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Claims  |
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The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An apparatus for processing a signal containing information about
arterial blood flowing in tissue, said signal having a relatively periodic
pulsatile component superimposed upon a varying baseline component, said
apparatus comprising:
first portion identification means for identifying a first portion of said
signal during which the sign of the slope of said signal changes from
positive to negative;
positive peak location means for locating the point along said first
portion of said signal having the largest amplitude, the point having said
largest amplitude defining a positive peak of said signal;
second portion identification means for identifying a second portion of
said signal during which the sign of the slope of said signal changes from
negative to positive;
negative peak location means for locating the point along said second
portion of said signal having the smallest amplitude, the point having
said smallest amplitude defining a negative peak of said signal and the
difference in signal amplitude between said positive peak and said
negative peak defining a pulse amplitude; and
analyzing means, responsive to said positive and negative peak location
means, for producing an output indicative of a characteristic of said
arterial blood.
2. The apparatus of claim 1, wherein said first portion identification
means and said second portion identification means further comprise:
derivative computation means for producing an indication of the first
derivative of said signal with respect to time;
first marker means for identifying an initial point on said signal at which
the absolute value of said indication crosses below a predetermined
threshold; and
second marker means for identifying the point on said signal at which the
absolute value of said indication first crosses back above said
predetermined threshold.
3. The apparatus of claim 2, wherein said indication produced by said
derivative computation means comprises an autonormalized convolution
derivative of the signal determined in accordance with the relationship:
##EQU14##
where: n is the sample time for which the autonormalized convolution
derivative is determined;
j is a summation index;
V(j) is the amplitude of said signal at a sample time j; and
k is an integer used to define the range over which the samples are summed.
4. The apparatus of claim 3, wherein S(n) is determined for k equal to
three.
5. The apparatus of claim 1, further comprising positive slope detection
means for determining when said slope of said signal has been positive for
some predetermined time, said first portion detection means being
inhibited from identifying said first portion of said signal until said
predetermined time has been exceeded.
6. The apparatus of claim 1, further comprising period determination means
for determining the time interval occurring between said positive peak and
said negative peak of said sample.
7. The apparatus of claim 1, wherein said first portion identification
means, positive peak location means, second portion identification means
and negative peak location means cooperatively produce a plurality of
pairs of said positive and negative peaks.
8. The apparatus of claim 1, wherein said characteristic indicated by said
output of said analyzing means includes pulse rate and oxygen saturation.
9. The apparatus of claim 1, further comprising rejection means for
rejecting said positive and negative peaks when said peaks fail to satisfy
a selection criterion.
10. The apparatus of claim 9, wherein said criterion comprises a pulse
amplitude template defining an allowable pulse amplitude range, said
rejection means rejecting said positive and negative peaks when said pulse
amplitude is outside of said allowable pulse amplitude range.
11. The apparatus of claim 10, wherein said allowable pulse amplitude range
is adjustable.
12. The apparatus of claim 11, wherein said allowable pulse amplitude range
is initialized at a predetermined level.
13. The apparatus of claim 11, wherein said allowable pulse amplitude range
is automatically increased in porportion to said pulse amplitude when said
pulse amplitude is outside of said allowable range and decreased in
portion to said pulse amplitude when said pulse amplitude is within said
allowable range.
14. The apparatus of claim 10, wherein said criterion further comprises a
systolic interval template, the time between said positive peak and said
negative peak defining a systolic interval, said systolic interval
template defining an allowable systolic interval range, said rejection
means rejecting said positive and negative peaks when said systolic
interval is outside of said allowable systolic interval range.
15. The apparatus of claim 9, wherein said criterion comprises a systolic
interval template, the time between said positive peak and said negative
peak defining a systolic interval, said systolic interval template
defining an allowable systolic interval range, said rejection means
rejecting said positive and negative peaks when said systolic interval is
outside of said allowable systolic interval range.
16. The apparatus of claim 15, wherein said allowable systolic interval
range is adjustable.
17. The apparatus of claim 16, wherein said allowable systolic interval
range is initialized at a predetermined level.
18. The apparatus of claim 16, wherein said allowable systolic interval
range is increased in proportion to said systolic interval when said
systolic interval is outside of said allowable systolic interval range and
decreased in proportion to said systolic interval when said systolic
interval is within said allowable range.
19. The apparatus of claim 9, further comprising means for comparing an
average of said pulse amplitudes determined at a first and third pulse
with an average determined at a second and fourth pulse, said first,
second, third and fourth pulses being consecutive pulses of said arterial
blood flowing in said tissue.
