U.S. patent application number 11/426512 was filed with the patent office on 2007-12-27 for active noise reduction engine speed determining.
This patent application is currently assigned to BOSE CORPORATION*EWC*. Invention is credited to Davis Pan.
Application Number | 20070297619 11/426512 |
Document ID | / |
Family ID | 38698423 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070297619 |
Kind Code |
A1 |
Pan; Davis |
December 27, 2007 |
ACTIVE NOISE REDUCTION ENGINE SPEED DETERMINING
Abstract
An active noise reduction system using adaptive filters. A
method of operation the active noise reduction system includes
smoothing a stream of leakage factors. The frequency of a noise
reduction signal may be related to the engine speed of an engine
associated with the system within which the active noise reduction
system is operated. The engine speed signal may be a high latency
signal and may be obtained by the active noise reduction system
over audio entertainment circuitry.
Inventors: |
Pan; Davis; (Arlington,
MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
BOSE CORPORATION*EWC*
Framingham
MA
|
Family ID: |
38698423 |
Appl. No.: |
11/426512 |
Filed: |
June 26, 2006 |
Current U.S.
Class: |
381/71.11 ;
381/71.14 |
Current CPC
Class: |
H04R 3/002 20130101;
G10K 2210/1282 20130101; G10K 11/17854 20180101; G10K 11/17885
20180101; G10K 11/1785 20180101; G10K 11/17825 20180101; G10K
11/17833 20180101; G10K 11/17883 20180101 |
Class at
Publication: |
381/71.11 ;
381/71.14 |
International
Class: |
A61F 11/06 20060101
A61F011/06 |
Claims
1. A method for operating an active noise reduction system
comprising: receiving a high latency signal representative of
engine speed; providing a noise reduction audio signal at a
reference frequency, the reference frequency related to the engine
speed; and generating a noise reduction audio signal at a frequency
corresponding to a predetermined multiple of the reference
frequency.
2. A method in accordance with claim 1, further comprising:
transducing acoustic energy in an enclosed space to provide a noise
signal representative of the noise in the enclosed space, and
determining, responsive to the noise signal, a phase and a
magnitude of the noise reduction signal.
3. A method in accordance with claim 2, wherein the determining the
phase and magnitude of the noise reduction signal is performed by
circuitry comprising an adaptive filter.
4. A method in accordance with claim 3, wherein the enclosed space
is a vehicle cabin.
5. A method for operating an active noise reduction system
comprising: receiving from a bus associated with an audio
entertainment system a signal representative of engine speed; and
responsive to the signal representative of engine speed, generating
a noise reduction audio signal having a frequency related to the
engine speed.
6. A method in accordance with claim 5, further comprising
receiving from the bus, an entertainment audio signal.
7. A method in accordance with claim 5, wherein the receiving the
signal representative of engine speed comprises receiving a high
latency signal.
8. A method in accordance with claim 7, further comprising
processing the entertainment audio signal to provide a processed
entertainment audio signal; and combining the processed
entertainment audio signal with the noise reduction audio
signal.
9. A method in accordance with claim 5, further comprising
receiving from the bus an entertainment system control signal.
10. A method in accordance with claim 9, further comprising
receiving from the bus, an entertainment audio signal.
11. A method in accordance with claim 10, further comprising
processing the entertainment audio signal to provide a processed
entertainment audio signal; and combining the processed
entertainment audio signal with the noise reduction audio
signal.
12. An audio system comprising: an input element for receiving a
signal representative of engine speed and an entertainment audio
control signal; and circuitry for generating a noise reduction
signal of a frequency related to the signal representative of
engine speed.
13. Apparatus in accordance with claim 12, further comprising audio
signal processing circuitry for processing the entertainment audio
signal to provide a processed entertainment audio signal; and an
acoustic driver, for radiating acoustic energy corresponding to the
noise cancellation signal and also corresponding to the processed
entertainment audio signal.
14. An audio system in accordance with claim 12, wherein the active
noise reduction system is associated with a vehicle.
Description
BACKGROUND
[0001] This specification describes an active noise reduction
system using adaptive filters. Active noise control is discussed
generally in S. J. Elliot and P. A. Nelson, "Active Noise Control"
IEEE Signal Processing Magazine, October 1993.
SUMMARY
[0002] In one aspect of the invention a method for operating an
active noise reduction system includes providing filter
coefficients of an adaptive filter in response to a noise signal;
determining leakage factors associated with the filter
coefficients; smoothing the leakage factors to provide smoothed
leakage factors; applying the smoothed leakage factors to the
filter coefficients to provide modified filter coefficients and,
responsive to the odorfied filter coefficients, providing an active
noise reduction signal characterized by a magnitude. The
determining may be responsive to a triggering condition. The
triggering condition may include the result of comparing the
magnitude of the active noise reduction signal in a first spectral
band with a fisrt threshold. The triggering condition may include
the result of comparing the magnitude of the active noise reduction
signal in a second spectral band with a second threshold. The
second threshold may have a predetermined relationship to the first
threshold. The first threshold may be related to causing a device
to operate non-linearly. The triggering condition may include the
result of monitoring the active noise reduction system to determine
if a predefined event has occurred. The predefined event may be
that an entertainment signal magnitude is within a predetermined
range of a magnitude that causes a device to operate non-linearly.
The predefined event may occur in an audio entertainment system.
The audio entertainment system may be associated with a vehicle.
