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Claims  |
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What is claimed is:
1. A device for detecting speech in an input signal comprising:
first determining means for determining a plurality of values
representative of a plurality of frequency band limited energy within the
signal, wherein the signal is sampled at a predetermined sampling rate in
a single frequency band over a first plurality of frames, wherein each
frame comprises a plurality of samples;
second determining means for receiving the plurality of values from said
first determining means, and determining a variance of the frequency band
limited energy of the signal the single frequency band over a second
plurality of frames; and
third determining means for determining beginning and ending points of
speech within the signal using the variance of the frequency band limited
energy, wherein the third determining means comprises:
fourth determining means for determining a beginning of speech as occurring
when the variance of the frequency band limited energy exceeds an upper
threshold level and for determining an ending of speech as occurring when
the variance of the frequency band limited energy falls below a lower
threshold level, the upper threshold level being greater than the lower
threshold level.
2. The device of claim 1, wherein the first determining means comprises:
fifth determining means for determining frequencies associated with the
signal;
selecting means for selecting portions of the signal having frequencies
within a preselected range; and
sixth determining means for determining a total energy within the selected
portions of the signal, the total energy being the frequency band limited
energy.
3. The device of claim 1, wherein the first determining means comprises:
first applying means for applying a Hamming window filter to a portion of
the signal to generate a filtered signal;
second applying means for applying a Fourier Transform to the filtered
signal to generate a transformed signal; and
summing means for summing the transformed signal to generate a value
representative of total energy in the portion of the signal, the value
representative of the total energy being the frequency band limited
energy.
4. The device of claim 1, wherein the device includes:
receiving means for receiving the signal;
storing means for storing a portion of the signal covering a continuous
period of m seconds where m is between 0.1 and 10 seconds; and
updating means for updating the stored portion of the signal as new signals
are received,
wherein the first determining means determines the value representative of
the frequency band limited energy from the stored portion of the signal,
the value representative of the frequency band limited energy being
updated as new signals are received.
5. The device of claim 4, wherein the storing means comprises a shift
register.
6. The device of claim 1, wherein the second determining means comprises:
first calculating means for calculating variance, V, wherein V is given by
V=g(A,B); where
##EQU4##
BLE(f) represents the plurality of values representative of the frequency
band limited energy, nv is a number of values of the frequency band
limited energy, f=nv, . . . , 3, 2, 1; and
BLE(1) is an oldest BLE value.
7. The device of claim 6, wherein the second determining means further
comprises:
second calculating means for calculating V=g(A', B') as new values of
BLE(nv) are received,
where
A'=A+[BLE(nv).times.BLE(nv)]-[BLE(0).times.BLE(0)];
B'=B+BLE(nv)-BLE(0);
where
A' is an updated value for A, and
B' is an updated value for B, and
BLE(nv) is a newest frequency band limited energy, and
BLE(1) is the oldest frequency band limited energy.
8. The device of claim 1, wherein upper and lower threshold levels are
predetermined, and wherein the beginning (B) of speech is determined at a
point in time when the variance initially exceeds the lower threshold
level, and the variance subsequently remains above the lower threshold
level until the variance also exceeds the upper threshold level.
9. The device of claim 1, wherein the upper and lower threshold levels are
predetermined, and wherein the ending of the signal is determined as a
point in time when the variance falls below the lower threshold level.
10. The device of claim 9, wherein the determination of the beginning and
ending points of the signal is rejected if the signal does not remain
above the upper threshold level for a predetermined time period.
11. The device of claim 10, wherein the predetermined time period is 0.3
seconds.
