Active noise control apparatus

• US 6,683,960 B1
• Filed: 08/18/2000
• Issued: 01/27/2004
• Est. Priority Date: 04/15/1998
• Status: Expired due to Fees

• Current Assignee: Fujitsu Limited
• Original Assignee: Fujitsu Limited
• Inventors: Fujii, Kensaku, Ohga, Juro
• Primary Examiner(s): Mei, Xu
• Application Number:
  US09/641,660
• Time in Patent Office: 1,257 Days
• Field of Search: 381/71.1, 381/71.2, 381/71.8, 381/71.9, 381/71.4, 381/71.12, 381/71.13, 381/71.14, 381/FOR.123, 381/94.1, 381/FOR.124
• US Class Current: 381/71.8
• CPC Class Codes: G10K 11/17819 between the output signals and the reference signals, e.g. to prevent howling
• CPC Class Codes: G10K 11/17854 the filter being an adaptive filter
• CPC Class Codes: G10K 11/17881 the reference signal being an acoustic signal, e.g. recorded with a microphone
• CPC Class Codes: G10K 2210/112 Ducts vehicle exhausts G10K2210/12822
• CPC Class Codes: G10K 2210/30232 Transfer functions, e.g. impulse response
• CPC Class Codes: H03H 2021/0089 System identification, i.e. modeling
• CPC Class Codes: H03H 2021/0092 Equalization, i.e. inverse modeling
• CPC Class Codes: H03H 21/0012 Digital adaptive filters

Abstract

An active noise control apparatus for transmitting, from a loud speaker, a secondary noise synthesized so as to have the same amplitude as and the opposite phase to a primary noise and for canceling the noise by acoustically overlapping the secondary noise. An overall system filter for simulating a characteristic of an overall system leading to an error detecting microphone from a noise detecting microphone is provided for the first and the second overall system filter. A second and a first noise control filter are connected to the first and the second overall system filter in cascade to form a noise control filter. A coefficient of an estimating noise transfer system filter, obtained when the difference between the differential output of both obtained when a white noise is applied to the circuit of cascade connections and the response difference of the first and the second overall system filter from a differential overall system filter becomes a minimum, is made the coefficient of the noise control filter.

Claims

1. An active noise control apparatus for updating a coefficient of a noise control filter which synthesizes a secondary noise having a same amplitude as and an opposite phase to a noise to be suppressed:

the noise control filter comprising a first and a second noise control filter respectively having a first and a second arbitrary coefficient,

the apparatus further comprising;

a first and a second overall system filter which form an overall system filter for simulating a characteristic of an overall system leading to an error detecting microphone from a noise detecting microphone that detects a noise component required for synthesizing the secondary noise, and which has a first and a second coefficient respectively obtained for the second and the first coefficient,

a differential overall system filter for outputting a response difference of the first and the second overall system filter, and

an estimating noise transfer system filter, having a variable coefficient, connected to the differential overall system filter in cascade,

a white noise generated by a white noise generation circuit being applied to respective cascade combinations of the first overall system filter and the second noise control filter, and the second overall system filter and the first noise control filter, and to the differential overall system filter;

the coefficient of the estimating noise transfer system filter being updated in order that a difference between the output of the differential overall system filter and a differential output between the first and the second noise control filter becomes a minimum, and the coefficient of the estimating noise transfer system filter obtained at a time when the difference becomes the minimum being provided as the coefficients of the noise control filters.

2. The active noise control apparatus as claimed in claim 1 wherein only upon an activation preliminarily stored coefficients are used for the first and the second coefficient respectively of the first and the second noise control filter.

3. The active noise control apparatus as claimed in claim 2 wherein after the activation a coefficient obtained upon a present coefficient update is substituted for one of the first and the second coefficient respectively of the first and the second noise control filter.

4. The active noise control apparatus as claimed in claim 2 wherein after the activation coefficients obtained upon a last and a present coefficient update are respectively provided for the first and the second coefficient respectively of the first and the second noise control filter.

5. The active noise control apparatus as claimed in claim 1 wherein when an output of the overall system filter becomes equal to or less than a threshold value, the coefficient update is stopped.

6. An active noise control apparatus having a feedback control filter which simulates a characteristic of a feedback path leading to a noise detecting microphone connected to an input of a noise control filter for synthesizing a secondary noise, through a loud speaker which transmits the secondary noise having a same amplitude as and an opposite phase to a primary noise for suppression comprising:

an inverse filter composing circuit which forms a filter having a characteristic including a transfer function opposite to a closed circuit starting from the noise control filter and returning to the noise control filter again through both of the feedback path and the feedback control filter, the transfer function expressing a generation process of the primary noise, and

a system identification circuit for providing two pairs of different fixed coefficients either to the feedback control filter or the noise control filter to update a coefficient of an adaptive filter in order that a difference becomes a minimum between an output of a first filter forming a numerator of a solution obtained by eliminating a transfer function component expressing the generation process of the primary noise, from simultaneous equations based on two transfer functions of an inverse filter formed within the inverse filter composing circuit for the two pairs of coefficients and an output of the adaptive filter connected to a second filter in cascade forming a denominator of the solution,

a coefficient of the feedback control filter being updated by using the coefficient of the adaptive filter obtained by operating the system identification circuit.