The predefined event may be the deactivation of the active noise
reduction system. The predefined event may be that a noise signal
is above a threshold associated with non-linear operation of an
input transducer.
[0003] The smoothing may include low pass filtering. Prior to the
smoothing, the determining may include selecting one of a discrete
number of predetermined values for the leakage factor. The discrete
number may be two. The discrete number may be greater than two. The
method may further include combining the active noise reduction
signal with an audio entertainment signal. The audio entertainment
signal may be associated with an audio system in an enclosed space.
The enclosed space may be a vehicle cabin.
[0004] The noise reduction system may be configured to be installed
in a vehicle.
[0005] The determining may be responsive to a plurality of
triggering conditions. The leakage factor determining may include
determining which of the plurality of triggering conditions exist;
responsive to determining that a first triggering condition exists,
selecting a first leakage factor value; and responsive to a
determining that a second triggering condition exists, selecting a
second leakage factor value.
[0006] In another aspect of the invention, an active noise
reduction system includes an adaptive filter, for providing an
active noise reduction signal; a coefficient calculator, for
providing filter coefficients for the adaptive filter; and a
leakage adjuster comprising a data smoother to provide smoothed
leakage factors to apply to the filter coefficients. The apparatus
may include circuitry for comparing the active noise reduction
signal magnitude to a threshold. The apparatus may further include
monitoring circuitry for monitoring the active noise reduction
system to determine if a redefined event has occurred. The leakage
adjust may be responsive to the monitoring circuitry. The apparatus
may further include an audio entertainment system. The monitoring
circuitry may include circuitry for monitoring the audio
entertainment system to determine if an entertainment audio signal
magnitude is within a predetermined range of a magnitude that
causes a device to operate non-linearly. The monitoring circuitry
may further include circuitry for determining if the active noise
reduction system has been deactivated. The active noise reduction
system may further include an input transducer for transducing
periodic vibrational energy to a noise signal and the monitoring
circuitry may include circuitry for determining if the magnitude of
the noise signal is above a threshold associated with non-linear
operation of the input transducer.
[0007] The data smoother may include a low pass filter. The leakage
adjuster may be constructed and arranged to select one of a
discrete number of values for the leakage factor.
[0008] The apparatus may further include an audio entertainment
system for providing an audio entertainment signal; and a combiner
for combining the noise reduction signal.
[0009] In another aspect of the invention, a method for operating a
noise reduction system includes providing a stream of leakage
factor values and smoothing the system of leakage values to provide
a smoothed stream of leakage factor values. The value of each of
the stream of leakage valued may be selected from a discrete number
of predefined values. The providing of each of the stream of
leakage values may be responsive to a detectible condition of the
active noise reduction system. The detectible condition may be that
the active noise reduction system has been deactivated. The
detectible condition may be that the active noise reduction system
has generated an audio signal having a magnitude greater than a
threshold magnitude. The detectible condition may be that the
magnitude of a noise signal above a threshold associated with
non-linear operation of an input transducer. The providing of each
of the stream of leakage values may include selecting a leakage
factor value from a plurality of predetermined leakage factor
values. The method may further include applying the smoothed stream
of leakage factor values to coefficients of an adaptive filter of
an active noise reduction system.
[0010] In another aspect of the invention, a method for operating
an adaptive filter of an active noise reduction system in which the
adaptive filter characterized by coefficients includes smoothing
the stream of leakage factor values to provide smoothed leakage
factor values and applying the smoothed leakage factor values to
the coefficients to provide modified coefficient values. The stream
of leakage factor values may include values selected from a
discrete number of predetermined leakage factor values. The
discrete number may be two. Providing the stream of leakage factors
may include calculating leakage factor values.
[0011] In another aspect of the invention, a method for operating
an active noise reduction system includes providing a first
threshold amplitude for a noise reduction signal corresponding to a
first noise amplitude limit for a first frequency; providing a
second threshold amplitude for noise reduction signal corresponding
to a second noise amplitude limit for a second frequency, wherein
the second noise amplitude limit has a predetermined relationship
to the first noise amplitude limit; calculating filter coefficents
associated with adaptive filters associated with the noise
reduction system to provide a noise reduction signal characterized
by a magnitude; and determining, responsive to a comparing of the
magnitude of the noise reduction signal to the first threshold
amplitude at the first frequency and to the second threshold
amplitude of the second frequency, leakage factors for modifying
the filter coefficients. The second frequency may be a
predetermined multiple of the first frequency. The second noise
amplitude limit may be non-zero. The active reduction system may be
associated with a sinusoidal noise source, such as an engine, which
may be associated with a vehicle. The first frequency may be
related to the frequency of the sinusoidal noise source, such as an
engine associated with the sinusoidal noise source.
[0012] In another aspect of the invention, an active noise
reduction system includes determining an amplitude of a first noise
reduction signal characterized by a first frequency and providing a
non-zero noise reduction amplitude limit for a second frequency,
wherein the second frequency has a predetermined relationship to
the first frequency and wherein the noise reduction amplitude limit
has a predetermined relationship to the first amplitude. The method
may further include, in response to a noise signal characterized by
the second frequency and by an amplitude, providing filter
coefficients of an adaptive filter to reduce the noise signal
amplitude; in the event that the noise signal amplitude is greater
than the noise reduction amplitude limit, applying a first leakage
factor to the filer coefficients; and in the event that the noise
signal amplitude is equal to or greater than the noise reduction
amplitude limit, applying a second leakage factor to the filter
coefficients.
