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Claims  |
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What is claimed is:
1. A method of identifying the endpoints of one or more utterances in an
interval of speech comprising the steps of
(1a) dividing the interval into a succession of time frames, each frame
having an identifying pointer,
(1b) selecting the frame over the interval which has the maximum speech
energy level,
(1c) defining the first frame preceding the selected energy frame which has
an energy level below a first threshold as the beginning frame of an
energy pulse,
(1d) defining the first frame following the selected energy frame which has
an energy level below a second threshold as the ending frame of the energy
pulse,
(1e) saving the pointers of the beginning and ending frame and the level of
the selected energy frame of the energy pulse if the number of frames
between the beginning and ending frame is greater than a predetermined
number and the level of the selected energy frame is greater than a third
threshold,
(1f) repeating steps (1b)-(1e), examining only those frames which are not
constituents of the current or prior energy pulses until no further energy
pulses are found, whereby the saved pointers correspond to the end points
of the utterances in the interval, whereby the endpoint determinations are
likely to be more effective in the presence of varying background noise
than in the prior art.
2. The method of claim 1 further comprising after step (1c)
designating the frame which follows the current beginning frame by a
predetermined number of frames as the new beginning frame it the energy
level in each of a predetermined number of frames following the current
beginning frame is below a fourth threshold.
3. The method of claim 1 further comprising after step (1d)
designating the frame which precedes the current ending frame by a
predetermined number of frames as the new ending frame is the energy level
in each of a predetermined number of frames preceding the current ending
frame is below a fifth threshold.
4. The method of claim 1 further comprising after step (1f)
combining the energy pulses according to predetermined criteria, and
saving the pointers of the beginning and ending frames of the combined
energy pulses.
5. The method of claim 4 wherein the energy pulse combining step comprises
(5a) selecting the energy pulse over the interval which has the maximum
energy level,
(5b) combining the selected energy pulse with the immediately preceding
energy pulse;
(5c) determining: if the slope of the energy level over a predetermined
number of frames before the ending frame of the preceding energy pulse is
greater than a predetermined threshold, and if the slope of the energy
level over a predetermined number of frames after the beginning frame of
the current selected energy pulse is greater than a predetermined value,
and if the number of frames between the ending frame of the preceding
energy pulse and the beginning frame of the current selected energy pulse
is less than a predetermined number;
(5d) then, when all the conditions of (5c) are satisfied, defining the
current combined energy pulse as a new energy pulse, eliminating the
current selected energy pulse and immediately preceding energy pulse from
further consideration, and repeating steps (5a)-(5d);
(5e) then, when any of the conditions of (5c) are not satisfied,
terminating the combining step;
(5f) then, when the current new energy pulse has been thus defined
selecting the energy pulse which has the next highest energy level, and
(5g) repeating steps (5a)-(5g), as if this next level were the maximum
energy level until all energy pulses that were found have been selected or
combined.
6. The method of claim 4 wherein the energy pulse combining step comprises
(6a) selecting the energy pulse over the interval which has the maximum
energy level,
(6b) combining the selected energy pulse with the immediately succeeding
energy pulse;
(6c) determining: if the slope of the energy level over a predetermined
number of frames after the beginning frame of the succeeding energy pulse
is greater than a sixth threshold, and if the slope of the energy level
over a predetermined number of frames before the ending frame of the
current selected energy pulse is greater than a seventh threshold, and if
the number of frames between the ending frame of the succeeding energy
pulse and the ending frame of the current selected energy pulse is less
than a predetermined number;
(6d) then, when all of the conditions of (6c) are satisfied, defining the
current combined energy pulse as a new energy pulse, eliminating the
current selected energy pulse and immediately succeeding energy pulse from
further consideration, and repeating steps (6a)-(6d);
(6e) then, when any of the conditions of (6c) are not satisfied,
terminating the combining step;
(6f) then, when the current new energy pulse has been thus defined
selecting the energy pulse which has the next highest energy level,
(6g) repeating steps (6a)-(6g), as if this next level were the maximum
energy level until all energy pulses that were found has been selected or
combined.