7. The active noise control apparatus as claimed in claim 6 wherein the system identification circuit gives the simultaneous equations by using a transfer function provided by the inverse filter composing circuit, from which a constant 1 is removed.

8. The active noise control apparatus as claimed in claim 6 wherein upon an activation of the apparatus the system identification circuit gives the simultaneous equations by setting the two pairs of fixed coefficients of the noise control filter or the feedback control filter to different taps.

9. The active noise control apparatus as claimed in claim 6 wherein upon an activation of the apparatus the system identification circuit uses coefficients obtained upon a last activation for the two pairs of fixed coefficients of the noise control filter or the feedback control filter.

10. The active noise control apparatus as claimed in claim 6 wherein the system identification circuit applies a transfer function corresponding to a coefficient used for a present coefficient update for the two pairs of fixed coefficients of the noise control filter or the feedback control filter and the transfer function of the inverse filter upon a subsequent coefficient update of the feedback control filter.

11. The active noise control apparatus as claimed in claim 6 wherein the system identification circuit sets the two pairs of fixed coefficients of the noise control filter or the feedback control filter with a time interval.

12. The active noise control apparatus as claimed in claim 11 wherein when a gain of the adaptive filter becomes equal to or less than a fixed value upon a coefficient update performed by providing the two pairs of fixed coefficients to the noise control filter, the system identification circuit does not update the coefficient of the feedback control filter.

13. The active noise control apparatus as claimed in claim 9 wherein when an output of a filter composing a component corresponding to a gain of the closed circuit becomes equal to or less than a fixed value on the inverse filter composing circuit upon a coefficient update performed by providing the two pairs of fixed coefficients to the noise control filter, the system identification circuit does not update the coefficient of the feedback control filter.

14. An active noise control apparatus of a feedback type having a feedback control filter which simulates a characteristic of a feedback path leading to a noise detecting microphone connected to an input of a noise control filter for synthesizing a secondary noise, through a loud speaker which transmits the secondary noise having a same amplitude as and an opposite phase to a primary noise for suppression comprising:

an inverse filter composing circuit which forms a filter having a characteristic including a transfer function opposite to a closed circuit starting from the noise control filter and returning to the noise control filter again through both of the feedback path and the feedback control filter, the transfer function expressing a generation process of the primary noise, and

a system identification circuit for providing two pairs of different fixed coefficients either to the noise control filter or the feedback control filter to update a coefficient of an adaptive filter in order that a difference becomes a minimum between an output of a first filter forming a numerator of a solution obtained by eliminating the transfer function component expressing the generation process of the primary noise, from simultaneous equations based on two transfer functions of an inverse filter formed within the inverse filter composing circuit for the two pairs of coefficients and an output of the adaptive filter connected to a second filter in cascade forming a denominator of the solution,

the system identification circuit further being repeatedly used for calculating the coefficient of the noise control filter optimally canceling the primary noise as the coefficient of the adaptive filter by making a numerator, which forms a ratio of the transfer function expressing the generation process of the primary noise and the transfer function of the feedback path previously identified on the system identification circuit, the first filter and making a denominator the second filter.

15. The active noise control apparatus of the feedback type as claimed in claim 14 wherein the system identification circuit gives the simultaneous equations by using a transfer function provided by the inverse filter composing circuit, from which a constant 1 is removed.

16. The active noise control apparatus of the feedback type as claimed in claim 14 wherein upon an activation of the apparatus the system identification circuit gives the simultaneous equations by setting the two pairs of fixed coefficients of the noise control filter or the feedback control filter to different taps.

17. The active noise control apparatus of the feedback type as claimed in claim 14 wherein the system identification circuit sets the two pairs of fixed coefficients of the noise control filter or the feedback control filter with a time interval.

18. The active noise control apparatus of the feedback type as claimed in claim 14 wherein upon an activation of the apparatus the system identification circuit uses coefficients obtained and stored upon a last activation for the two pairs of fixed coefficients of the noise control filter or the feedback control filter.

19. The active noise control apparatus of the feedback type as claimed in claim 17 wherein when a difference between a new and an old coefficient of the feedback control filter becomes equal to or less than a fixed value upon a coefficient update performed by providing the two pairs of coefficients to the feedback control filter, the system identification circuit does not update the coefficient of the feedback control filter.

20. The active noise control apparatus of the feedback type as claimed in claim 14 wherein the system identification circuit applies a transfer function corresponding to a coefficient used for a present coefficient update for the two pairs of coefficients of the noise control filter or the feedback control filter and the transfer function of the inverse filter upon a subsequent coefficient update of the feedback control filter.

21. The active noise control apparatus of the feedback type as claimed in claim 14 wherein the inverse filter composing circuit is composed of a non-recursive filter.