[0013] The active noise reduction system may be associated with a
sinusoidal noise source and the first frequency may be related to
the vehicle. The sinusoidal noise source may be an engine, which
may be associated with a vehicle. The method may further include
nulling the first noise reduction signal.
[0014] In another aspect of the invention, a method for operating
an active noise reduction system includes providing filter
coefficients of an adaptive filter in response to a noise signal
and determining leakage factors associated with the filter
coefficents. The determining includes in response to a first
triggering condition, providing a first leakage factor; in response
to a second triggering condition, providing a second discrete
leakage factor; and in the absence of the first triggering
condition and the second triggering condition, providing a default
leakage factor.
[0015] In another aspect of the invention, a method for operating
an active noise reduction system includes receiving a high latency
signal representative of engine speed; providing a noise reduction
audio signal at a reference frequency, the reference frequency
related to the engine speed; and generating a noise reduction audio
signal at a frequency corresponding to a predetermined multiple of
the reference frequency.
[0016] The method may further include transducing acoustic energy
in an enclose space to provide a noise signal representative of the
noise in the enclosed space, and determining, responsive to the
noise signal, a phase and a magnitude of the noise reduction
signal. The determining the phase and magnitude of the noise
reduction signal may be performed by circuitry comprising an
adaptive filter. The enclosed space may be a vehicle cabin.
[0017] In another aspect of the invention, a method for operating
an active noise reduction system includes receiving from a bus
associated with an audio entertainment system a signal
representative of engine speed and responsive to the signal
representative of engine speed, generating a noise reduction audio
signal having a frequency related to the engine speed. The method
may further include receiving from the bus, an entertainment audio
signal. The receiving the signal representative of engine speed may
include receiving a high latency signal. The method may further
include processing the entertainment audio signal to provide a
processed entertainment audio signal and combining the processed
entertainment audio signal with the noise reduction audio signal.
The method may further include receiving from the bus an
entertainment system control signal. The method may further include
receiving from the bus, an entertainment audio signal. The method
may still further include processing the entertainment audio signal
to provide a processed entertainment audio signal and combining the
processed entertainment audio signal with the noise reduction audio
signal.
[0018] In another aspect of the invention, an audio system includes
an input element for receiving a signal representative of engine
speed and entertainment audio control signal circuitry for
generating a noise reduction signal of a frequency related to the
signal representative of engine speed.
[0019] The audio system may further include audio signal processing
circuitry for processing the entertainment audio signal to provide
a processed entertainment audio signal; and an acoustic driver, for
radiating acoustic energy corresponding to the noise cancellation
signal and also corresponding to the processed entertainment audio
signal
[0020] Other features, objects, and advantages will become apparent
from the following detailed description, when read in connection
with the following drawing, in which:
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0021] FIG. 1A is a block diagram of an active noise reduction
system;
[0022] FIG. 1B is a block diagram including elements of the active
noise reduction system of FIG. 1A implemented as an active acoustic
noise reduction system in a vehicle;
[0023] FIG. 2A is a block diagram of a delivery system of the
reference frequency and an implementation of the delivery system of
the entertainment audio signal of FIG. 1B;
[0024] FIG. 2B is a block diagram of another implementation of the
delivery system of the reference frequency and the delivery system
of the entertainment audio signal of FIG. 1B;
[0025] FIG. 3A is a block diagram showing the logical flow of the
operation of the leakage adjuster of FIGS. 1A and 1B;
[0026] FIG. 3B is a block diagram showing the logical flow of the
operation of another implementation of a leakage adjuster,
permitting a more complex leakage adjustment scheme; and
[0027] FIG. 4 is a frequency responsive curve illustrating an
example of a specific spectral profile.
DETAILED DESCRIPTION
[0028] Through the elements of several views of the drawing may be
shown and described as discrete elements in a block diagram and may
be referred to as "circuitry", unless otherwise indicated, the
elements may be implemented as one of, or a combination of, analog
circuitry, digital circuitry, or one or more microprocessors
executing software instructions. The software instructions may
include digital signal processing (DSP) instruction. Unless
otherwise indicated, signal lines may be implemented as discrete
analog or digital signal lines. Multiple signal lines may be
implemented as one discrete difficult signal line with appropriate
signal processing to process separate streams of audio signals, or
as elements of a wireless communication system. Some of the
processing operations may be expressed in terms of the calculation
and application of coefficients. The equivalent of calculating and
applying coefficients can be performed by other analog or DSP
techniques and are included within the scope of this patent
application. Unless otherwise indicated, audio signals may be
encoded in either digital or analog form; conventional
digital-to-analog and analog-to-digital converters may not be shown
in circuit diagrams. This specification describes an active noise
reduction system. Active noise reduction systems are typically
intended to eliminate undesired noise (i.e. the goal is zero
noise). However in actual noise reduction systems undesired noise
is attenuated, but complete noise reduction is not attained. In
this specification "driving toward zero" means that the goal of the
active noise reduction system is zero noise, though it is
recognized that actual result is significant attenuation, not
complete elimination.
[0029] Referring to FIG. 1A, there is shown a block diagram of an
active noise reduction system. Communication path 38 is coupled to
noise reduction reference signal generator 19 for presenting to the
noise reduction reference signal generator a reference frequency.