7. The method of claim 4 wherein the energy pulse combining step comprises
(7a) selecting the energy pulse over the interval which has the maximum
energy level,
(7b) combining the selected energy pulse with the immediately adjacent
energy pulse to either side thereof;
(7c) determining: if the slope of the energy level over a predetermined
number of frames receding from the nearest frame of and receding within
the adjacent energy pulse is greater than a sixth threshold, and if the
slope of the energy level over a predetermined number of frames receding
from the nearest frame of and receding within the current selected energy
pulse is greater than a seventh threshold, and if the number of frames
between the nearest frame of the adjacent energy pulse and the nearest
frame of the current selected energy pulse is less than a predetermined
number;
(7d) then, when all the conditions of (7c) are satisfied, defining the
current combined energy pulse, as a new energy pulse, eliminating the
current selected energy pulse and the immediately adjacent energy pulse
from the further consideration, and repeating steps (7a)-(7d);
(7e) then, when all the conditions of (7c) are not satisfied for a pulse to
the first side of the selected pulse, terminating combining to said first
side;
(7f) combining the current combined energy pulse, if any, or the current
selected energy pulse, with the immediately adjacent energy pulses to the
second side thereof;
(7g) then, determining: if the slope of the energy level over a
predetermined number of frames receding from the nearest frame of and
receding within the adjacent energy pulse is greater than a sixth
threshold, and if the slope of the energy level over a predetermined
number of frames receding from the nearest frame of and receding within
the current combined energy pulse or the current selected energy pulse is
greater than a seventh threshold, and if the number of frames between the
nearest frame of the adjacent energy pulse and the nearest frame of the
current combined energy pulse or the current selected energy pulse is less
than a predetermined number;
(7h) then, when all of the conditions of (7g) are satisfied, defining the
current combined energy pulse as a new energy pulse, eliminating the
current selected energy pulse, and the immediately adjacent energy pulse
from further consideration, and repeating steps (7f)-(7h);
(7i) then, when any of the conditions of (7g) are not satisfied,
terminating the combining step;
(7j) then, when a new energy pulse has been thus defined, selecting the
energy pulse which has the next highest energy level, and repeating steps
(7a)-(7c) as if this next energy level were the maximum energy level until
all energy pulses that were found have been selected or combined.
8. Apparatus for identifying the endpoints of one or more utterances in an
interval of speech comprising
(8a) means for dividing the interval into a succession of time frames, each
frame having an identifying pointer,
(8b) means for selecting the frame over the interval which has the maximum
speech energy level,
(8c) means for defining the first frame preceding the selected energy frame
which has an energy level below a first threshold as the beginning frame
of an energy pulse,
(8d) means for defining the first frame following the selected energy frame
which has an energy level below a second threshold as the ending frame of
the energy pulse,
(8e) means for saving the pointers of the beginning and ending frames and
the level of the selected energy frame of the energy pulse if the number
of frames between the beginning and ending frame is greater than a
predetermined number, and the level of the selected energy frame is
greater than a third threshold, and
(8f) means for controlling means (8b)-(8e) to repeat processing on only
those frames which are not constituents of current or prior energy pulses
until no further energy pulses are found,
whereby the saved pointers correspond to the endpoints of the utterances in
the interval and the endpoints are likely to be more effectively
determined in the presence of varying background noise than in the prior
art.
9. The apparatus of claim 8 wherein the means (8c) for defining the first
frame preceding the selected energy frame which has an energy level below
a first threshold further comprises
means for designating the frame which follows the current beginning frame
by a predetermined number of frames as the new beginning frame if the
energy level in each of a predetermined number of frames following the
current beginning frame is below a fourth threshold.
10. The apparatus of claim 8 wherein the means (8d) for defining the first
frame following the selected energy frame which has an energy level below
a second threshold as the ending frame of the energy pulse further
comprises
means for designating the frame which precedes the current ending frame by
a predetermined number of frames the new ending frame if the energy level
in each of a predetermined number of frames preceding the current ending
frame is below a fifth threshold.