12. In a device for recognizing speech within an input signal, with the
device having means for receiving the signal, means for determining
beginning and ending points of speech within the signal, and means for
determining content of speech within the signal between the beginning and
ending points, an improvement to the means for determining the beginning
and ending points of the speech comprising:
first determining means for determining a plurality of values
representative of a plurality of frequency band limited energy within the
signal, wherein the signal is sampled at a predetermined sampling rate in
a single frequency band over a first plurality of frames, wherein each
frame comprises a plurality of samples;
second determining means for receiving the plurality of values from said
first determining means, and determining a variance of the frequency band
limited energy of the signal in the single frequency band over a second
plurality of frames; and
third determining means for determining beginning and ending points of
speech within the signal based on the variance of the frequency band
limited energy, wherein the third determining means comprises:
fourth determining means for determining a beginning of speech as occurring
when the variance of the frequency band limited energy exceeds an upper
threshold level and for determining an ending of speech as occurring when
the variance of the frequency band limited energy falls below a lower
threshold level, the upper threshold level being greater than the lower
threshold level.
13. A device for the detection of speech in an input signal, comprising:
first determining means for determining a variance of a frequency band
limited energy of the signal; and
speech interval decision means for deciding start and end points of speech
within the signal based on said variance, wherein said speech interval
decision means comprises:
second determining means for determining a beginning of speech as occurring
when the variance of the frequency band limited energy exceeds an upper
threshold level and for determining an ending of speech as occurring when
the variance of the frequency band limited energy falls below a lower
threshold level, the upper threshold level being greater than the lower
threshold level, wherein the first determining means for determining a
variance comprises:
third means for providing a plurality of determined values representative
of a plurality of frequency band limited energy at a predetermined
sampling rate in a single frequency band over a first plurality of frames,
wherein each frame comprises a plurality of samples; and
fourth means for calculating the variance from the plurality of determined
values provided from the third means in the single frequency band over a
second plurality of frames.
14. The device of claim 13, wherein the frequency band limited energy is
derived from routing the signal through a Fourier transform.
15. The device of claim 13, wherein said variance is determined from the
frequency band limited energy over a continuous period of m seconds, where
m is between 0.1 second and 10 seconds.
16. The device of claim 13, wherein the variance of the frequency band
limited energy is determined by maintaining a sum of m seconds of
frequency band limited energy, wherein m is between 0.1 to 10 seconds, and
a sum of squares of said m seconds of frequency band limited energy and,
for a new variance determination, the sum of squares of frequency band
limited energy is updated by adding a square of a newest frequency band
limited energy and subtracting a square of the frequency band limited
energy value m seconds past, and wherein the sum of said m seconds of
frequency band limited energy is updated by adding the newest frequency
band limited energy and subtracting the frequency band limited energy
value m seconds past.
17. A device for detecting speech in an input signal comprising:
first determining means for determining a plurality of values
representative of a plurality of frequency band limited energy within the
signal, wherein the signal is sampled at a predetermined sampling rate in
a single frequency band over a first plurality of frames, wherein each
frame comprises a plurality of samples, the first determining means using
a frequency band limiter;
second determining means for receiving the plurality of values from said
first determining means, and determining a variance of the frequency band
limited energy of the signal in the single frequency band over a second
plurality of frames; and
third determining means for determining beginning and ending points of
speech within the signal using the variance of the frequency band limited
energy. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to a device for the detection of the start
and end of a segment containing speech within an input audio signal which
contains both speech segments and nonspeech noise or background segments.
2. Description of Related Art
Detection of speech in real time is a necessary component for many devices,
including but not limited to voice-activated tape recorders, answering
machines, automatic speech recognizers, and processors for removing speech
from music. Many of these applications have noise inseparably mixed with
the speech. Detection of speech requires a more sophisticated speech
detection capability than provided by conventional devices that simply
detect when energy levels rise above or fall below a preset threshold.
In the field of automatic speech recognition, the speech detection
component is most critical. In practice, more speech recognition errors
arise from errors in speech detection than from errors in pattern
matching, which is commonly used to determine the content of the speech
signal. One proposed solution is to use a word spotting technique, in
which the recognizer is always listening for a particular word. Howewer,
if word spotting is not preceded by speech detection, the overall error
rate can be high.