The noise reduction reference signal generator is coupled to filter
22 and adaptive filter 16. The filter 22 is coupled to coefficient
calculator 20. Input transducer 24 is coupled to control block 37
and to coefficient calculator 20, which is in turn bidirectionally
coupled to leakage adjuster 18 and adaptive filter 16. Adaptive
filter 16 is coupled to output transducer 28 by power amplifier 26.
Control block 37 is coupled to leakage adjuster 18. Optionally,
there may be additional input transducers 24' coupled to coefficent
calculator 20, and optionally, the adaptive filter 61 may be
coupled to leakage adjuster 18. If there are additional input
transducers 24', there typically will be a corresponding filter 23,
25.
[0030] In operation, a reference frequency, or information from
which a reference frequency can be derived, is provided to the
noise reduction reference signal generator 19. The noise reduction
reference signal generator generates a noise reduction signal,
which may be in the form of a periodic signal, such as a sinusoid
having a frequency component related to the engine speed, to filter
22 and to adaptive filter 16. Input transducer 24 detects periodic
vibrational energy having a frequency component related to the
reference frequency and transduces the vibrational energy to a
noise signal, which is provided to coefficient calculator 20.
Coefficient calculator 20 determines coefficients for adaptive
filter 16. Adaptive filter 16 uses the coefficients from
coefficient calculator 20 to modify the amplitude and/or phase of
the noise cancellation reference signal from noise reduction
reference signal generator 19 and provides the modified noise
cancellation signal to power amplifier 26. The noise reduction
signal is amplified by power amplifier 26 and transduced to
vibrational energy by output transducer 28. Control block 37
controls the operation of the active noise reduction elements, for
example by activating or deactivating the active noise reduction
system or by adjusting the amount of noise attenuation.
[0031] The adaptive filter 16, the leakage adjuster 18, and the
coefficient calculator 20 operate repetitively and recursively to
provide a stream of filter coefficient that cause the adapting
filter 16 to modify a signal that, when transduced to periodic
vibrational energy, attenuates the vibrational energy detected by
input transducer 24. Filter 22, which can be characterized by
transfer function H(s), compensates for effects in the energy
transduced by input transducer 24 of components of the active noise
reduction system (including power amplifier 26 and output
transducer 28) and of the environment in which the system
operates.
[0032] Input transducer(s) 24, 24' may be one of many types of
devices that transduce vibrational energy to electrically or
digitally encoded signals, such as an accelerometer, a microphone,
a piezoelectric device, and others. If there is more the on input
transducer 24, 24', the filtered inputs from the transducers may be
combined in some manner, such as by averaging, or the input from
one may be weighted more heavily than the others. Filter 22,
coefficient calculator 20, leakage adjuster 18, and control block
37 may be implemented as instructions executed by a microprocessor,
such as a DSP device. Output transducer 28 can be one of many
electromechanical or electroacoustical devices that provide
periodic vibrational energy, such as a motor or an acoustic
driver.
[0033] Referring to FIG. 1B, there is shown a block diagram
including elements of the active nose reduction system of FIG. 1A.
The active noise reduction system of FIG. 1B is implemented as an
active acoustic noise reduction system in an enclosed space. FIG.
1B is described as configured for a vehicle cabin, but and also be
configured for use in other enclosed spaces, such as a room or
control station. The system of FIG. 1B also includes elements of an
audio entertainment or communications system, which may be
associated with the enclosed space. For example, if the enclosed
space is a cabin in a vehicle, such as a passenger car, van, truck,
sport utility vehicle, construciton or farm vehicle, military
vehicle, or airplane, the audio entertainment or communications
system may be associated with the vehicle. Entertainment audio
signal processor 10 is communicatingly applied to signal 40 to
receive an entertainment audio signal and/or entertainment system
control signal, and is coupled to combiner 14 and may be coupled to
leakage adjuster 18. Noise reduction reference signal generator 19
is communicatingly coupled to signal line 38 and to adaptive filter
16 and cabin filter 22', which corresponds to the filter 22 of FIG.
1A. Adaptive filter 16 is coupled to combiner 14, to coefficient
calculator 20, and optionally may be directly coupled to leakage
adjuster 18. Coefficient calculator 20 is coupled to value filter
22', to leakage adjuster 18, and to microphones 24'', which
correspond to the input transducers 24, 24' of FIG. 1A. Combiner 14
is coupled to power amplifier 26 which is coupled to acoustic
driver 28', which corresponds to output transducer 28 of FIG. 1A.
Control block 37 is commmunicatingly coupled to leakage adjuster 18
and to microphones 24.DELTA.. In many vehicles, entertainment audio
signal processor 10 is coupled to a plurality of combiners 14, each
of which is coupled to a power amplifier 26 and an acoustic driver
28'.
[0034] Each of the plurality of combiners 14, power amplifies 26,
and acoustic drivers 28' may be coupled, through elements such as
amplifiers and combiners to one of a plurality of adaptive filters
16, each of which has associated with it a leakage adjuster 18, a
coefficient calculator 20, and a cabin filter 22. A single adaptive
filter 16, associated leakage adjuster 18, and coefficient
calculator 20 may modify noise cancellation signals presented to
more than one acoustic driver. For simplicity, only one combiner
14, one power amplifier 26, ad one acoustic driver 28' are shown.
Each microphone 24'' may be coupled to more than one coefficient
calculator 20.
