Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation
Reexamination Certificate
1999-06-16
2003-10-28
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
C381S094700
Reexamination Certificate
active
06639986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a built-in microphone device for reducing the influence of internal noise of an apparatus in which the built-in microphone device is incorporated.
2. Description of the Related Art
In an audio visual apparatus, such as a video camera, having a built-in main microphone for picking up a sound, internal noise generated by a mechanism section is undesirably received by the main microphone. In order to reduce the influence of such internal noise, a built-in microphone device has been developed. A built-in microphone device includes a noise reference microphone provided in a housing of the apparatus. An internal noise signal which is output from the noise reference microphone is given to an adaptive filter, and the adaptive filter generates a control audio signal. The control audio signal is subtracted from the output signal from the main microphone. Thus, the internal noise is cancelled.
A conventional built-in microphone device operating in this manner will be described with reference to
FIGS. 9 and 10
.
FIG. 9
is a block diagram of a conventional built-in microphone device, and
FIG. 10
is a schematic isometric view of the conventional built-in microphone device shown in FIG.
9
and an audio visual apparatus, such as a video camera, in which the built-in microphone device is incorporated.
FIG. 10
illustrates the positional relationship between a main microphone
1001
and a noise reference microphone
1005
of the conventional built-in microphone device.
In
FIGS. 9 and 10
, the main microphone
1001
is provided for picking up an external sound for recording and is provided on an outer surface of a wall of a housing
1010
of the audio visual apparatus. The housing
1010
accommodates a magnetic recording and reproduction section including a tape transfer mechanism and a rotary head. The magnetic recording and reproduction section generates internal noise and is referred to as a mechanism section
1020
. The noise reference microphone
1005
is provided in the housing
1010
and is directed toward the mechanism section
1020
. The noise reference microphone
1005
picks up internal noise such as sound noise caused by vibration mainly generated from the mechanism section
1020
.
An adaptive filter
1030
shown in
FIG. 9
identifies a transfer characteristic of internal noise transferred from the noise reference microphone
1005
to the main microphone L
001
. The adaptive filter
1080
also receives an internal noise signal from the noise reference microphone
1005
and generates a control audio signal based on the internal noise signal. A signal subtraction section
1040
subtracts the control audio signal generated by the adaptive filter
1030
from the output signal from the main microphone
1001
. Thus, an audio signal having a reduced internal noise component is output.
The conventional built-in microphone device having ouch a structure operates as follows. The main microphone
1001
, which is provided on the wall of the housing
1010
. efficiently picks up external sound around the apparatus. Since the mechanism section
1020
operates at this point, internal noise, which should not be picked up, is generated. The internal noise is received by the main microphone
1001
through the housing
1010
, as a result of which the signal-to-noise ratio of the sound picked up by the main microphone
1001
is lowered.
The noise reference microphone
1005
captures the internal noise generated by the mechanism section
1020
. The adaptive filter
1020
estimates a signal identical with an internal noise signal received by the main microphone
1001
based an the internal noise signal output from the noise reference microphone
1005
, and outputs the estimated signal as a control audio signal. The signal subtraction section
1040
subtracts the control audio signal from the output signal from the main microphone
1001
, thus removing the internal noise component from the output signal. As a result, an audio signal having a reduced internal noise component is obtained. As an adaptive algorithm used by the adaptive filter
1030
, a well known LMS (least means square) algorithm or the like is used.
However, the conventional built-in microphone device having the above-described structure has a problem in that a filter coefficient of the adaptive filter
1030
often is not updated optimally in practical use. For example, the filter coefficient is not converged in the condition of canceling the internal noise, resulting in time-consuming filter coefficient learning. In so-no cases, the filter coefficient is diverged, and thus the internal noise is not sufficiently cancelled.
When one internal noise source is not specified, i.e., when a plurality of internal noise sources are present, there are a plurality of transfer characteristics from the plurality of internal noise sources to the noise reference microphone
1005
and also a plurality of transfer characteristics from the plurality of internal noise sources to the main microphone
1001
. Accordingly, the effect of suppressing the internal noise is difficult to obtain.
The conventional built-in microphone device has another problem in that, when the noise reference microphone
1005
picks up the external sound, the built-in microphone device adds an echo to the audio signal. This deteriorates the sound quality. These problems will be described in detail.
(1) When internal noise from the mechanism section
1020
has a sufficiently high sound pressure level, the adaptive filter
1030
accurately estimates (i.e., learns) the transfer characteristic from the noise reference microphone
1005
to the main microphone
1001
. However, when the filter coefficient of the adaptive filter
1030
is updated in the, state where the level of the internal noise from the mechanism section
1020
is lower than the level of the external sound or where the operation of the mechanism section
1020
is in pause (i.e., where the level of the internal noise signal from the noise reference microphone
1005
is significantly lower than the level of the output signal from the main microphone
1001
), the filter coefficient diverges from a desired characteristic. As a result, the internal noise cannot be cancelled.
(2) In the case where the mechanism section
1020
generating the internal noise operates Intermittently, for example, in the case where recording of video and audio data is started and paused repeatedly in a video camera, an internal noise signal required for learning is not obtained while the apparatus is in a pause. Accordingly, it is difficult to cancel the internal noise from the start of recording of video and audio data.
(3) In the conventional structure, the filter coefficient of the adaptive filter
1030
is converged so as to reproduce the transfer characteristic of the internal noise from the noise reference microphone
1005
to the main microphone
1001
. As a result, the internal noise is cancelled. However, when either one or both of the main microphone
1001
and the noise reference microphone
1005
are vibrated, such a vibration acts as a signal disturbing the convergence of the filter coefficient. Then, the filter coefficient of the adaptive filter
1030
does not converge so as to cancel the internal noise. Accordingly, the internal noise is not cancelled.
(4) When internal noise is generated by one mechanism section
1020
, the adaptive filter
1030
normally performs the learning operation. However, when there are a plurality of internal noise sources, for example, when the video camera generates a noise of the rotary head and noise created when the lens is zoomed, the following problem occurs. In the case where the noise reference microphone
1005
is located in the vicinity of either one of the internal noise sources, the noise reference microphone
1005
cannot capture the internal noise from the other internal noise source or sources. Even when the noise reference microphone
1005
is located at an equal distance from the plurality of inte
Ibaraki Satoru
Kanamori Takeo
Kawamura Takashi
Isen Forester W.
Matsushita Electric - Industrial Co., Ltd.
Pendleton Brian
RatnerPrestia
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