11. The method of claim 8 further comprising after step (8f)
means for combining the energy pulses according to predetermined criteria,
and
means for saving the pointers of the beginning and ending frames of the
combined energy pulses.
12. The apparatus of claim 11 wherein the energy pulse combining means
comprises
(12a) means for selecting the energy pulse over the interval which has the
maximum energy level,
(12b) means for combining the selected energy pulse with the immediately
preceding energy pulse;
(12c) means for determining: if the slope of the energy level over a
predetermined number of frames before the ending frame of the preceding
energy pulse is greater than a sixth threshold, and if the slope of the
energy level over a predetermined number of frames after the beginning
frame of the current selected energy pulse is greater than a seventh
threshold, and if the number of frames between the ending frame of the
preceding energy pulse and the beginning frame of the current selected
energy pulse is less than a predetermined number;
(12d) means responsive to a positive output from the determining means for
defining the current combined energy pulse as a new energy pulse,
eliminating the current selected energy pulse and immediately preceding
energy pulse from further consideration, and means for controlling means
(12a)-(12d) to repeat operation thereof;
(12e) means responsive to a non-positive output from the determining means
for terminating the repetition of operation by means (12a)-(12d),
(12f) means responsive to such termination by the terminating means for
selecting the energy pulse which has the next highest energy level, and
(12g) means for controlling means (12a)-(12g) to repeat operation thereof
as if this next level were the maximum energy level until all energy
pulses that were found have been selected or combined.
13. The apparatus of claim 11 wherein the energy pulse combining means
comprises
(13a) means for selecting the energy pulse over the interval which has the
maximum energy level,
(13b) means for combining the selected energy pulse with the immediately
succeeding energy pulse;
(13c) means for determining: if the slope of the energy level over a
predetermined number of frames after the beginning frame of the succeeding
energy pulse is greater than a sixth threshold, and if the slope of the
energy level over a predetermined number of frames before the ending frame
of the current selected energy pulse is greater than a seventh threshold,
and if the number of frames between the ending frame of the succeeding
energy pulse and the ending frame of the current selected energy pulse is
less than a predetermined number;
(13d) means responsive to a positive output from the determining means for
defining the current combined energy pulse as a new energy pulse,
eliminating the current selected energy pulse and immediately succeeding
energy pulse from further consideration, and controlling means (13a)-(13d)
to repeat operation thereof;
(13e) means responsive to a non-positive output from the determining means
for terminating the repetition of operation by means (13a)-(13d);
(13f) means responsive to such termination by the terminating means for
selecting the energy pulse which has the next highest energy level, and
(13g) means for controlling means (13a)-(13g) to repeat operation thereof
as if this next level were the maximum energy level until all energy
pulses that were found have been selected or combined.
14. The apparatus of claim 11 wherein the energy pulse combining means
comprises
(14a) means for selecting the energy pulse over the interval which has the
maximum energy level,
(14b) means for combining the selected energy pulse with the immediately
adjacent energy pulse to either side thereof;
(14c) means for determining: if the slope of the energy level over a
predetermined number of frames receding from the nearest frame of and
receding within the adjacent energy pulse is greater than a sixth
threshold, and if the slope of the energy level over a predetermined
number of frames receding from the nearest frame of and receding within
the current selected energy pulse is greater than a seventh threshold, and
if the number of frames between the nearest frame of the adjacent energy
pulse and the nearest frame of the current selected energy pulse is less
than a predetermined number;
(14d) means responsive to a positive output from the determining means for
defining the current combined energy pulse, as a new energy pulse,
eliminating the current selected energy pulse and the immediately adjacent
energy pulse from further consideration and for controlling means
(14a)-(14d) to repeat operation thereof;
(14e) means responsive to a non-positive output from the determining means
for a pulse to the first side of the selected pulse for terminating the
operation of the combining means for pulses to said first side;
(14f) means for combining the current combined energy pulses, if any, or
the current selected energy pulse, with the immediately adjacent energy
pulse to the second side thereof;
(14g) means for controlling the operation of the determining means to