[0035] All or some of the entertainment audio signal processor 10,
the noise reduction reference signal generator 19, the adaptive
filter 16, the cabin filter 22', the coefficient calculator 20 the
leakage adjuster 18, the control block 37, and the combiner 14 may
be implemented as software instructions executed by one or more
microprocessors or DSP chips. The power amplifier 26 and the
microprocessor or DSP chip may be components of an amplifier
30.
[0036] In operation, some of the elements of FIG. 1B operate to
provide audio entertainment and audibly presented information (such
as navigation instructions, audible warning indicators, cellular
phone transmission, operational information [for example, low fuel
indication], and the like) to occupants of the vehicle. An
entertainment audio signal from signal line 40 is processed by
entertainment audio signal processor 10. A processed audio signal
is combined with an active noise reduction signal (to be described
later) at combiner 14. The combined signal is amplified by power
amplifier 26 and transduced to acoustic energy by acoustic driver
28'.
[0037] Some elements of the device of FIG. 1B operate to actively
reduce noise in the vehicle compartment caused by the vehicle
engine and other noise sources. The engine speed, which is
typically represented as pulses indicative of the rotational speed
of the engine, also referred to as revolutions per minute or RPM,
is provided to noise reduction reference signal generator 19, which
determines a reference frequency according to
f ( Hz ) = engine_speed ( rpm ) 60 . ##EQU00001##
The reference frequency is provided to cabin filter 22'. The noise
reduction reference signal generator 19 generates a noise
cancellation signal, which may be in the form of a periodic signal,
such as a sinusoid having a frequency component related to the
engine speed. The noise cancellation signal is provided to adaptive
filter 16 and in turn to cabin filter 22'. Microphone 24''
transduces acoustic energy, which may include acoustic energy
corresponding to entertainment audio signals, in the vehicle cabin
to a noise audio signal, which is provided to the coefficient
calculator 20. The coefficient calculator 20 modifies the
coefficients of adaptive filter 16. Adaptive filter 16 uses the
coefficents to modify the amplitude and/or phase of the noise
cancellation signal from noise reduction reference signal generator
19 and provides the modified noise cancellation signal to signal
combiner 14. The combined effect of some electro-acoustic elements
(for example, acoustic driver 28', power amplifier 26, microphone
24'' and of the environment within which the noise reduction system
operates) can be characterized by a transfer function H(s). Cabin
filter 22' models and compensates for the transfer function H(s).
The operation of the leakage adjuster 18 and control block 37 will
be described below.
[0038] The adaptive filter 16, the leakage adjuster 18, and the
coefficent calculator 20 operate repetitively and recursively to
provide a stream of filter coefficients that cause the adaptive
filter 16 to modify an audio signal that, when radiated by the
acoustic driver 28', drives the magnitude of specific spectral
components of the signal detected by microphone 24'' to some
desired value. The specific spectral components typically
correspond to fixed multiples of the frequency derived from the
engine speed. The specific desired value to which the magnitude of
the specific spectral components is to be driven may be zero, but
may be some other value as will be described below.
[0039] The elements of FIGS. 1A and 1B may also be replicated and
used to generate and modify noise reduction signal for more than
one frequency. The noise reduction signal for the other frequencies
is generated and modified in the same manner as described
above.
[0040] The content of the audio signals from the entertainment
audio signal source includes conventional audio entertainment, such
as for example, music, talk radio, news and sports broadcasts,
audio associated with multimedia entertainment and the like, and,
as stated above, may include forms of audible information such as
navigation instructions, audio transmissions from a cellular
telephone network, warning signals associated with operation of the
vehicle, and operational information about the vehicle. The
entertainment audio signal processor may include stereo and/or
multi-channel audio processing circuitry. Adaptive filter 16 and
coefficient calculator 20 together may be implemented as one of a
number of filter types, such as an n-tap delay line: a Leguerre
filter; a finite impulse response (FIR) filter; and others. The
adaptive filter may use one of a number of types of adaptation
schemes, such as a a least mean square (LMS) adaptive scheme; a
normalized LMS scheme; a block LMS scheme; or a block discrete
Fourier transform scheme; and others. The combiner 14 is not
necessarily a physical element, but rather may be implemented as a
summation of signals.
[0041] Though shown as a single element, the adaptive filter 16 may
include more than one filter element. In some embodiments of the
system of FIG. 1B, adaptive filter 16 includes two FIR filter
elements, one each for a sine function and a cosine function with
both sinusoid inputs at the same frequency, each FIR filter using
an LMS adaptive scheme with a single tap, and a sample rate which
may be related to the audio frequency sampling rate r (for
example
r 28 ) , ##EQU00002##
Suitable adaptive algorithms for use by the coefficent calculator
20 may be found in Adaptive Filter Theory, 4.sup.th Edition by
Simon Haykin, ISBN 013091261. Leakage adjuster 18 will be described
below.
[0042] FIG. 2A is a block diagram showing devices that provide the
engine speed to noise reduction reference signal generator 19 and
that provide the audio entertainment signal to audio signal
processor 10. The audio signal delivery elements may include an
entertainment bus 32 coupled to audio signal processor 10 of FIG.
1B by signal line 40 and further coupled to noise reduction
reference signal generator 19 by signal line 38. The entertainment
bus may be a digital bus that transmits digitally encoded audio
signals among elements of a vehicle audio entertainment system.
