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Description  |
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FIELD OF THE INVENTION
This invention concerns a method and a device for attenuating wideband
spurious noise and specifically a device to improve the signal-to-noise
ratio obtained from a signal sensor disturbed by spurious noises.
BACKGROUND OF THE INVENTION
Attenuation of noises disturbing tracked signals is typically performed in
the prior art by using self-adaptive transverse filters which are
effective as long as there is only a single noise source and the noise is
carried by a plane wave. The noise components are estimated with the help
of a transverse filter and are subtracted from the signal supplied by the
disturbed sensor. The tracked signal, as corrected by subtraction, is
continually put to use in the filter to adjust the various filter
coefficients so as to obtain an optimum attenuation. In practice, the
tracked signals received by a sensor, such as a microphone or a
hydrophone, are usually drowned in noises comprising spectrum lines
originating from different sources and the noises are picked up after
having travelled along a variety of unidentified acoustic paths.
In these conditions, the results obtained are thus far inferior to those
obtained when the above-mentioned conditions are observed.
Other relevant prior art, described in U.S. Pat. No. 4,232,381, provides
for attenuation of spurious noises from the engine of a vehicle equipped
with an acoustic signal receiver. The device comprises a rotation sensor
which generates pulses for use by a synchronous filter which selects the
fundamental and harmonic frequencies of the engine noise for the purpose
of generating a correcting signal enabling static removal from the signal
received by the acoustic signal receiver.
It is the object of the present invention to provide a method and two
devices for attenuating wideband noises disturbing a tracked acoustic
signal, which are both simpler and more efficient than the above-mentioned
devices.
SUMMARY OF THE INVENTION
The method according to the invention makes use of two sensors: a first,
acoustic type sensor picking up the tracked acoustic signal and a second
sensor designed to supply a noise signal alone. The signal from the second
sensor is filtered, then subtracted from the signal from the first sensor
as in the prior art.
According to one feature of the invention, the filtered signal obtained
from the second sensor--the sensor which is insensitive to the tracked
acoustic signal due to its location or its nature--is the sum of a
plurality of correction signals each resulting from the correlation of the
signal from the first sensor with a different spectrum-line energy density
signal obtained by narrow, spectral-line filtering of the signal output
from the second sensor, the number of correction signals being selected to
equal the number of spectral lines in which disturbing noise components
occur.
By application of the method defined above, the present application also
proposes a device for attenuating wideband noises disturbing an acoustic
signal received by a first, acoustic-type sensor, wherein said attenuation
is obtained by subtracting from the signal sent by the first sensor a
correction signal obtained from a second sensor designed to deliver a
signal corresponding to the noise alone.
In one embodiment of the invention, said attenuation device comprises:
means of filtering the signal from the second sensor on a spectral line
basis in order to supply an energy density signal for each spectral line
where a disturbing noise component is present;
means of correlating each of the energy density signals supplied by the
filtering means, following its dephasing, with the signal sent by the
first sensor, in order to provide as many correction signals as there are
energy density signals being simultaneously provided;
means of adding together the simultaneously obtained correction signals;
and means of subtracting from the signal sent by the first sensor the
signal obtained from the adding means, in order to provide an electrical
image of the noise-free acoustic intelligence signal.
In another embodiment of the invention, said attenuation device comprises:
means of converting into digital form the output signals from both sensors;
means for computing the fast Fourier transforms of the output signals from
both sensors to yield a sum signal of the correlated corrections, said
corrections being being equal in number to the number of spectral lines
requiring attenuation, to subtract the sum signal from the product of the
fast Fourier transform of the output signal from the first sensor and to
derive an electrical image of the acoustic intelligence signal by inverse
fast Fourier transforming the signal obtained by subtraction.
The features and advantages of the invention are described in greater
detail below with reference to the figures listed below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the attenuation device according to the
invention.
FIG. 2 is a block diagram of the attenuation device according to the
invention, in digital form.
FIG. 3 is a diagram of a correlation module for the device according to the
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
The wideband spurious noise attenuation device according to the invention
is essentially designed to improve the signal-to-noise ratio obtained for
a signal supplied by an acoustic sensor 1, such as a microphone or a
hydrophone, in the case where the intelligence or tracked signal picked up
by said sensor is drowned in spurious noise.
It is indeed typically not practicable to remove the noise sources
disturbing reception of an acoustic signal, as in the case, for example,
of hydrophone use aboard a moving ship, especially when the signal to be
picked up is of the same nature as the noise.
In fact, although the use of a directional and/or narrow field microphone
or hydrophone makes it possible to favour one source in relation to other
sources located outside the preferential field or out of range, there are
many cases where the disturbance sources are nearer and more powerful than
the wanted signal source and where the acoustic paths followed by the
noise waves lead them into the sensor's preferential field.
It is generally possible however to find zones wherein spurious noises are
present while the tracked signal can no longer be picked up, making it
possible to dispose one or several sensors 2 therein to pick up these
noises alone. Specifically, the noise sources may be so powerful as to
make it possible to easily pick up the noise alone, or the type of sensor
2 can be selected such that it is not, as opposed to sensor 1, sensitive
to the tracked signal.