determine if the slope of the energy level over a predetermined number of
frames receding from the nearest frame of and receding within the adjacent
energy pulse is greater than a sixth threshold, and if the slope of the
energy level over a predetermined number of frames receding from the
nearest frame of and receding within the current combined energy pulse or
the current selected energy pulse is greater than a seventh threshold, and
if the number of frames between the nearest frame of the adjacent energy
pulse and the nearest frame of the current combined energy pulse or the
current selected energy pulse is less than a predetermined number;
(14h) means responsive to a positive output from the determining means for
an energy pulse to the second side of the current combined energy pulse
for defining the current combined energy pulse as a new energy pulse,
eliminating the current combined energy pulse, and the immediately
adjacent energy pulse from further consideration, and controlling means
(14a)-(14h) to repeat operation thereof;
(14i) means responsive to a non-positive output from the determining means
for an energy pulse to the second side of the current combined energy
pulse for terminating the operation of the combining means;
(14j) means responsive to the operation of the last said terminating means
for selecting the energy pulse which has the next highest energy level,
and for controlling means (14a)-(14j) as if this next energy level were
the maximum energy level until all energy pulses that were found have been
selected or combined. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Our invention relates to automatic speech recognition, and more
particularly, to arrangements for detecting the endpoints or boundaries of
the speech portion of an input signal.
An automatic speech recognizer identifies an unknown spoken utterance by
matching an input signal which corresponds to the unknown utterance, to
reference template signals which correspond to known utterances. The
reference template which matches best is selected as the identity of the
unknown utterance. The reference templates typically include only
information-bearing or speech portions. On the other hand, in many
commercially important environments, the input signal often includes both
speech and nonspeech sounds. An input signal from the switched telephone
network, for example, may have clicks, pops, tones and other background
noise.
Whereas human listeners are comparatively tolerant of noise and distortion,
current machine recognizers generally are not. Accurate location of the
beginning and ending, the "endpoints" of spoken words and phrases, is thus
important for reliable and robust automatic speech recognition. The
endpoint detection problem is relatively less complex for high level
speech signals in a low level, stationary noise environment, for example,
where the signal-to-noise ratio is greater than about 30 dB. The problem
is considerably more difficult, however, if the speech signal level is low
relative to the background noise, or if the level and spectral content of
the background noise is nonstationary. Such conditions may be encountered
in the switched telephone network, especially in the long distance
network, due to transmission line characteristics and transients in line
signal generators.
In a prior endpoint detector, disclosed in U.S. Pat. No. 4,370,521, issued
Jan. 25, 1983 to Johnston et al. and assigned to the present assignee, an
input signal interval which contains speech is divided into a sequence of
time frames. The energy level of the signal in each time frame is
computed. Responsive to the energy levels, one or more energy pulses are
identified over the signal interval. Each energy pulse consists of a group
of contiguous time frames which correspond to a potential speech portion
of the input signal. For example, an input signal interval containing the
spoken words "one eight" ideally yields three distinct energy pulses: the
first corresponding to the voiced portion "one"; the second corresponding
to the voiced portion "eigh"; and the third corresponding to the unvoiced
portion "t".
Next, certain of the raw energy pulses are "combined", that is, the
constituent frames of two or more adjacent energy pulses are grouped
together to form a longer energy pulse. In the above example, the second
and third energy pulses may be combined to form a single energy pulse
corresponding to "eight". Finally, the endpoints of the energy pulses
remaining after the combining steps are passed to a speech recognizer.
In more detail, the identification of the raw energy pulses according to
Johnston proceeds as follows. The energy levels are considered frame by
frame in temporal sequence. If the energy level rises above a first
threshold, and then above a second threshold before falling below the
first threshold, the frame in which the energy level first rose above the
first threshold is designated as the beginning frame of an energy pulse.
Subsequently, the first frame in which the energy level falls below a
third threshold is designated as the ending frame of the energy pulse.