Devices such as a CD player, an MP3 player, a DVD player or similar
devices or a radio receiver (none of which are shown) may be
coupled to the entertainment bus 32 to provide an entertainment
audio signal. Also coupled to entertainment bus 32 may be sources
of audio signals representing information such as navigation
instructions, audio transmissions from a cellular telephone
network, warning signals associated with operation of the vehicle,
and other audio signals. The engine speed signal delivery elements
may include a vehicle data bus 34 and a bridge 36 coupling the
vehicle data bus 34 and the entertainment bus 32. The example has
ben described with reference to a vehicle with an entertainment
system; however the system of FIG. 2A may be implemented with noise
reducing systems associated with other types of sinusoidal noise
sources, for example a power transformer. The system may also be
implemented in noise reduction systems that do not include an
entertainment system, by providing combinations of buses, signal
lines, and other signal transmission elements that result in
latency characteristics similar to the system of FIG. 2A.
[0043] In operation, the entertainment bus 32 transmits audio
signals and/or control and/or status information for elements of
the entertainment system. The vehicle data bus 34 may communicate
information about the status of the vehicle, such as the engine
speed. The bridge 36 may receive engine speed information and may
transmit the engine speed information to the entertainment bus,
which in turn may transmit a high latency engine speed signal to
the noise reduction reference signal generator 19. As will be
described more fully below, in FIGS. 2A and 2B, the terms "high
latency" and "low latency" apply to the interval between the
occurrence of an event, such as a change in engine speed, and the
arrival of an information signal indicating the change in engine
speed at the active noise reduction system. The buses may be
capable of transmitting signals with low latency, but the engine
speed signal may be delivered with high latency, for example
because of delays in the bridge 36.
[0044] FIG. 2B illustrates another implementation of the signal
delivery elements of the engine speed signal and the signal
delivery elements of the entertainment audio signal of FIG. 1B. The
entertainment audio signal delivery elements include entertainment
audio signal bus 49 coupled to audio signal processor 10 of FIG. 1B
by signal line 40A. Entertainment control bus 44 is coupled to
audio entertainment processor 10 of FIG. 1B by signal line 40B. The
engine speed signal delivery elements include the vehicle data bus
34 coupled to an entertainment control bus 44 by bridge 36. The
entertainment control bus 44 is coupled to noise reduction
reference signal generator 19 by signal line 38.
[0045] The embodiment of FIG. 2B operates similarly to the
embodiment of FIG. 2A, except that the high latency engine speed
signal is transmitted from the bridge 36 to the entertainment
control bus 44 and then to the noise reduction reference signal
generator 19. Audio signals are transmitted from the entertainment
audio signal bus 49 to entertainment audio signal processor 10 over
signal line 40A. Entertainment control signals are transmitted from
entertainment control bus 44 to entertainment audio signal
processor 10 of FIG. 1 by signal line 40B. Other combinations of
vehicle data buses, entertainment bases, entertainment control
buses, entertainment audio signal buses, and other types of buses
and signal lines, depending on the configuration of the vehicle,
may be used to provide the engine speed signal to reference signal
generator 19 and the audio entertainment signal to entertainment
signal processor 20.
[0046] Conventional engine speed sources include a sensor, sensing
or measuring some engine speed indicator such as crankshaft angle,
intake manifold pressure, ignition pulse, or some other condition
or event. Sensor circuits are typically low latency circuits but
require the placement of mechanical, electrical, optical or
magnetic sensors at locations that may be inconvenient to access or
may have undesirable operating conditions, for example high
temperatures, and also require communications circuitry, typically
a dedicated physical connection, between the sensor and noise
reduction reference signal generator 19 and/or adaptive filter 16
and/or cabin filter 22'. The vehicle data bus is typically a high
speed, low latency bus that includes information for controlling
the engine or other important components of the vehicle.
Interfacing to the vehicle data bus adds complexity to the system,
and in addition imposes constraints on the devices that interface
to the vehicle data bus so that the interfacing device does not
interfere with the operation of important components that control
the operation of the vehicle. Engine speed signal delivery systems
according to FIGS. 2A and 2B are advantageous over other engine
speed signal sources and engine speed signal delivery systems
because they permit active noise reduction capability without
requiring any dedicated components such as dedicated signal lines.
Arrangements according to FIGS. 2A and 2B are further advantageous
because the vehicle data bus 34, bridge 36, and one or both of the
entertainment bus 32 of FIG. 2A or the entertainment control bus 44
of FIG. 2B are present in many vehicles so no additional signal
lines for engine speed are required to perform active noise
reduction. Arrangements according to FIG. 2A or 2B also may use
existing physical connection between the entertainment bus 32 or
entertainment control bus 44 and the amplifier 30 and require no
additional physical connections, such as pins or terminals for
adding active noise reduction capabaility. Since entertainment bus
32 or entertainment control bus 44 may be implemented as a digital
bus, the signal lines 38 and 40 of FIG. 2A and signal lines 38, 40A
and 40B of FIG. 2B may be implemented as a single physical
elements, for example a pin or terminal, with suitable circuitry
from routing the signals to the appropriate component.
[0047] An engine speed signal delivery system according to FIGS. 2A
and 2B may be a high latency delivery system, due to the bandwith
of the entertainment bus, the latency of the bridge 36, or both.
