Electrical audio signal processing systems and devices – Directive circuits for microphones
Reexamination Certificate
2003-04-24
2004-06-29
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Directive circuits for microphones
C381S111000, C381S091000
Reexamination Certificate
active
06757394
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microphone array for detecting the direction and the position of a sound source, enhancing a desired signal and suppressing noise by performing signal processing based on signals inputted from arrayed microphones.
2. Description of the Related Art
A microphone array includes a plurality of real microphones connected in an array and processes signals received by the real microphones so that directivity can be provided.
In a microphone array, an SN(signal-to-noise) ratio can be improved by two approaches, namely, enhancement of a desired signal coming from a look direction and suppression of unnecessary noise. A conventional microphone array according to each approach will be described below.
FIG. 25
is a view showing an example of the structure of a conventional microphone array, which is a so-called delay-and-sum array. The delay-and-sum array shown in
FIG. 25
includes a plurality of real microphones
2501
, a plurality of delay units
2502
corresponding to the respective real microphones and an adder
2503
.
The delay-and-sum array enhances a desired signal coming from a look direction by utilizing a time lag generated when a sound wave coming from the look direction reaches the plurality of real microphones.
FIG. 26
is a view illustrating enhancement of a desired signal in the delay-and-sum array. In
FIG. 26
, a sound wave that can be approximated to a plane wave is received at two microphones
2601
and
2602
in a free space. In
FIG. 26
, a bold arrow denotes a propagation direction of the sound wave, and a broken line denotes a wavefront. The two real microphones
2601
and
2602
are separated by a distance d.
It is assumed that a sound wave comes from a look direction &thgr;, and that the signal received at the real microphone
2602
is delayed against the signal received at the real microphone
2601
by a time lag &tgr; during which the sound wave travels a distance &xgr;. This can be expressed by the following equations:
x
2
(
t
)=
x
1
(
t−&tgr;
)
&tgr;=&xgr;/
c=d
·(sin &thgr;)/
c,
where c represents the velocity of sound. When the signal received at the real microphone
2601
is delayed for a delay period &tgr;, the two received signals that were previously separated by a time lag become in-phase on the time axis. On the other hand, sound waves coming from directions other than the look direction are received at the real microphones with time lags different from the time lag &tgr;, so that the signals are not processed to be in-phase by this delay operation. In other words, the above-described delay operation makes it possible to enhance the desired signal coming from the look direction.
The delay-and-sum array shown in
FIG. 25
processes an input signal from each real microphone
2501
to be in-phase with the delay unit
2502
, and then the signals are added by the adder
2503
, so that the desired signal coming from the look direction can be enhanced.
Next, a conventional microphone array according to the approach of noise suppression will be described.
FIG. 27
shows an example of the structure of a microphone array that suppresses noise. The microphone array shown in
FIG. 27
is called a subtraction type array. The subtraction type array shown in
FIG. 27
includes two real microphones
2701
and
2702
, a delay unit
2703
, a subtracter
2704
, and a desired signal correction filter
2705
.
In the subtraction type array, when noise coming only from a direction &thgr; are received at the two microphones
2701
and
2702
, the relationship expressed by the equation: x
2
(t)=x
1
(t−&tgr;) is satisfied. In this case, x
1
(t) is delayed by time &tgr; so as to process noise components included in the two received signals to be in-phase as in the case of the delay-and-sum array. Then, the noise that is in-phase is subtracted so that those noise components can be erased.
However, the direction &thgr; of the noise is unknown in many cases. Therefore, the value of &tgr; is unknown. Then, as shown in
FIG. 27
, information about an output e(t) from the subtracter
2704
is fed back to the delay unit
2703
so that an amount of delay is adjusted to minimize the power of the output e(t).
If the received signals consist only of noise coming from the direction &thgr;, e(t) becomes zero, which is the minimum, when the amount of delay becomes &tgr;. According to this approach, even if a value of &thgr; is unknown, noise can be erased by a subtraction process.
On the other hand, if a desired signal comes from a direction other than the direction &thgr;, the desired signals are not processed to be in-phase by the above-described operation. Therefore, the signals of the desired signal cannot be erased by subtraction. The frequency components of the signals of the desired signal, however, are changed by subtraction. Therefore, as shown in
FIG. 27
, a desired signal correction filter
2705
is provided to correct this change.
When noise comes from a small number of directions, the subtraction type array can provide an effective improvement in the SN ratio, even if the subtraction type array is small.
However, when using the delay-and-sum array or the subtraction type array, it is necessary to increase the number of real microphones in order to improve the enhancement of a desired signal, the suppression of noise and the performance for detecting the position of the sound source, thus causing the problem of upsizing the array.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a compact and high-performance microphone array with a small number of real microphones that can provide substantially the same quality as a microphones array with a large number of real microphones.
In order to achieve the object, a microphone array of the present invention comprises a plurality of real microphones arranged in predetermined positions, at least one virtual microphone, and a sound signal estimator for estimating a sound signal received by the virtual microphone. The sound signal estimator comprises a sound signal divider for dividing, based on sound signals received by the plurality of real microphones, a sound signal received by a predetermined real microphone into components, each component corresponding to one coordinate axis direction in a coordinate system that is defined on the basis of positions of the plurality of real microphones, a sound signal component estimator for estimating a virtual microphone sound signal component corresponding to a predetermined coordinate axis direction in the coordinate system, based on the sound signal received by the predetermined real microphone and the sound signal component corresponding to the predetermined coordinate axis direction divided by the sound signal divider; and a sound signal component adder for adding the sound signal component corresponding to the coordinate axis direction divided by the sound signal divider and the sound signal component, each component corresponding to one coordinate axis direction estimated by the sound signal component estimator.
In one embodiment of the present invention, the microphone array further comprises at least one delay element for performing delay processing to each sound signal so that sound signals received by the plurality of real microphones and sound signals estimated by the sound signal estimator are in-phase; and an adder for adding signals that have been processed by the delay elements. This embodiment makes it possible to enhance a desired signal by using the estimated sound signal. Furthermore, by subtracting the signal that has been processed in the delay element, it is possible to suppress noises by using the estimated signal.
In another embodiment of the present invention, the microphone array further comprises a correlation coefficient calculator for calculating correlation coefficients based on sound signals received by the predetermined real microphone and a sound signal estimated by the s
Armstrong, Quintos, Kratz, Hanson & Brooks, LLP
Grier Laura A.
Harvey Minsun Oh
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