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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention, generally, relates to a method and a system for
cancelling noise from noise-corrupted speech and, more particularly, to an
improved method and system for rendering speech recognizable in a high
noise environment, particularly where noise is distributed.
One glance into the cockpit of today's commercial airliner would give an
idea of the hands-busy, eyes-busy environment that exists there, and this
is more true of the cockpit in today's military aircraft. The military has
solved their problem somewhat by the use of voice-actuated controls for
many activities, such as located in the cockpit of a fighter aircraft, and
this has been accomplished through the use of voice recognition systems.
It was realized early that, due to the relatively high noise in the cockpit
of a fighter aircraft, some form of noise cancellation was required, and
from that need, an adaptive filter noise cancellation technique was
developed that has become a standard in the industry. More recently, that
technique was tried in military helicopters, and it was found to be
ineffective.
2. Description of the Prior Art
It is understandable that the presence of high levels of noise in an audio
signal will produce a substantial reduction in the intelligibility of
speech, and it has been found that the most advanced voice recognition
equipment is seriously ineffective in recognizing the simplest words in
the high noise levels encountered in the cockpit of today's tactical
fighter aircraft. A technique that was proposed by Bernard Widrow et al.
in 1975, known as Adaptive Noise Cancellation (or ANC), has been tested
extensively at the Research Laboratory of Electronics at the Massachusetts
Institute of Technology.
The Widrow technique is described in an article that is entitled "Adaptive
Noise Cancelling: Principles and Applications", Proc. IEEE, Vol. 63, No.
12, December, 1975.
During the M.I.T. tests, some improvements were developed in the Widrow
technique, such as placements for the two microphones in a fighter cockpit
environment as being one inside the oxygen facemask of the pilot and the
second microphone outside the facemask. The one microphone, called the
"primary" microphone, is located to sense, or to detect, the voice of the
pilot plus the noise.
The second, or "reference", microphone is located to sense, or detect,
principally the noise. By locating the reference microphone outside the
oxygen facemask, very little of the pilot's voice is picked up.
The engineers at M.I.T. learned also that it is better to have the
signal-to-noise ratio of the primary microphone large compared to the
signal-to-noise ratio of the reference microphone, so that the adaptive
filter can be kept as small as possible. Otherwise, the adaptive filter
must either estimate the delay between the primary and reference signals
or have a long impulse response in order to provide good cancellation of
the noise from the primary signal.
A report of the M.I.T. engineers is given in a paper entitled "Adaptive
Noise Cancellation in a Fighter Cockpit Environment" by Harrison, Lim and
Singer, 1984 IEEE, pages 18A.4.1 through 18A.4.4.
With all of the expertise of these M.I.T. engineers, the conclusion was
that the Adaptive Noise Cancellation technique of Widrow, while effective
enough in an environment with a localized noise source, degrades in
performance when there is more than one noise source present or when the
noise source is distributed over a region. Actually, the many sources of
noise in a helicopter make the Adaptive Noise Cancellation technique
virtually ineffective in that high noise environment where the noise
sources are distributed over a wide region. While those experts in the
field departed to study the use of additional reference microphones in a
distributed noise environment, the present invention proceeds with the
development of a unique solution to this perplexing problem.
A review of the prior patent art reveals very little to assist in
developing a solution such as provided by the present invention. For
example, U.S. Pat. No. 4,625,083 to Poikela is concerned with providing a
voice operated switch that is capable of distinguishing between voice and
noise. By using one microphone primarily for speech and one microphone
primarily for ambient noise signals, each of these groups of signals have
a certain sound pressure level, and since it is desired to have the sound
pressure level of the speech signal always exceed that of the noise
signal, this is accomplished in two ways. One way is by placing the two
microphones in predetermined locations so that the sound pressure level
distinctions are realized, and another way is by limiting the width of the
frequencies, like that customarily used in telephone receivers. A typical
frequency range is 100 hertz to 4 kilohertz, but a narrower frequency
range of 250 hertz to 3.5 kilohertz is termed as being satisfactory. By
connecting both signals to a differential amplifier, an output will result
when there is speech, and there is no output when there is no speech.