"High latency," in the context of this specification, means a
latency between the occurance of an event, such as an ignition
event or a change in engine speed, and the arrival at noise
reduction reference signal generator 19 of a signal indicating the
occurrence of the event, of 10 ms or more.
[0048] An active noise reduction system that can operate using a
high latency signal is advantageous because providing a low latency
signal to the active noise reduction system is typically more
complicated, difficult, and expensive than using an already
available high latency signal.
[0049] The leakage adjuster 18 will now be described in more
detail. FIG. 3A is a block diagram showing the logical flow of the
operation of the leakage adjuster 18. The leakage adjuster selects
a leakage factor to be applied by the coefficient calculator 20. A
leakage factor is a factor .alpha. applied in adaptive filters to
an existing coefficient value when the existing coefficient value
is updated by an update an amount; for example
(new_value)=.alpha.(old_value)+(update_amount)
Information on leakage factors may be found in Section 13.2 of
Adaptive Filter Theory by Simon Haykin, 4.sup.th Edition, ISBN
013091261. Logical block 52 determines if a predefined triggering
event has occurred, or if a predefined triggering condition exists,
that may cause it to be desirable to use an alternate leakage
factor. Specific examples of events or conditions will be described
below. If the value of the logical block 52 is FALSE, the default
leakage factor is applied at leakage factor determination logical
block 48. If the value of logical block 52 is TRUE, an alternate,
typically lower, leakage factor may be applied at leakage factor
determination logical block 48. The alternate leakage factor may be
calculated according to an algorithm, or may operate by selecting a
leakage factor value from a discrete number of predetermined
leakage factor values based on predetermined criteria. The stream
of leakage factors may optionally be smoothed (block 50), for
example by low pass filtering, to prevent abrupt changes in the
leakage factor that have undesirable results. The low pass
filtering causes leakage factor applied by adaptive filter 16 to be
bounded by the default leakage factor and the alternate leakage
factor. Other forms of smoothing may include slew limiting or
averaging over time.
[0050] FIG. 3B is a block diagram showing the logical flow of the
operation of a leakage adjuster 18 permitting more than one, for
example n, alternate leakage factor permitting the n alternate
leakage factors to be applied according to a predetermined
priority. At logical block 53-1, it is determined if the highest
priority triggering conditions exist or events have occurred. If
the value of logical block 53-1 is TRUE, the leakage factor
associated with the triggering conditions and events of logical
block 53-1 is selected at logical block 55-1 and provided to the
coefficient calculator 20 through a data smoother 50, if present.
If the value of logical block 53-1 is FALSE, it is determined at
logical block 53-2 if the second highest priority triggering
conditions exist or events have occurred. If the value of logical
block 53-2 is TRUE, the leakage factor associated with the
triggering conditions and events of logical block 53-2 is selected
at logical block 55-2 and provided to the coefficient calculator 20
through the data smoother 50, if present. If the value of logical
block 53-2 is FALSE, then it is determined if the next highest
priority triggering conditions exist or events have occurred. The
process proceeds until at logical block 53-n, it is determined if
the lowest (or nth highest) priority triggering conditions exist or
events have occurred. If the value of logical block 53-n is TRUE,
the leakage factor associated with the lowest priority triggering
conditions or events is selected at logical block 55-n and provided
to the coefficient calculator 20 through the data smoother 50, if
present. If the value of logical block 53-n is FALSE, at logical
block 57 the default leakage factor is selected and provided to the
coefficient calculator 20 through the data smoother 50, if
present.
[0051] In one implementation of FIG. 3B, there are 2 sets of
triggering conditions and events and two associated leakage factors
(n=2). The highest priority triggering conditions or events include
the system being deactivated, the frequency of the noise reduction
signal being out of the spectral range of the acoustic driver, or
the noise detected by an input transducer such as a microphone
having a magnitude that would induce non-linear operation, such as
clipping. The leakage factor associated with the highest priority
triggering conditions is 0.1. The second highest priority
triggering conditions or events include the cancellation signal
magnitude from adaptive filter 16 exceeding a threshold magnitude,
the magnitude of the entertainment audio signal approaching (for
example coming within a predefined range, such as 6 dB) the signal
magnitude at which one of more electro-acoustical elements of FIG.
1B, such as the power amplifier 26 or the acoustic drive 28' may
operate non-linearly, or some other event occurring that may result
in an audible artifact, such as a click or pop, or distortion.
Events that may cause an audible artifact, such as a click, pop, or
distortion may include output levels being adjusted or the noise
reduction signal having an amplitude or frequency that is known to
cause a buzz or rattle in the acoustic driver 28 or some other
component of the entertainment audio system. The leakage factor
associated with the second highest priority triggering conditions
and events is 0.5. The default leakage factor is 0.999999.
[0052] Logical blocks 53-1 . . . 53-n receive indication that a
triggering event has or is about to occur that a triggering
condition exists from an appropriate element of FIG. 1A or 1B, as
indicated by arrows 59-1 . . . 59-n. The appropriate element may be
control block 37 of FIG. 1B; however the indication may come from
other elements. For example if the predefined event is that the
magnitude of the entertainment audio signal approaches a non-linear
operating range of one of the elements of FIG. 1B, the indication
may originate in the entertainment audio signal processor 10 (not
shown in this view).
[0053] The processors of FIGS. 3A and 3B are typically implemented
by digital signal processing instructions on a DSP processor.