Many speech detection devices are based on a certain parameter of the
input, such as energy, pitch, and zero crossings. The performance of the
speech detector depends heavily on the robustness of that parameter to
background noise. For real time speech detection, these parameters must be
quickly extracted from the signal.
OBJECTS AND SUMMARY OF THE INVENTION
One of the objects of the present invention is to provide a device for the
detection of speech which is capable of operation at a speed fast enough
to keep up with the arrival of the input, i.e., real time.
Another object of the present invention is to provide a device for the
detection of speech that can be implemented with a conventional digital
signal processing circuit board.
Another object of the present invention is to provide a device for the
detection of speech which is effective despite various types of noise
mixed with the speech.
Another object of the present invention is to provide a speech detection
device for various applications, including, but not limited to: isolated
word automatic speech recognizers, continuous speech recognizers (to
detect pauses between phrases or sentences), voice-controlled tape
recorders, answering machines, and the processing of voice embedded in a
recording with background noise or music.
These and other objects of the invention are achieved by the provision of a
device for detecting speech in an input signal which includes means for
determining a value representative of frequency band limited energy within
the signal, means for determining a variance of the value representative
of the frequency band limited energy of the signal, and means for
determining the beginning and ending points of speech within the signal
based on the variance of the band limited energy.
The invention exploits the variance in frequency band limited energy to
detect the beginning and end of speech within an input speech signal.
Variance of the frequency band limited energy is employed based on the
observation that for foreground speech occurring in a difficult
background, such as a lead vocalist against a background of music, there
is a noticeable fluctuation of the energy level above a "noise floor" of
relatively low fluctuation. This effect occurs although the level of the
foreground and the level of the background may be high. Variance
quantifies that fluctuation of energy.
In accordance with the preferred embodiment, the device calculates
frequency band limited energy using a Hamming window and a Fourier
transform. The variance is calculated as a function of time from frequency
band limited energy values stored in a shift register. To determine the
beginning and ending points of speech within an input signal, the device
compares the variance as a function of time with two predetermined
threshold levels, an upper threshold level and a lower threshold level. If
the variance exceeds the lower threshold level, the device tentatively
determines that speech has begun. However, if the variance does not
subsequently rise above the upper threshold level before falling below the
lower threshold level, then the tentative determination of the beginning
of speech is discarded. When the variance is between the lower and upper
threshold levels, the device characterizes the signal as being in a
beginning (B) :speech state. Once the variance exceeds the upper threshold
level, the device characterizes the signal as being within a speech (S)
state. If the variance does not remain within speech state(s) for at least
a predetermined period of time, such as 0.3 seconds, the speech is
rejected as being too short. If the variance remains above the upper
threshold level for at least the predetermined period of time, then the
determination of the beginning point of the speech is retained. Finally,
the ending point of the speech is determined when the variance falls below
the lower threshold level.
By employing upper and lower threshold levels and by testing whether the
variance remains within the speech state for at least a predetermined
period of time, the error rate in detecting speech is minimized.
Preferably, the device is implemented within integrated circuit hardware
such that the processing of the input signal to determine the beginning
and ending points of speech based on the variance of the frequency band
limited energy can be performed in real time.
BRIEF DESCRIPTION OF THE DRAWINGS
The exact nature of this invention, as well as its objects and advantages,
will become readily apparent upon reference to the following detailed
description when considered in conjunction with the accompanying drawings,
in which like reference numerals designate like parts throughout the
figures thereof, and wherein:
FIG. 1 provides a block diagram of an automatic speech recognizer,
employing a speech detection device in accordance with a preferred
embodiment of the invention;
FIG. 2 is a block diagram of the speech detection device of FIG. 1;
FIG. 3 provides a flow chart illustrating a method for initially
determining the variance of the frequency band limited energy employed by
the speech detection device of FIG. 2;
FIG. 4 is a state diagram illustrating the speech detection device of FIG.