This process is repeated over the remainder of the input signal interval
whereby a plurality of energy pulses may be detected.
The Johnston arrangement attempts to find endpoints based on the energy of
speech rising above the energy of the background noise. This may be
conveniently characterized as a "bottom-up" approach. The bottom-up
endpoint detector works well where the background noise is stationary.
Where the level and spectral content of the background noise fluctuates,
however, the bottom-up detector may be less effective.
It is thus an object of the invention to provide an endpoint detector which
improves the accuracy of a speech recognizer where the input signal
include nonstationary noise.
SUMMARY OF THE INVENTION
We have discovered that the endpoints of information bearing portions of an
input signal which includes nonstationary noise can be reliably detected
by finding the high energy frame in local regions of the input signal and
then analyzing the energy values of frames surrounding the local high
energy frames to define energy pulse boundaries. This may be characterized
as a "top-down" approach.
An interval of speech is divided into time frames. The frame having the
maximum energy level over the interval is selected. The first frame
preceding the maximum energy level frame which has an energy level below a
threshold is defined as the beginning frame of an energy pulse. The first
frame following the maximum energy level frame which has an energy level
below a threshold is defined as the ending frame of the energy pulse. The
process is repeated, excluding in each repetition frames that became
energy pulse constituents in a prior repetition, until the entire interval
has been considered.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a general block diagram of an endpoint detector in accordance
with the invention.
FIGS. 2-10 show flow charts of endpoint detection in accordance with the
invention.
DETAILED DESCRIPTION
FIG. 1 shows a general block diagram of a top-down endpoint detector in
accordance with the invention. The system of FIG. 1 may be used to provide
the beginning and ending points of the information-bearing components of
an input signal to a utilization device, such as a speech recognizer. The
endpoint detector may comprise a programmed general purpose digital
computer such as the MV8000 made by Data General Incorporated.
Alternatively, the endpoint detector may be implemented with special
purpose digital hardware, as is well known in the art.
Referring to FIG. 1, an interval of an input signal s(t) which includes
speech is applied to the input of coder 104. In coder 104 the input signal
is first bandpass filtered and sampled. If the input signal is a telephone
bandwidth signal, for example, the input signal is bandpass filtered from
100 Hz to 3200 Hz and sampled at 6.67 kHz. The sampled speech is then
quantized and converted to digital form. The digitized speech from coder
104 is applied to frame and window processor 106. There, the digitized
speech is preemphasized using a simple first-order digital filter with a
z-transform:
H(z)=1-az.sup.-1 (1)
where a=0.95. The digitized signal interval is then blocked into frames of
N samples, with a shift or overlap between frames of L samples. N may be,
for example, 300 samples and L may be 100 samples. This translates to a
frame duration of 45 milliseconds with a 15 millisecond shift between
frames. Each frame may then be weighted by a Hamming window of the form:
w(n)=0.54-46cos (2.pi.n/N),
0.ltoreq.n.ltoreq.N-1. (2)
The output of frame and window processor 106 is a preemphasized, windowed
signal s(l,n) wherein the index l denotes the frame, the frames ranging
from 0 to L-1. The index n denotes the particular sample within a frame,
wherein n ranges from 0 to N-1.
The windowed signals s(l,n) are applied to energy level generator 108.
Generator 108 forms signals e(1) representative of the energy in each
frame of the windowed signal:
e(1)=10 log R(1)0,
1=1,2 . . . NF (3)
where NF is the total number of frames in the input signal interval, and
R(1)0 is the zero'th order correlation coefficient:
##EQU1##
The output signal e(1) from energy level generator 108 is applied to
equalizer-normalizer 110. Unit 110 performs adaptive level equalization to
compensate for the mean background noise level. The member of e(1), where
1=1,NF, having the minimum value, e(min), is subtracted from each member
e(1) to yield, enorm(1), a normalized energy level array:
enorm(1)=e(1)-e(min),
1=1,NF. (5)
A second normalization is performed in unit 110 to obtain the energy level
signal E(1):
E(1)=enorm(1)-MODE (6)
where MODE is the mode of a histogram of the lowest NP values of E(1). NP
may be, for example, 15.