Specific values for the default leakage factor and the alternate
leakage factor may be determined empirically. Some systems may not
apply a leakage factor in default situations. Since the leakage
factor to multiplicative, not applying a leakage factor is
equivalent to applying a leakage factor of 1. Data smoother 50 may
be implemented, for example as a first order lowpass filter with a
tuneable frequency cutoff that may be set, for example, at 20
Hz.
[0054] An active noise reduction system using the devices and
methods of FIGS. 1A, 1B, 3A, and 3B is advantageous because it
significantly reduces the number of occurrences of audible clicks
or pops, and because it significantly reduces the number of
occurrences of distortion and nonlinearities.
[0055] The active noise reduction system may control the magnitude
of the noise reduction audio signal, to avoid overdriving the
acoustic driver or for other reasons. One of those other reasons
may be to limit the noise present in the enclosed space to a
predetermined non-zero target value, or in other words to permit a
predetermined amount of noise in the enclosed space. In some
instances it may be desired to cause the noise in the enclosed
space to have a specific spectral profile to provide a distinctive
sound or to achieve some effect.
[0056] FIG. 4 illustrates an example of a specific spectral
profile. For simplicity, the effect of the room and characteristics
of the acoustic driver 28 will be omitted from the explanation. The
effect of the room is modeled by the filter 22 of FIG. 1A or the
cabin filter 22' of FIG. 1B. An equalizer compensates for the
acoustic characteristisc of the acoustic driver. Additionally, to
facilitate describing the profile in terms of ratios, the vertical
scale of FIG. 4 is linear, for example volts of the noise signal
from microphone 24''. The linear scale can be converted to a
non-linear scale, such as dB, by standard mathematical
techniques.
[0057] In FIG. 4, the frequency f may be related to the engine
speed, for example as
f ( Hz ) = engine_speed ( rpm ) 60 . ##EQU00003##
Curve 62 represents the noise signal without the active noise
cancellation elements operating. Curve 64 represents the noise
signal with the active noise cancellation elements operating.
Numbers n.sub.1, n.sub.2, and n.sub.3 may be fixed numbers so that
n.sub.1f, n.sub.2f, and n.sub.3f are fixed multiples of f. Factors
n.sub.1, n.sub.2, and n.sub.3 may be integers so that frequencies
n.sub.1f, n.sub.2f, and n.sub.3d can conventionally be described as
"harmonics", but do not have to be integers. The amplitudes
a.sub.1, a.sub.2, and a.sub.3 at frequencies n.sub.1f, n.sub.2f,
and n.sub.3f may have a desired characteristic relationship, for
example a.sub.2=0.6a.sub.1 or
a 2 a 1 = 0.6 ##EQU00004##
and a.sub.3=0.5a.sub.1 or
a 3 a 1 = 0.5 . ##EQU00005##
These relationships may vary as a function of frequency.
[0058] There may be little acoustic energy at frequency f. It is
typical for the dominant noise to be related to the cylinder
firings, which for a four cycle, size cylinder engine occurs three
times each engine rotation, so the dominant noise may be at the
third harmonic of the engine speed, so in this example n.sub.3=3.
It may be desired to reuse the amplitude at frequency 3f
(n.sub.1=3) as much as possible because noise at frequency 3f is
objectionable. To achieve some acoustic effect, it may be desired
to reduce the amplitude at frequency 4.5f (so in this example
n.sub.2=4.5) but not as far as possible, for example to amplitude
0.5 a.sub.2. Similarly, it may be desired to reduce the amplitude
at frequency 6f (so in this example n.sub.3=6) to, for example
0.4a.sub.3. In this example, referring to FIG. 1B, noise reduction
reference signal generator 19 receives the engine speed from the
engine speed signal delivery system and generates a noise reduction
reference signal at frequency 3f. The coefficent calculator 16
determines filter coefficents appropriate to provide a noise
reduction audio signal to drive the amplitude at frequency 3f
toward zero, thereby determining amplitude a.sub.1. In instances in
which the noise at frequency 3f is not objectionable, but rather is
desired to achieve the acoustic effect, the adaptive filter may
null the signal at frequency 3f numerically and internal to the
noise reduction system. This permits the determination of amplitude
a.sub.1 without affecting the noise at frequency 3f. Noise
reduction reference signal generator 19 also generates a noise
reduction signal of frequency 4.5f and coefficient calculator 20
determines filter coefficents appropriate to provide a noise
reduction signal to drive the amplitude a.sub.2 toward zero.
However, in this example, it was desired that the amplitude at
frequency 4.5f to be reduced to no less than 0.5 a.sub.2. Since it
is known that a.sub.2=0.6a.sub.1, the alternate leakage factor is
applied by the leakage adjuster 18 when the noise at frequency 4.5f
approaches (0.5) (0.6)a.sub.1 or 0.3a.sub.1. Similarly, the
alternate leakage factor is applied by leakage adjuster 18 when the
noise at frequency 6f approaches (0.4) (0.5) a.sub.1 or 0.2a.sub.1.
Thus, the active noise reduction system can achieve the desired
spectral profile interims amplitude a.sub.1.
[0059] Numerous uses of and departures from the specific apparatus
and techniques disclosed herein may be made without departing from
the inventive concepts. Consequently, the invention is to be
construed as embracing each and every novel feature and novel
combination of features disclosed herein and limited only by the
spirit and scope of the appended claims.
* * * * *