U.S. Pat. No. 4,649,505 to Zinser, Jr. et al. is an example of another
attempt to improve on the basic adaptive filter of Widrow, identified
supra, but this effort is for the purpose of eliminating crosstalk between
speech and noise signals. It discloses the use of a speech input, a noise
input and a reference input with a reference noise portion and a crosstalk
speech portion to a digital signal processing microcontroller, a
read-only-memory and a random access memory, from which the signals are
processed digitally. After the inputs are converted first from analog to
digital signals, they are converted next from digital serial signals to
digital parallel signals for further processing. There is no mention of
the problem with which the present invention is concerned.
U.S. Pat. No. 4,658,426 to Chabries et al. discloses several different
forms of noise suppression devices for use where the signal-to-noise ratio
is poor at the input and where the characteristics of the adaptive filter
adjust automatically to variations in the input signal. These adjustments
utilize time and frequency domains in making the adaptive filter
adjustments in order to filter noise, and a mathematical description is
given in substantial detail for devices constructed to take advantage of
such premises. A use for such devices is given as one tuned to filter out
the normal operating sound of machinery as "noise" and to detect the
unusual sound of a worn or failed component of the machinery. However,
these are illustrations of localized noise, with which the adaptive filter
type of device is capable of functioning quite adequately, according to
the M.I.T. reference, supra.
U.S. Pat. No. 4,672,674 to Clough et al. discloses a system utilizing two
specially built microphones that have good near field response and poor
far field response to produce signals with noise components having high
correlation. Like the Poikela U.S. Pat. No. 4,625,083 above, the outputs
from these microphones are connected to a filter to remove frequencies
outside the range of 300 Hz to between 5 and 8 kHz. The signals then pass
to analog-to-digital converters, to micro-processor circuitry having delay
and other capability, to achieve weighted-factor-samples for further
processing. While this prior patent discloses the use of two microphones,
it also suggests that a logical extension of this use is to use three or
more microphones, one for speech and the outputs of the other microphones
being used to cancel the noise in the signal from the one microphone.
On the other hand, the present invention takes a different approach to
providing a solution to the problem of cancelling distributed noise from a
speech signal, because tests show that the Adaptive Noise Cancellation
technique of the prior art degrades in performance when the noise is
distributed over a region.
OBJECTS AND SUMMARY OF THE INVENTION
It is a principal object of the present invention to provide a system for
cancelling distributed noise from a signal that contains noise-degraded
speech.
An important object of the invention is to provide a method for cancelling
distributed noise from a voice signal.
Another object of the present invention is to provide a new and improved
method and means for cancelling distributed noise from a voice signal.
Yet another object of the invention is to provide a noise cancellation
method and system that is effective in a high distributed noise
environment.
Still another object of the invention is to provide an effective noise
cancellation method and system for use with a speech (or voice)
recognition system.
A further object of the present invention is to provide a noise
cancellation method and system that will function effectively with
standard speech (or voice) sensing pickups.
A still further object of the present invention is to provide a noise
cancellation method and system that will function effectively with a
standard speech (or voice) recognition system in a helicopter environment.
Briefly, a method and system that is constructed and arranged in accordance
with the present invention includes two sensors, or microphones, located
so that a first sensor will detect both voice and noise and a second
sensor will detect principally only the noise. The voice picked up at the
second sensor is negligible, and the noise that is picked up at both
sensors is correlated. The signal output from each sensor is connected to
means to divide each respective signal output into a predetermined number
of frequencies. Then, both signal outputs are connected to a circuit to
cancel effectively the noise component from the signal output with both
voice and noise.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described with reference to the accompanying
drawings, in which:
FIG. 1 is an illustration of a conventional noise cancellation circuit that
has become an industry standard.
FIG. 2 is an illustration of a noise cancellation system that embodies the
features of the invention.
FIG. 3 is a curve for use in describing the operation of the system of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 of the drawings, the conventional, or "standard", noise
cancellation technique is illustrated in the form it was introduced first
by Bernard Widrow et al. in 1975, and is identified generally by the
reference numeral 10. As a system, this technique is considered usually as
the input for a voice recognition system. Noise cancellation is performed
in a substract circuit 11 between one signal received directly from one
microphone 12 and the output from a second microphone 13 after it is
passed through an adaptive filter 14. The output from the substract
circuit 11 is connected directly to a voice recognition system 15.