2; and
FIG. 5 is an exemplary input signal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is provided to enable any person skilled in the
art to make and use the invention and sets forth the best modes
contemplated by the inventor of carrying out his invention. Various
modifications, however, will remain readily apparent to those skilled in
the art, since the generic principles of the present invention have been
defined herein specifically to provide a speech detection device which
detects the beginning and ending points of speech based on the variance of
the frequency band limited energy of an input signal.
A preprocessor for an isolated word automatic speech recognition system
using the present invention is illustrated in FIG. 1. Analog input 101,
from a microphone, is voltage-amplified and converted to digital form by
an analog-to-digital converter 102 at a rate equal to a sampling frequency
(typically 10,000 samples per second). A resulting digital signal 103 is
saved in a memory area 104 that can store up to m seconds of speech where
m is between 0.1 and 10 seconds. In the preferred embodiment m is 6.5536
seconds--a period longer than any single word utterance. If the capacity
of 104 is exceeded, then old data is erased as new data is saved. Thus,
memory 104 contains the most recent 6.5536 seconds of input data. The
digital signal 103 also serves as input to a speech detection device 105.
An output decision signal 106 triggers a gate 107 to pass a portion of
memory 104 which has been determined by 105 to contain speech, to an
output 108. For different applications, the length of buffer memory 104
can be modified and, in some applications such as an answering machine,
buffer memory 104 can be eliminated, and signal 106 can control a tape
drive directly.
Speech detection device 105 is illustrated in detail in FIGS. 2, 3, and 4.
The digital input signal 103 of FIG. 1 is shown as input signal 103 of
FIG. 2. Signal 103 enters a delay line shift register 202 keeps nf
consecutive samples of the input (e.g. 256). When register 202 is filled,
a frequency band limiter CO3 starts processing the signal. When nf/2 (e.g.
128) new samples of input data 103 have been received, a delay line shift
register 202 shifts 128 to the right, erasing the 128 oldest samples, and
fills the left half with 128 new samples. Thus, shift register 2O2 always
contains 256 consecutive samples of the input and overlaps 50% with the
previous contents. the unit of time for the 128 new samples to be ready is
a frame, and one frame is, e.g., 0.0128 seconds.
The frequency band limited energy is calculated by frequency band limiter
203. After multifplying the elements of the delay line shift register 202
by a Hamming window 204, a Fourier transform, 205, extracts the frequency
spectrum of the contents of shift register 202. The spectral components
corresponding to frequencies between 250 Hz and 3500 Hz, the band that
contains the most important speech information, are converted to units of
decibels by dB converter unit 206, and are summed to together in summer
unit 267 producing the frequency band limited energy on output line 208.
Alternatively, band limiting may be performed by a method other than
summing the portions of a frequency spectrum converter. For example, the
input signal 103 may be digitally filtered by convolution or by passing
through a digital filter, which replaces shift register 202 and all of
frequency band limiter 203 of FIG. 2. Then, the resulting energy of the
signal may be measured by a method described below.
Also, band limiting may be performed in the analog domain, with the energy
obtained directly from the band limiting filter, or by a method described
below. The analog band limiter may consist of a band-pass filter, a low
pass filter, or another spectral shaping filter, or may arise from
frequency limiting inherent in an amplifier or microphone, or may take the
form of an antialiasing filter. The energy may be obtained directly from
the filter or by a method described in the following paragraph. The signal
resulting from either of these alternative techniques is hereafter
referred to as the frequency band limited signal.
Any quantity that varies generally monotonically with the energy of the
frequency band limited signal is hereafter called the frequency band
limited energy. Instead of the method described in FIG. 2, the frequency
band limited energy may be calculated by: (a) calculating the variance of
the frequency band limited signal over a short period of time; (b) summing
the absolute value, magnitude, rectified value, or square or other even
power of the frequency band limited signal over a short period of time; or
(c) determining the peak of the value, the magnitude, the rectified value,
or square or other power of the frequency band limited signal over a short
period of time.