Further background information with respect to coder 104, frame and window
processor 106, energy level generator 108 and equalizer-normalizer 110 may
be found in U.S. Pat. No. 4,370,521, Johnston et al., herein incorporated
by reference.
The energy level signals E(1) from equalizer-normalizer 110 are collected
in frame energy store 112. Responsive to controller 120, all of the energy
level signals E(1), 1=1,NF, are applied to maximum energy detector 116.
Detector 116 finds the frame with the maximum energy over all frames in
the input interval. Next, the energy level signals E(1) of frames
surrounding the maximum energy frame are applied to begin-end detector
114. Detector 114 finds the first frame prior to the maximum energy frame
which has an energy level less than a threshold K1. Threshold K1 may be,
for example, 3 dB. Detector 114 then finds the first frame following the
maximum energy frame which has an energy level less than a threshold K3.
Threshold K3 may be, for example, 5 dB. At this point, a set of possible
beginning and ending frames for an energy pulse has been found. These
endpoints are applied from detector 114 along with the maximum energy
frame from detector 116 to pulse store 118.
Controller 120 next checks the first IT1 frames and last IT2 frames of the
pulse for consistently low energy content which indicates breath noise.
IT1 and IT2 may be, for example, 5 frames. Any low energy frames are
eliminated by adjusting the endpoints in store 118. Then the adjusted
energy pulse is tested to guarantee that its duration is greater than a
minimum length threshold and that its maximum energy level frame is above
a minimum level. The pulse is considered invalid if either test is failed.
Controller 120 repeats the preceding steps starting with the next highest
energy level frame over the input interval. All frames in previously
detected pulses are eliminated from consideration in the current
iteration. The process is complete when all frames over the input interval
have been considered.
Controller 120 next applies a pulse combiner algorithm to the energy pulses
in store 118. The algorithm attempts to combine two or more adjacent
pulses to form longer pulses. The first current pulse is the pulse having
the highest peak energy frame of all the pulses in store 118. The first
pulse preceding the current pulse is combined with the current pulse if
the downward slope DS over the last IGAP frames of the preceding pulse is
greater than a threshold and if the last frame of the preceding pulse is
within NFW frames of the first frame of the current pulse. IGAP may be,
for example, 3 frames. NFW may be set adaptively according to the value of
DS. Similarly, the first pulse following the current pulse is combined
with the current pulse if the downward slope of the current pulse is
greater than a threshold and if the following pulse is within NFW frames
of the current pulse. Other pulse combining restrictions may be applied as
would now be apparent to those skilled in the art. For example, the
duration of any combined pulse may be constrained to be less than a
predetermined maximum. Also, an upward slope minimum value could be
imposed.
The above process is repeated with the current pulse being the pulse which
has the next highest peak energy frame of the pulses in store 118. The
process terminates when all possible pulses have been considered. The
final output to utilization device 122 is the beginning and ending frames
IPB(J) and IPE(J) for each energy pulse.
A program for implementing the instant endpoint detector invention may be
structured, for example, in accordance with flow charts 200-1000 in FIGS.
2-10. In particular, flow charts 200-600 show a detailed example of
finding the beginning and ending frames which define an energy pulse. Flow
charts 700-900 show a detailed example of combining the raw energy pulses
to form longer energy pulses.
Referring to FIG. 2, energy pulse detection starts (202) with pulse counter
NPULSE=0 and frame counter J=1 (204). If the frame energy level E(J) is
less than or equal to threshold K2 (206), J is incremented by 1 (208). If
J is greater than the number of frames NF in the interval (210), the
process terminates (216). If J is less than or equal to NF, E(J) is again
compared to K2. If E(J) is greater than K2 (206), frame counter I is set
equal to J (212). If I is less than NF (218), I is incremented by 1 (226).