The outputs from the two microphones 12 and 13 cover the entire audible
voice frequency range; for example, from 100 to 3,200 Hz. The single
adaptive filter 14 in this standard technique, therefore, must be capable
of performing effectively over the entire audible voice frequency range.
The adaptive filter 14 in the conventional technique must provide
compensating amplitude and phase capabilities that vary greatly from one
end of the voice frequency range to the other end. In addition, such an
adaptive filter 14 would require a large number of adjustable elements;
for example, 100 tap coefficient adjustments, or just "taps", all of which
leads to problems, such as:
(1) The adjustment of a large number of control elements (using the
conventional gradient method, or the like) is a very slow process.
(2) Efforts to speed up the process of working with a large number of
control elements can produce other problems, such as numerical instability
due to truncation errors, rounding errors, statistical averaging errors,
etc.
Noise that is detected by the microphones 12 and 13 from a single,
localized source will be the same "noise" at each microphone; that is, it
will be the same frequency or frequencies, but it will be displaced in
time due to differencies in length of the paths it must travel. This is
the meaning of the term "correlated" as applied to the two noise
frequencies.
It is an important function that is performed by the adaptive filter 14,
therefore, when it compensates for the differences in time between the two
noise frequencies. It is this compensation between the two signals that
results in an effective cancellation when they are combined in the
substract circuit 11.
When the circuit illustrated in FIG. 1, therefore, was tried with noise
that was distributed over a region, it was immediately apparent that its
performance was degraded seriously relative to its performance with a
single localized noise source. Although much effort has been devoted to
solving this problem in recent years, none has been effective until the
present invention.
In FIG. 2 of the drawings there is illustrated a circuit arrangement to
solve the problem of effectively cancelling the noise from voice, or
similar information signals, sufficiently for a voice recognition system
to be useful reliably. A noise cancellation system in accordance with the
invention is sufficiently effective to be useful in every known
environment where noise-degraded speech renders a voice recognition system
ineffective; such as, for example, in a factory, on a manufacturing floor,
in large office areas, at airports, etc., etc.
Referring now to FIG. 2, a system that is constructed and arranged in
accordance with the principles of the invention is identified generally by
the reference numeral 16. Two standard sensors 17 and 18, that are readily
available commercially, such as, for example, microphones, are located so
that the sensor 17 detects both voice and noise. It is contemplated that
the sensor 17 will be located so that it will detect as much voice as
possible, even though that signal is degraded by noise.
The sensor 18, however, is located so that it will detect principally noise
and very little of the voice. When used in a pilot's environment, the
sensor 17 is located inside of the pilot's oxygen facemask and the sensor
18 is located outside the oxygen facemask. In other environments, where a
wire-like headset is used, the sensor 17 is located close to the mouth of
a speaker, and the sensor 18 is located also on the headset but as far as
possible from the mouth of the speaker and is pointed in such a way that
it detects principally noise. It is important to note, however, that the
distance between the two sensors 17 and 18 is quite small, a matter of
inches, so that the two sensors pick up effectively the same noise but
displaced relative to each other a small amount.
The signals detected by each of the sensors 17 and 18 are connected to a
suitable device to divide them into a number of frequencies. For example,
each signal is divided into a predetermined number of frequency signals
having limited bandwidths, and in FIG. 2, the number that is illustrated
is 15.
In FIG. 2, the signal output from each of the sensors 17 and 18 is
connected to 15 respective narrowband filters. It is important that the
same number used for one sensor be used for the other. The narrowband
filters that are connected to receive the signal output from the sensor 17
are in a group that is identified generally by the reference numeral 19,
and the narrowband filters that are connected to receive the signal output
from the sensor 18 are in a group that is identified generally by the
reference numeral 20.
Since the usual frequency range for the voice signals spans approximately 3
kHz (or 3000 Hertz), by dividing this range into 15 different bandwidths,
each one of the narrowband filters in the two groups 19 and 20 will be
approximately 200 Hertz wide in this example. In tests that have been made
on this technique, the voice frequency has been divided into as many as 25
different narrowband frequencies with exceptional results, a good range
for the number of narrowband filters being about 10 to about 25. This
range covers most instances of their use.