20. The apparatus of claim 1, further comprising means for band-limiting
said signals.
21. The apparatus of claim 1, further comprising a differential,
current-to-voltage amplifier for amplifying said signals.
22. A method for processing a signal containing information about arterial
blood flowing in tissue, said signal having a relatively periodic
pulsatile component superimposed upon a varying baseline component, said
method comprising the steps of:
identifying a first portion of said signal during which the sign of the
slope of said signal changes from positive to negative;
locating the point along said first portion of said signal having the
largest amplitude, the point having said largest amplitude defining a
positive peak of said signal;
identifying a second portion of said signal during which the sign of the
slope of said signal changes from negative to positive;
locating the point along said second portion of said signal having the
smallest amplitude, the point having said smallest amplitude defining a
negative peak of said signal and the difference in signal amplitude
between said positive peak and said negative peak defining a pulse
amplitude; and
producing an output indicative of a characteristic of said arterial blood
in response to said positive and negative peaks.
23. The method of claim 22, wherein said step of identifying said first and
second portions of said signal further comprises the steps of:
producing an indication of the first derivative of said signal with respect
to time;
identifying an initial point on said signal at which the absolute value of
said indication crosses below a predetermined threshold; and
identifying the point on said signal at which the absolute value of said
indication first crosses back above said predetermined threshold.
24. The method of claim 23, wherein said step of producing an indication of
the first derivative of said signal with respect to time comprises
producing an autonormalized convolution derivative of the signal
determined in accordance with the relationship:
##EQU15##
where: n is the sample time for which the autonormalized convolution
derivative is determined;
j is a summation index;
V(j) is the amplitude of said signal at a sample time j; and
k is an integer used to define the range over which the samples are summed.
25. The method of claim 24, wherein S(n) is determined for k equal to
three.
26. The method of claim 22, further comprising the step of determining when
said slope of said signal has been positive for some predetermined time
before said step of identifying said first portion of said signal is
performed.
27. The method of claim 22, further comprising the step of determining the
time interval occurring between said positive peak and said negative peak
of said sample.
28. The method of claim 22, wherein said steps of identifying said first
portion of said signal, locating the point along said first portion of
said signal having the largest amplitude, identifying said second portion
of said signal, and locating the point along said second portion of said
signal having the smallest amplitude cooperatively produce a plurality of
pairs of said positive and negative peaks.
29. The method of claim 22, wherein said characteristic of said arterial
blood flow indicated by said output includes pulse rate and oxygen
saturation.
30. The method of claim 22, further comprising the step of rejecting the
positive and negative peaks when said peaks fail to satisfy a selection
criterion.
31. The method of claim 30, wherein said criterion comprises a pulse
amplitude template defining an allowable pulse amplitude range, said
positive and negative peaks being rejected when said pulse amplitude is
outside of said allowable pulse amplitude range.
32. The method of claim 31, wherein said allowable pulse amplitude range is
adjustable.
33. The method of claim 32, wherein said allowable pulse amplitude range is
initialized at a predetermined level.
34. The method of claim 32, wherein said allowable pulse amplitude range is
automatically increased in proportion to said pulse amplitude when said
pulse amplitude is outside of said allowable range and decreased in
proportion to said pulse amplitude when said pulse amplitude is within
said allowable range.
35. The method of claim 31, wherein said criterion further comprises a
systolic interval template, the time between said positive peak and said
negative peak defining a systolic interval, said systolic interval
template defining an allowable systolic interval range, said positive and
negative peaks being rejected when said systolic interval is outside of
said allowable systolic interval range.
36. The method of claim 30, wherein said criterion comprises a systolic
interval template, the time between said positive peak and said negative
peak defining a systolic interval, said systolic interval template
defining an allowable systolic interval range, said positive and negative
peaks being rejected when said systolic interval is outside of said
allowable systolic interval range.
37. The method of claim 36, wherein said allowable systolic interval range
is adjustable.
38. The method of claim 37, wherein said allowable systolic interval range
is initialized at a predetermined level.
39. The method of claim 37, wherein said allowable systolic interval range
is increased in proportion to said systolic interval when said systolic
interval is outside of said allowable systolic interval range and
decreased in proportion to said systolic interval when said systolic
interval is within said allowable range.
40. The method of claim 30, further comprising the step of comparing an
average of said pulse amplitudes determined at a first and third pulse
with an average determined at a second and fourth pulse, said first,
second, third and fourth pulses being consecutive pulses of said arterial
blood flowing in said tissue.
41. The method of claim 22, further comprising the step of band-limiting
said signals.
