Electrical audio signal processing systems and devices – Noise or distortion suppression
Reexamination Certificate
1999-02-26
2003-09-23
Williams, Kimberly A. (Department: 2644)
Electrical audio signal processing systems and devices
Noise or distortion suppression
C381S096000, C381S098000, C381S074000, C381S071100, C381S071600, C381S071130, C327S110000, C330S256000
Reexamination Certificate
active
06625287
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to noise reduction systems. In particular, the invention relates to negative output resistance amplifiers in noise reduction systems, and more particularly to temperature-compensated negative output resistance amplifiers in noise reduction systems.
BACKGROUND
Automatic Noise Reduction (ANR) systems cancel or reduce unwanted acoustic waves by generating an out-of-phase response, thereby canceling out the unwanted waves.
FIG. 1
depicts an ANR system
100
having a microphone
110
, a filter
120
and a speaker
130
.
In referring to
FIG. 1
, the combination of the microphone
110
, filter
120
and speaker
130
form a transfer function G(f)=Output(f)/Input(f). This creates a closed-loop control system that reduces ambient noise around the microphone according to the function 1/(1−G(f)).
ANR can be used in a variety of applications. For example, an ANR system may be placed near the muffler of a motor vehicle to reduce vehicle noise emissions. Also, an ANR system can be incorporated in a headset. Such an ANR headset can be worn by construction workers to protect their hearing. Similarly, the ANR headset can be worn by airplane pilots whose ability to hear may suffer from engine noise.
FIG. 2
illustrates one embodiment of an ANR headset
200
worn by a user
201
. The ANR headset
200
includes two cups
240
, each of which fits over an ear
202
of the user
201
. Each cup
240
is enclosed by a cup wall
235
. The cup
240
is sealed about the ear
202
by a cushion
205
to diminish undesired noise from reaching the user's ear
202
, and to provide the user
201
with a comfortable fit.
The cup
240
also includes a speaker
220
. The speaker
220
broadcasts the out-of-phase audio signal. The speaker
220
also defines front and rear cavities,
245
and
250
respectively, in the cup
240
.
A microphone
210
is inserted in the front cavity
245
proximate to the user's ear
202
. The microphone
210
receives the audible noise. The microphone
210
is coupled through a filter
225
to the speaker
220
. Optionally, for ANR headsets
200
worn by users that must receive audio communication signals, a signal summer
215
is inserted between the microphone
210
and filter
225
. The signal summer
215
is connected to an audio output
230
that permits the user
201
to listen to desired audio signals while reducing undesired ambient noise. For example, this technique permits an airplane pilot to listen to radio communications even when ambient noise is being suppressed by the ANR system. The filter
225
and summer
215
can be incorporated in the ANR headset
200
, such as in the cups
240
, or they may be positioned externally with respect to the cups
240
.
The electroacoustic combination of each cup's speaker, and front and rear cavities create relatively high Q resonances in the audio frequency response of the speaker. The resonances' amplitudes and frequencies can readily change as a result of variations in cup and speaker construction. Further, the resonances' amplitudes and frequencies can also readily change as a result of variations in cavity dimensions which may result from varying headset positions on different users, and varying shapes of users' heads and ears.
A speaker
220
, and its resonances, can be modeled by a lumped equivalent circuit, as illustrated in
FIG. 3. R
E
represents the resistance of the wire coil of the speaker. A represents the area of the speaker's diaphragm. M
M
represents the moving mass of the speaker. R
M
represents the speaker's mechanical damping associated with suspension of the wire coil. C
M
represents the speaker's compliance associated with suspension of the diaphragm. Z
C
is the acoustic impedance that terminates the speaker's diaphragm. Finally, Z
LOAD
is the input impedance seen across the speaker input terminals.
To permit relatively uniform ANR across the audible frequency range, the high Q response of the speaker is equalized, or diminished. To this end, an equalization filter is included in the filter
225
of the ANR system, described above. The equalization filter typically must cancel complex pole-zero pairs because of the cup's high Q frequency response. Because of the cup's high Q frequency response, the equalization is sensitive to, and can be diminished by, minor variations in operating parameters, such as headset fit on a user and component variations. To diminish the relatively high Q response of the cup, fabric is often placed over vents in the back of the speakers. The fabric dampens the frequency response of the speakers, thus reducing the Qs of the resonances. However, as a result, the fabric also undesirably diminishes the efficiency of the speakers, and provides variable changes in performance.
Further, such an equalization filter is relatively costly because of the number of required parts necessary to cancel the complex pole-zero pairs. One embodiment of an ANR filter
225
incorporating an equalization filter
410
and a noise reduction filter
420
is illustrated in FIG.
4
.
The ANR filter
225
provides the correct open-loop response for G(f) so that the closed-loop response of the ANR headset
200
provides high gain (i.e., high noise cancelation) and closed-loop stability.
It has been proposed by St{dot over (a)}hl in U.S. Pat. No. 4,118,600, issued Oct. 3, 1978, that the bass response of a loudspeaker can be improved by including a negative impedance in series with a plurality of impedances connected in parallel, such that the negative impedance (including negative resistance) is chosen to be substantially equal to the impedance of the voice-coil of the loudspeaker. St{dot over (a)}hl proposed that the plurality of parallel impedances have values which cause the loudspeaker to exhibit apparent mechanical parameters which are substantially different from the actual mechanical parameters in the bass response of the loudspeaker.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for ANR systems capable of diminishing the Q of the frequency response of the speaker, without reducing speaker efficiency.
SUMMARY
The present invention provides a method of enhancing automatic noise reduction in a headset speaker using a negative output resistance to substantially eliminate the coil resistance of the speaker. In one embodiment, the method includes generating a negative output resistance substantially equal in magnitude to the coil resistance of the speaker, and serially combining the negative output resistance with the coil resistance of the speaker. In another embodiment, generating a negative output resistance includes generating a negative output resistance using a negative output resistance amplifier. In a further embodiment, generating a negative output resistance includes generating a negative output resistance using a single-ended negative output resistance amplifier. In yet another embodiment, generating a negative output resistance includes generating a negative output resistance using a balanced negative output resistance amplifier.
The invention further provides a method of temperature compensating a system having a negative output resistance amplifier and a resistive load. In one embodiment, the method includes coupling a negative output resistance amplifier to the resistive load, and temperature compensating the negative output resistance amplifier so that a temperature coefficient of the negative output resistance is approximately equivalent to a temperature coefficient of the resistive load. In another embodiment, temperature compensating the negative output resistance amplifier includes implementing a resistor in the negative output resistance amplifier having a temperature coefficient substantially equivalent to the temperature coefficient of the resistive load, wherein the output resistance of the negative output
Graham Andrew
Schwegman Lundberg Woessner & Kluth P.A.
Williams Kimberly A.
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