Any particular number of narrowband filters 19 and 20 may be used, or to be
more accurate, the signal output from each sensor 17 and 18 can be divided
into any number of signals. It is important, however, that the number of
the divisions be the same for the signals from the two sensors 17 and 18,
because one of these group of divided signals is subtracted from the other
to provide a substantially noise-free voice signal.
Each of the narrowband filters in the group 20 is connected to an adaptive
filter in a group that is identified by the reference numeral 21. Each of
the adaptive filters in the group 21 functions to compensate for the
amplitude and phase differences in the signal detected by the sensor 18.
By this means, when each of the divided signals is combined in each
circuit in a group that is identified by the reference numeral 22, the
noise signal from the sensor 18 is subtracted from the voice-plus-noise
signal from the sensor 17 to provide the substantially noise-free voice
signal.
While each circuit in the group 22 is indicated as being a "subtract"
circuit, it will be apparent to one skilled in the art that other
procedures are available for obtaining a "difference" action, such as, the
signals from the adaptive filters 21 can readily be inverted and then
"added" to the voice-plus-noise signal from the narrowband filters 19.
Other ways of obtaining a difference action also will give a similar
result.
The output from each of the individual subtract circuits in the group 22,
as illustrated in FIG. 2 of the drawings, is connected to a voice
recognition system 23. With a system 16 constructed and arranged in
accordance with the present invention, the voice recognition system 23 has
no difficulty responding to spoken commands in noisy environments and even
with noises that are distributed over a wide region.
FIG. 3 of the drawings illustrates a waveform to show this division of the
signal from either sensor 17 or 18 into individual component frequencies.
For example, the entire curve in FIG. 3 can be an illustration of the
output signal from either one of the sensors 17 or 18. The number "1",
identified also by the reference numeral 24, is illustrative of a signal
that is divided by the narrowband filter in either group 19 or 20.
Similarly, the reference numeral 25 in FIG. 3 identifies the number "2"
that corresponds to the narrowband filter "2" in either the group 19 or
20, in FIG. 2, and the reference numeral 26 identifies the number "15"
that corresponds to the narrowband filter "15" shown in either group 19 or
20, also in FIG. 2. Therefore, in accordance with the present invention,
the noise cancellation system 16, FIG. 2, divides the total signal that is
detected by each of the sensors 17 and 18 into a plurality of narrow band
frequencies each of which covers only a small fraction of the total signal
frequency.
Of course, this dividing of the total signal into a plurality of smaller
frequencies may be accomplished through a variety of hardware component
parts. For example, it is always acceptable to use a plurality of
individual narrowband filters, but the presently preferred way the
division is accomplished is by means of a computer, because a computer
permits the number of the divided frequencies to be changed readily and
quickly.
Tests that have been performed on the invention show that it is possible to
obtain a substantially noise-free signal by dividing the total signal into
a predetermined number of individual frequencies before the cancellation
is attempted. By dividing the noise signal into a plurality of narrow
bands, then there is less noise in each narrow band. Now, it has been
discovered that it is much easier to cancel the noise by this division
technique.
A system arranged in accordance with the invention has the following unique
advantage. Since each individual adaptive filter in the group 21, FIG. 2,
must compensate for only the frequency in its own narrow band, each of the
adaptive filters in the group 21 of the invention needs only a small
number of adjustable elements; such as, 4 tap coefficients, for example.
Now, it will be more readily apparent that such an adaptive filter as
needed in a system of the invention can be adjusted easily, rapidly and
much more accurately.
The system of the present invention, therefore, offers a solution to a
problem that has been heretofore impossible technically. Moreover,
published statements by researchers in this field indicate that they are
considering other and materially different arrangements to solve the
problem of cancelling noise from distributed sources.
Having described the invention completely with reference to the presently
preferred embodiment, it will be apparent to those skilled in this art
that modifications and changes can be made, but it is understood that all
such modifications and changes that come within the spirit and scope of
the claims appended hereto are within the present invention.
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