42. An apparatus for processing a signal containing information about
arterial blood, said signal having a relatively periodic pulsatile
component superimosed upon a variable baseline component, said apparatus
comprising:
positive peak identification means for employing an autonormalized
convolution derivative of said signal to identify a positive peak of said
signal;
negative peak identification means for employing an autonormalized
convolution derivative of said signal to identify a negative peak of said
signal; and
analyzing means, responsive to said positive and negative peak idenfication
means, for producing an output indicative of a characteristic of said
arterial blood.
43. A method of processing a signal containing information about arterial
blood, said signal having a relatively periodic pulsatile component
superimposed upon a variable baseline component, said method comprising
the steps of:
determining the autonormalized convolution derivative of said signal;
employing said autonormalized convolution derivative to identify a positive
peak of said signal and a negative peak of said signal; and
producing an output indicative of a characteristic of said arterial blood
in response to said positive and negative peaks. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to oximetry and, more particularly, to
signal-processing techniques employed in oximetry.
The arterial oxygen saturation and pulse rate of an individual may be of
interest for a variety of reasons. For example, in the operating room
up-to-date information regarding oxygen saturation can be used to signal
changing physiological factors, the malfunction of anaesthesia equipment,
or physician error. Similarly, in the intensive care unit, oxygen
saturation information can be used to confirm the provision of proper
patient ventilation and allow the patient to be withdrawn from a
ventilator at an optimal rate.
In many applications, particularly including the operating room and
intensive care unit, continual information regarding pulse rate and oxygen
saturation is important if the presence of harmful physiological
conditions is to be detected before a substantial risk to the patient is
presented. A noninvasive technique is also desirable in many applications,
for example, when a home health care nurse is performing a routine
check-up, because it increases both operator convenience and patient
comfort. Pulse transmittance oximetry is addressed to these problems and
provides noninvasive, continual information about pulse rate and oxygen
saturation. The information produced, however, is only useful when the
operator can depend on its accuracy. The method and apparatus of the
present invention are, therefore, directed to the improved accuracy of
such information without undue cost.
As will be discussed in greater detail below, pulse transmittance oximetry
basically involves measurement of the effect arterial blood in tissue has
on the intensity of light passing therethrough. More particularly, the
volume of blood in the tissue is a function of the arterial pulse, with a
greater volume present at systole and a lesser volume present at diastole.
Because blood absorbs some of the light passing through the tissue, the
intensity of the light emerging from the tissue is inversely proportional
to the volume of blood in the tissue. Thus, the emergent light intensity
will vary with the arterial pulse and can be used to indicate a patient's
pulse rate. In addition, the absorption coefficient of oxyhemoglobin
(hemoglobin combined with oxygen, HbO.sub.2) is different from that of
deoxygenated hemoglobin (Hb) for most wavelengths of light. For that
reason, differences in the amount of light absorbed by the blood at two
different wavelengths can be used to indicate the hemoglobin oxygen
saturation, % SaO.sub.2 (OS), which equals ([HbO.sub.2 ]/([Hb]+[HbO.sub.2
])).times.100%. Thus, measurement of the amount of light transmitted
through, for example, a finger can be used to determine both the patient's
pulse rate and hemoglobin oxygen saturation.
As will be appreciated, the intensity of light transmitted through a finger
is a function of the absorption coefficient of both "fixed" components,
such as bone, tissue, skin, and hair, as well as "variable" components,
such as the volume of blood in the tissue. The intensity of light
transmitted through the tissue, when expressed as a function of time, is
often said to include a DC component, representing the effect of the fixed
components on the light, and an AC pulsatile component, representing the
effect that changing tissue blood volume has on the light. Because the
attenuation produced by the fixed tissue components does not contain
information about pulse rate and arterial oxygen saturation, the pulsatile
signal is of primary interest. In that regard, many of the prior art
transmittance oximetry techniques eliminate the DC component from the
signal analyzed.
For example, in U.S. Pat. No. 2,706,927 (Wood) measurements of light
absorption at two wavelengths are taken under a "bloodless" condition and
a "normal" condition. In the bloodless condition, as much blood as
possible is squeezed from the tissue being analyzed. Then, light at both
wavelengths is transmitted through the tissue and absorption measurements
made. These measurements indicate the effect that all nonblood tissue
components have on the light. When normal blood flow has been restored to
the tissue, a second set of measurements is made that indicates the
influence of both the blood and nonblood components. The difference in
light absorption between the two conditions is then used to determine the
average oxygen saturation of the tissue, including the effects of both
arterial and venous blood. As will be readily apparent, this process
basically eliminates the DC, nonblood component from the signal that the
oxygen saturation is extracted from.