If E(I) is greater than or equal to K2 (224), the process returns to test
whether I is greater than or equal to NF (218). If E(I) is less than K2
(224), mark counter MK is set to I (228). If I is less than NF (232), and
E(I) is less than threshold K3 (230), and E(I) is greater than or equal to
K2 (220), the process returns to test I (218). If E(I) is less than K2
(220), I is incremented (222) and the process returns to test I (232). If
I is greater than or equal to NF (232) or if E(I) is less than K3 (230),
and if I minus MK is greater than slope parameter IT slope center frame
IPE(NPULSE+1) is set to MARK(238). If I minus MK is less, than or equal to
IT2 (234), IPE(NPULSE+1) is set to I (236). The outputs of blocks 236 and
238 are connected to control downward slope generation in block 242. The
values of E, IGAP, ISLOPE and IPE (244) are provided to generate the
downward slope (242). The slope generation is shown in block Z, FIG. 5.
Referring to FIG. 5, in block Z (518), I is set to END minus 1 (520). If
E(I) is greater than or equal to E(END) plus ISLOPE (522), NSEP is set to
NSEP2 (516) and the subroutine returns the value of NSEP (514). If E(I) is
less than E(END) plus ISLOPE (522), I is decremented (524). If I is
greater than or equal to END minus IGAP (526), the process returns to test
E(I) (522). If I is less than END minus IGAP (526), NSEP is set to NSEP1
(512) and the subroutine returns NSEP (514).
Referring to FIG. 3, which is joined at connector A (302) to FIG. 2
connector A (240), I is set equal to J (304). If I is greater than 1
(306), I is decremented (308) and the subroutine block X is performed
(310).
Referring to the block X subroutine (605) in FIG. 6, if NPULSE is equal to
0 (610), block X returns a "NO" value (640). If NPULSE is not 0 (610), K
is set to 1 (615). If I is less than IPE(K) (620), block X returns a "YES"
value (635). If I is greater than or equal to IPE(K) (620), K is
incremented (625). If K is greater than NPULSE (630), the subroutine
returns "NO" (640). If K is less than or equal to NPULSE, the test on I is
repeated (620).
Returning to FIG. 3, I is incremented (312) only if the block X subroutine
returns a "YES" (310). If E(I) is greater than or equal to K2(314), the
test on I is repeated (306). If I is less than or equal to 1, or if E(I)
is less than K2 (314), MK is set to I (322). If the block X subroutine
returns "NO" (320), and if I is greater than to 1 (318), and if E(I) is
greater than or equal to K2 (316), the process returns to test I (306). If
block X returns "YES" (320), I is incremented (336). If MK minus I plus 1
is greater than IT1 (326), IPB(NPULSE+1) is set to MK (332); otherwise
IPB(NPULSE+1) is set to I (328). If block X returns "NO" (320) and I is
less than or equal to 1 (318), or if I is greater than 1 (318), and E(I)
is less than K2 (316) and K1 (324), the test on MK minus minud I plus 1 is
run (326). If E(I) is greater than or equal to K1 (324), I is decremented
(330) and MK is set to I (322). The outputs of both blocks 328 and 332
flow into point B, which is the same as point B of FIG. 4.
Referring to FIG. 4, which is joined at connector B (401) to connector B
(334) in FIG. 3, J is set to IPE(NPULSE+1) (402). The maximum peak energy
of the pulse is computed and output as XL (403). XLS(NPULSE+1) is set to
XL (404). If IPE(NPULSE+1) minus IPB(NPULSE+1) plus 1 is greater than IT3
(405), then NPULSE is incremented (406); otherwise NPULSE remains the
same. If NPULSE is equal to the maximum pulse number NPMAX (407), the
process terminate (408); otherwise the process repeats as shown by
connector F (409) which joins to connector F (214) in FIG. 2.
Referring to FIG. 7, the pulse combiner process begins (702) by testing the
number of pulses NPULSE is equal to 0 (704). If NPULSE is 0, the process
terminates (712). If NPULSE is greater than 0, the maximum energy XLS for
each of the NPULSE pulses are sorted in order of decreasing peak energy
(706). The output IXL is the index of the pulse with the highest peak
energy. Next, I and IS are set to 1 (708). All pulses are initially marked
as unused (710). J is set to IXL(I) (716). If pulse J is not currently
marked (718), pulse J is marked used (720). If I is not equal to
NPULSE(722), the process continues in FIG. 8, as shown by connector P
(726) in FIG. 7 and connector P (856) in FIG. 8.
