as music or speech
• CPC Class Codes: G10K 2210/1081 Earphones, e.g. for telephones, ear protectors or headsets
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: G10K 2210/3039 Nonlinear, e.g. clipping, numerical truncation, thresholding or variable input and output gain
• CPC Class Codes: G10K 2210/3045 Multiple acoustic inputs, single acoustic output
• CPC Class Codes: H04R 1/1083 Reduction of ambient noise active noise reduction per se G10K11/175; protective devices for the ear, e.g. providing acoustic protection A61F11/06

Abstract

In order to overcome problems in an active noise reduction system of sound buffets at low frequency and signal enhancement caused by imperfect transfer functions of a noise cancelling sound generator and a microphone, one or more high pass filters for reducing low frequency signals are provided in a feedback loop between the sound generator and microphone. A low pass filter is provided for extending the bandwidth of the system but which does not introduce unduly large phase shifts.

Claims

1. An active noise reduction system comprising:

a noise-cancelling sound generator, a microphone acoustically coupled to said generator, and a feedback loop connected between said microphone and said generator, wherein said feedback loop comprises;

loop stabilization means for inverting the phase of microphone signals and filtering the microphone signals, and means for amplifying the phase inverted and filtered microphone signals; and

first high pass frequency filter means for filtering out sound energy from high pressure sound pulses arising from flow frequency buffets, said first high pass frequency filter means comprising a resistive-capacitive combination and an amplifier having inverting and non-inverting inputs with a negative feedback loop, an input of the filter means being provided on one side of said resistive-capacitive combination and the other side of said combination being connected to said inverting input and the non-inverting input being connected to reference potential via a resistive connection.

2. An active noise reduction system comprising:

a noise-cancelling sound generator, a microphone acoustically coupled to said generator, and a feedback loop connected between said microphone and said generator, wherein said feedback loop comprises;

loop stabilization means for inverting the phase of microphone signals and filtering the microphone signals, and means for amplifying the phase inverted and filtered signals; and

first high pass frequency filter means for filtering out sound energy from high pressure sound pulses arising from low frequency buffets, wherein the first filter means has a gain characteristic which is relatively low in a first frequency band and then rises continuously to a relatively high value in a second frequency band, said first filter means introducing a phase shift which rises to a minimum value in a transitional region of said gain characteristic of said first high pass frequency filter means.

3. A system according to claim 2, wherein said loop stabilisation means includes means for limiting the amplitude of the signals in the feedback loop.

4. A system as claimed in claim 2, including second high pass filter means coupled between the loop stabilisation means and the amplifying means for increasing loop gain and/or adjusting phase shift by predetermined amounts within one or more predetermined frequency bands to compensate for transfer characteristics of the microphone and generator.

5. A system as claimed in claim 4 wherein the second filter means has a gain characteristic which is relatively low in a first frequency band and then rises continuously to a relatively high value in a second frequency band, the phase shift introduced by the filter rising to a maximum value in the transitional region of filter gain.

6. A system as claimed in claim 5 wherein the second filter means includes one or more stages, each stage comprising an amplifier with a negative resistive feedback loop, and a resistive capacitive path to reference potential connected to the inverting amplifier input.

7. A system as claimed in claim 4 wherein the functions of the first and second filters are provided by a single high pass filter means, with the transfer characteristics selected to filter out sound energy from high pressure sound pulses and to increase loop gain and/or adjust phase shift to compensate for transfer characteristics of the microphone and generator.

8. A system according to claim 2, and further comprising means for injecting a speech signal into the feedback loop between the further filter means and the amplifying means.

9. A system according to claim 5, wherein the second high pass filter means includes one or more stages, each stage including a first order high pass active filter, the active filter network coupled to the operational amplifier for providing a high pass characteristic.

10. A system according to claim 2 including a lowpass frequency filter means, the gain of the filter in the cut-off region decreasing from a relatively high constant gain region to a second relatively low constant gain region in a transitional region where the gain decreases continuously from the high region to the low region.

11. A system as claimed in claim 10, wherein the low pass frequency filter means comprises a second order transitional filter comprising an operational amplifier with a resistive-capacitive feedback loop connected between the output and non-inverting amplifier input, and a resistive feedback loop connected between the amplifier output and inverting output.

12. A system according to claim 10, wherein the low pass filter means comprises a second order transitional filter.

13. A system according to claim 12, wherein the transitional filter comprises a second order Sallen and Key filter configuration and further including in parallel a positive feedback loop including a resistive-capacitive combination.

14. A system according to claim 2, wherein the first filter means comprises one or more stages, each stage comprising a first order high pass active filter, the active filter including an operational amplifier, and a resistive-capacitive network coupled to the operational amplifier for providing a high pass characteristic.

15. A system according to claim 2, wherein the first high pass frequency means is connected between the microphone and said loop stabilization means.

16. An active noise reduction system comprising:

a noise-cancelling sound generator, a microphone acoustically coupled to said generator, and a feedback loop connected between said microphone and said generator, wherein said feedback loop comprises;

loop stabilization means for inverting the phase of microphone signals and filtering the microphone signals, and means for amplifying the phase inverted and filtered signals; and

further filter means coupled between the phase inverting means and the amplifying means for increasing loop gain and/or adjusting phase shift by predetermined amounts within one or more predetermined frequency bands, wherein the further filter means has a gain characteristic which is relatively low in a first frequency band and then rises continuously to a relatively high value in a second frequency band, the phase shift introduced by the further filter means rises to a maximum value in the transitional region of filter gain characteristic.

17. A system as claimed in claim 16 wherein the further filter means includes one or more stages, each stage comprising an amplifier with a negative resistive feedback loop, and a resistive capacitive path to reference potential connected to the amplifier inverting input.

18. A system as claimed in claim 16, and further comprising means for injecting a speech signal into the feedback loop between the further filter means and the amplifying means.

19. A system according to claim 16, and further including a low pass frequency filter means, the gain of the low pass frequency filter means in the cut-off region, decreasing from a relatively high constant gain region to a relatively low constant gain region in a transitional region where the gain decreases continuously from the high region to the low region.

20. A system according to claim 19, wherein the low pass filter frequency filter means comprises a second order transitional filter comprising an operational amplifier with a resistive-capacitive feedback loop connected between the output and the non-inverting amplifier input, and a resistive feedback loop connected between the amplifier output and inverting input.

21. A system according to claim 16, wherein the further filter means includes one or more stages, each stage including a first order high pass active filter, the active filter including an operational amplifier, and a resistive-capacitive network coupled to the operational amplifier for proving a high pass characteristic.