For a number of reasons, however, the Wood method fails to provide the
necessary accuracy. For example, a true bloodless condition is not
practical to obtain. In addition, efforts to obtain a bloodless condition,
such as by squeezing the tissue, may result in a different light
transmission path for the two conditions. In addition to problems with
accuracy, the Wood approach is both inconvenient and time consuming.
A more refined approach to pulse transmittance oximetry is disclosed in
U.S. Pat. No. 4,086,915 (Kofsky et al.). The Kofsky et al. reference is of
interest for two reasons. First, the technique employed automatically
eliminates the effect that fixed components in the tissue have on the
light transmitted therethrough, avoiding the need to produce bloodless
tissue. More particularly, as developed in the Kofsky et al. reference
from the Beer-Lambert law of absorption, the derivatives of the intensity
of the light transmitted through the tissue at two different wavelengths,
when multiplied by predetermined pseudocoefficients, can be used to
determine oxygen saturation. Basic mathematics indicate that such
derivatives are substantially independent of the DC component of the
intensity. The pseudocoefficients are determined through measurements
taken during a calibration procedure in which a patient first respires air
having a normal oxygen content and, later, respires air of a reduced
oxygen content. As will be appreciated, this calibration process is at
best cumbersome.
The second feature of the Kofsky et al. arrangement that is of interest is
its removal of the DC component of the signal prior to being amplified for
subsequent processing. More particularly, the signal is amplified to allow
its slope (i.e., the derivative) to be more accurately determined. To
avoid amplifier saturation, a portion of the relatively large DC component
of the signal is removed prior to amplification. To accomplish this
removal, the signal from the light detector is applied to the two inputs
of a differential amplifier as follows. The signal is directly input to
the positive terminal of the amplifier. The signal is also passed through
a low-resolution A/D converter, followed by a D/A converter, before being
input to the negative terminal of the amplifier. The A/D converter has a
resolution of approximately 1/10 that of the input signal. For example, if
the signal is at 6.3 volts, the output of the A/D converter would be 6
volts. Therefore, the output of the converter represents a substantial
portion of the signal, which typically can be used to approximate the DC
signal level. Combination of that signal with the directly applied
detector signal at the amplifier produces an output that can be used to
approximate the AC signal. As will be readily appreciated, however, the
process may be relatively inaccurate because the output of the A/D
converter is often a poor indicator of the DC signal.
Another reference addressed to pulse transmittance oximetry is U.S. Pat.
No. 4,407,290 (Wilber). In that reference, light pulses produced by LEDs
at two different wavelengths are applied to, for example, an earlobe. A
sensor responds to the light transmitted through the earlobe, producing a
signal for each wavelength having a DC and AC component resulting from the
presence of constant and pulsatile absorptive components in the earlobe. A
normalization circuit employs feedback to scale both signals so that the
DC nonpulsatile components of each are equal and the offset voltages
removed. Decoders separate the two signals, so controlled, into channels A
and B where lowpass filters remove the DC component from each. The
remaining AC components of the signals are amplified and combined at a
multiplexer prior to analog-to-digital (A/D) conversion. Oxygen saturation
is determined by a digital processor in accordance with the following
relationship.
U.S. Pat. No. 4,167,331 (Nielson) disclose another pulse transmittance
oximeter. The disclosed oximeter is based upon the principle that the
absorption of light by a material is directly proportional to the
logarithm of the light intensity after having been attenuated by the
absorber, as derived from the Beer-Lambert law. The oximeter employs
light-emitting diodes (LEDs) to produce light at red infrared wavelengths
for transmission through tissue. A photosensitive device responds to the
light produced by the LEDs and attenuated by the tissue, producing an
output current. That output current is amplified by a logarithmic
amplifier to produce a signal having AC and DC components and containing
information about the intensity of light transmitted at both wavelengths.
Sample-and-hold circuits demodulate the red and infrared wavelength
signals. The DC components of each signal are then blocked by a series
bandpass amplifier and capacitors, eliminating the effect of the fixed
absorptive components from the signal. The resultant AC signal components
are unaffected by fixed absorption components, such as hair, bone, tissue,
skin. An average value of each AC signal is then produced. The ratio of
the two averages is then used to determine the oxygen saturation from
empirically determined values associated with the ratio. The AC components
are also used to determine the pulse rate:
##EQU1##
wherein empirically derived data for the constants X.sub.1, X.sub.2,
X.sub.3 and X.sub.4 is stored in the processor.