Referring to FIG. 8, if J is not equal to NPULSE (824), and pulse J+1 is
not marked (826), NS is set to NSEP(J) (828). If J is equal to NPULSE
(824), or if pulse J+1 is marked (826), or if IPB(J+1) minus IPE(J) plus 1
is greater than NS (830), IS is incremented (832) and I is incremented
(834). If I is greater than NPULSE (836), IS is decremented (838) and the
process terminates (840). If IPB(J+1) minus IPE(J) (940) plus 1 is less
than or equal to NS (830), and if IPE(J+1) minus IPB(J) plus 1 is greater
than NFMAX (842), IS is incremented (832). If IPE(J+1) minus IPB(J) plus 1
is less than or equal to NFMAX (842), the process continues in FIG. 9, as
shown by connector A' (846) in FIG. 8 and connector A' (905) in FIG. 9.
Referring to FIG. 9, if NS equals NSEP2 (910), the pulses are not combined
(915), and the process continues in FIG. 8, as shown by connector N (920)
in FIG. 9 and connector N (852) in FIG. 8. If NS does not equal NSEP2
(910), the upward slope NT of pulse J+1 is computed around frame IPB (J+1)
(925) by subroutine block Y, as shown in FIG. 5.
Referring to FIG. 5, in block Y (502), I is set to BEG plus 1 (504). If
E(I) is greater than or equal to E(BEG) plus ISLOPE (506), NSEP is set to
NSEP2 (516) and returned (514). If E(I) is less than E(BEG) plus ISLOPE
(506), I is incremented (508). If I is less than or equal to BEG plus IGAP
(510), the test on E(I) is performed (506). If I is greater than BEG plus
IGAP (510), NSEP is set to NSEP1 (512) and returned (514).
Returning to FIG. 9, if upward slope NT is equal to NSEP1 (930), the
process continues in FIG. 8, as shown by connector N (852) in FIG. 8. If
NT is not equal to NSEP1 (930), pulse J+1 is marked and combined with
pulse J. The process continues as above in FIG. 8 (935).
Returning to FIG. 8, if I is less than or equal to NPULSE (836), the
process continues in FIG. 7, as shown by connector M (854) in FIG. 8 and
connector M (728) in FIG. 7. In FIG. 7, if pulse J is marked (718), the
process continues in FIG. 8, as shown by connector E (714) in FIG. 7 and
connector E (844) in FIG. 8.
FIG. 10 is a flow chart showing the top-down approach to energy pulse
detection in accordance with the invention. First, the maximum energy
frame over the interval is found (1002). Surrounding frames are examined
to determine the beginning and ending frames of a pulse (1004). The pulse
is checked for validity (1006). Frames comprising the pulse are eliminated
from further consideration (1008). If any frames remain in the interval
(1010), the above process is repeated, otherwise the process terminates
(1012).
While the invention has been shown and described with reference to a
preferred embodiment, various modifications may be made by those skilled
in the art without departing from the spirit and scope of the invention.
Additional decision rules may be incorporated that reflect the
characteristics of a specialized vocabulary. For example, if only digit
strings are to be detected, only two words, the digits 6 and 8, may
contain a stop gap; all other digits can be represented by a single energy
pulse with no other pulses attached. Also, for the digits 6 and 8, the
maximum energy pulse is always the first pulse when a secondary pulse is
added. This further implies that no pulse should be added to precede a
maximum energy pulse. Further, digits 6 and 8 have at most only one stop
gap, implying that at most one pulse can be added to follow a maximum
energy pulse. In addition, any of the aforementioned thresholds may be
dynamically determined, instead of being fixed values. For example, energy
threshold K3 may be set responsive to the average signal energy over a
prior time period.
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