Continuing with the preferred embodiment of the invention, frequency band
limited energy 208 enters a delay line shift register 209 which differs
from delay line shift register 202 in that (a) it receives one (not 128)
new entry every frame, and (b) it shifts right by one (not by 128) when
each new entry arrives. The length of this delay line shift register 209
is nv, which corresponds to a pause length of, for example, 0.64 seconds,
or 50 frames:
##EQU1##
Variance calculation unit 210 calculates the variance of the values in
delay line shift register 209. The variance of the frequency band limited
energy, is:
V=g(A, B)
##EQU2##
and
V is the output 211 of the variance calculation 210;
and
BLE(f) is the contents of delay line shift register 209 at locations f=nv,
. . . , 3, 2, 1; BLE (1) is the oldest BLE value; and BLE is the frequency
band limited energy.
FIG. 3 shows a faster way to calculate the variance V, replacing the
variance calculation unit 210 and delay line shift register 209. This
preferred technique updates, rather than recalculates, quantities A and B
as follows:
A'=A+[BLE(nv).times.BLE(nv)]-[BLE(0).times.BLE(0)]
B'=B+BLS(nv)-BLS(0)
where
A' is the updated value for A, shown as signal 302,
and
B' is the updated value for B, shown as signal 303,
and
BLE(nv) is the newest frequency band limited energy, 208 (FIG. 2),
and
BLE(0) is the oldest frequency band limited energy signal 304.
The square of BLE is delayed in the delay line memory 305. This delay line
memory can be removed and replaced by squaring the value from 304 in
situations where memory is expensive but multiplication is inexpensive.
The delay line memories 305 and 306 should be cleared to zero upon
initialization. Also, note that the delay line memories 306 and 305 are
one longer than delay line shift register 209 of FIG. 2.
FIG. 4 shows a state diagram that describes how the variance 211 is used in
detecting the existence of speech. FIG. 5 shows an example of a speech
signal as an aid in understanding the state diagram.
The state diagram begins in the N or Noise state (502). As long as the
variance V, which is from 211 of FIG. 2, stays below the lower threshold
501, transition 402 is taken, and state N is not exited. When V rises
above threshold 501, transition 403 is taken, and state B (beginning of
speech) is entered. One of three transitions can be taken from state B,
depending on the conditions, as follows:
th<V: transition 405 (advance to S, speech)
t1<V<th: transition 404 (stay in B)
O<V<t1: transition 406 (rejected: go to N)
where th is the upper, high threshold 506 and t1 is 501 the low threshold.
Segments 502, 503, and 504 show how these transition conditions make the
device wait for a sizable rise in variance before entering the S, or
speech, state. The conditions and transitions for exiting the state S are:
##EQU3##
The conditions for exiting state S depend on the low threshold t1, not th
the high threshold 506, to avoid instabiity when V is near th. Transition
409 rejects utterances that are too short to be a single word. Segment 507
shows the usual case: staying in state S until the variance decreases
below t1, taking transition 408 to state E.
State E triggers the output decision signal 106 of FIG. 1, showing that the
end of the utterance has been found. Because the variance depends on the
past nv (FIG. 3) frames, it will decrease about nv frames after the
frequency band limited energy fluctuations decrease. After state E the
state recycles to state N, to be ready for the next utterance.
Thresholds t1, 501, and th, 506 are determined early in a first N state, by
examining the level of the variance there. They are set as follows:
th=3.0.times. average of variance of 10 frames of N state;
t1 =1.2.times. average of variance of 10 frames of N state.
What has been described is a device for detecting the presence of speech
within an input signal. The device calculates the beginning and the ending
points of speech based on the variance of the frequency band limited
energy within the signal. By utilizing the variance of the frequency band
limited energy, the presence of speech is effectively detected in real
time. The device is particularly useful for detecting a segment of a
recording that contains speech, such that the segment can be extracted and
further processed.
Those skilled in the art will appreciate that various adaptations and
modifications of the just-described preferred embodiment can be configured
without departing from the scope and spirit of the invention. Therefore,
it is to be understood that, within the scope of the appended claims, the
invention may be practiced other than as specifically described herein.
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