For example, it is possible in order to eliminate the spurious noises
interfering with hydrophonic listening aboard a ship, when said noises are
due to the ship itself, such as in the case of sound waves generated by
vibrations from the ship's machinery, to employ accelerometers mounted on
the machines or on the hull as noise sensors 2, instead of hydrophones.
Clearly, the number of sensors 1 and 2 used to pick up tracked and noise
signals may vary according to need. However, for purposes of simplifying
the description, only one sensor 1 of disturbed tracked signals and one
sensor 2 of spurious noise are shown in the drawings and taken into
account herein.
In accordance with the invention, the noise signal b'(t) generated by the
noise-only sensor 2 is used to correct the signal s(t)+b(t) generated by
the signal sensor 1, said signal being amplified as required by
conventional means.
The signal b'(t) from sensor 2, amplified as required, is applied to a
plurality of narrow-band filters 3a to 3n, called spectrum-line filters,
which are individually centered on different frequencies throughout the
spectrum corresponding to the intelligence signal to be extracted.
Thus, in an embodiment of the invention where the useful bandwidth is 100
to 1000 Hz, sixteen spectrum-line filters 3, adjustable in 1 Hz steps,
would be used.
Each spectrum-line filter 3 supplies a reference value b'(f) standing for
the spectral energy density of the noise picked up by sensor 2 in the
frequency under consideration, for example b'(fa) for the spectrum-line
filter 3a centered on frequency fa. Each filter 3 is connected to one
input of a dedicated correlation module 4a to 4n having a second input
connected to the output of sensor 1 wherefrom it receives signal
S(t)+b(t).
The correlation modules 4 may, for example, be of the type described in
same applicant's French Pat. No. 2215 005, in which case each correlation
module 4, such as 4a, comprises:
a conventional normalization circuit 48, normalizing the power of the
signal supplied by the associated spectrum-line filter 3;
a phase shifter 40 connected to the output of normalization circuit 48 with
which it is associated and from which it receives a signal b'(fa) and
shifts the phase thereof by a value of .pi./2;
two input multipliers 41, 42, one of which 41 has its inputs connected
respectively to sensor 1 and to the output of filter 3a, and the other of
which has its inputs connected respectively to sensor 1 and to the output
of phase shifter 40;
two integrators 43 and 44 respectively connected to the outputs of the
input multipliers 41 and 42;
two output multipliers 45 and 46, one of which 45 has its inputs connected
respectively to the outputs of integrator 43 and of filter 3a and the
other of which 46 has its inputs connected respectively to the outputs of
sensor 1 and of phase shifter 40;
an adder 47 whose inputs are connected to the outputs of output multipliers
45 and 46 such as to supply a signal b(fa) corresponding to the correction
to be applied to signal S(t)+b(t) in order to remove the noise component
of the frequency fa covered by filter 3a.
Unit 8 making up the attenuation device according to the invention and
therefore containing the filters 3 and the correlation modules 4 further
comprises an adder 5, input-connected to the outputs of the correlation
modules 4, adding up the correction signals b(fa), b(fb), . . .b(fn)
supplied by said modules into a correction signal synthesizing the
spurious noises affecting the tracked signal.
A subtractor circuit 6, fed by both sensor 1 and adder 5, subtracts the
correction signal from the signal S(t)+b(t) sent by sensor 1 and provides
an electrical signal equivalent of the noise-filtered tracked signal.
Clearly, in practice, the number of spectrum-line filters 3 will
necessarily be limited if the signals are not converted into digital form
and it is the adjustability of the filters which enables specific spectral
lines to be selected for correction, since the spurious noise is typically
not a white noise and only the energy of certain specific spectral lines
needs to be taken into account, the noise component of the other lines
being either negligible or null.
An attenuation device applying the method according to the invention can be
implemented to advantage, after conversion from analog to digital form of
the signals delivered by the sensors, using Fourier transforms or more
specifically, fast Fourier transforms (FFT).
In this way it is possible to obtain the equivalent of a large number of
spectrum-line filters distributed throughout the useful bandwidth, the
equivalent for example of one thousand spectrum-line filters, using a
thousand-point transform, which in the digital example suggested above
enables the correction of all the whole frequencies of the useful band.
As shown in FIG. 2, each sensor 1 and 2 is linked to a suitable
analog-to-digital converter 7 and each converter feeds the results of its
conversions to a unit 8' consisting of a computer capable of computing a
double fast Fourier transform (FFT1, FFT2) based upon the signals received
from each of the converters to yield an .epsilon.0 sum signal of the
correlated corrections carried out for each spectral line or transform
point.
An inverse transform FFT.sup.-1 of the yield of the subtraction, i.e.
transform FFT1 minus .epsilon. sum, then enables the tracked signal to be
reconstituted in digital form and, if need be, to be converted back to
analog form via a digital-to-analog converter.
Clearly, the processing possibilities afforded by the use of fast Fourier
transforms also enable a variety of sensors 1 and 2 to be more easily
combined as the need arises.
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