The removal of the DC component of the signal, as typically employed by
prior art devices, leaves the AC pulsatile component substantially
centered about zero. This makes it easier to identify the peaks in the
pulsatile waveform corresponding to systole and diastole because they will
have substantially the same absolute value at each pulse. When the signal
being analyzed includes the slowly varying DC component, however, the
change in the DC level between pulses may cause the voltage level
associated with a particular feature of the pulsatile waveform to be
continually increasing or decreasing. The extraction of information from a
signal including both the AC and DC components is, therefore, more
complicated.
In addition, it should be noted that undesirable complicating features may
be present in the signal representation of the intensity of light
transmitted through the tissue. For example, the blood flow in certain
individuals may exhibit a secondary pressure wave following systole, known
as a dicrotic notch, that causes the signal to include a pair of slope
reversals that must be distinguished from the peaks associated with
systole and diastole. This phenomena is not exhibited by all individuals
and the morphology of the pressure wave may vary both between different
individuals and during the course of monitoring the same individual. Other
undesirable features to be rejected include those produced by movement of
the sensor relative to the patient, 50 Hz and 60 Hz power source
interference, 120 Hz fluorescent lighting interference, and
electrosurgical interference.
The present invention is directed to the accurate extraction of information
from such signals including both the AC and DC components in a manner
substantially unaffected by the complicating factors noted above.
SUMMARY OF THE INVENTION
The invention discloses an apparatus for processing signals containing
information about the pulse rate and oxygen saturation of arterial blood
flowing in tissue. These signals have a relatively periodic pulsatile
component superimposed upon a varying baseline component. The apparatus
includes an identifier for identifying a first portion of the signal
during which the sign of the signal's slope changes from positive to
negative. A positive peak locater then searches the first portion of the
signal to locate the point having the largest amplitude. This point is
defined as the positive peak of the signal. Similarly, a second portion
identifier is included to identify a second portion of the signal during
which the sign of the signal's slope changes from negative to positive. A
negative peak locater then examines the second portion of the signal to
locate the point having the smallest amplitude. This point is defined as
the negative peak of the signal. The difference in signal amplitude
between the positive and negative peaks is defined as a phase amplitude.
In accordance with a particular aspect of the invention, the first portion
and second portion identifiers include a derivative identifier for
producing an output indicative of the first derivative of the signal with
respect to time. A first marker then identifies an initial point on the
signal at which the absolute value of that output crosses below a
predetermined threshold. A second marker identifies the point on the
signal at which the absolute value of the output first crosses back above
the predetermined threshold. Preferably, the output produced by the
derivative processor is the autonormalized convolution derivative of the
signal determined in accordance with the relationship:
##EQU2##
where n is the sample time for which the autonormalized convolution
derivative is determined; j is a summation index;
V(j) is the amplitude of the signal at a sample time j; and
k is an integer used to define the range over which the samples are summed.
Preferably, S(n) is determined for k equal to three.
In accordance with additional aspects of the invention, a positive slope
detector is included to determine whether the slope of the signal has been
positive for some predetermined time before a first portion of the signal
is identified. The arrangement can be used to cooperatively produce a
plurality of pairs of positive and negative peaks. A period analyzer can
be included to determine the time interval occurring between the positive
and negative peaks of the sample and an additional analyzer included to
produce an output of the pulse rate and oxygen saturation from the
positive and negative peak information.
In accordance with another aspect of the invention, a rejection apparatus
is included that rejects positive and negative peaks when those peaks fail
to satisfy a selection criterion. For example, the criterion may include a
pulse amplitude template defining an allowable pulse amplitude range.
Thus, when the pulse amplitude computed for the signal is outside of this
allowable range, the positive and negative peaks producing that pulse
amplitude are rejected. The allowable pulse amplitude range may be
adjustable and initialized at a predetermined level. Preferably, the range
is increased in proportion to the pulse amplitude when the pulse amplitude
is outside of the range and decreased in proportion to the pulse amplitude
when the pulse amplitude is within the allowable range. A comparator may
also be included for comparing the average of the pulse amplitudes
determined at a first and third pulse with an average determined at a
second and fourth pulse.
The rejection criterion may also include a systolic interval template. This
template defines an allowable systolic interval range for comparison to
the time interval between the positive and negative peaks. If that
systolic interval is outside of the allowable systolic interval range, the
positive and negative peaks from which the systolic interval is determined
are rejected. Preferably, the allowable systolic interval range is
adjustable and initialized at a predetermined level. The allowable range
may be increased in proportion to the systolic interval when the systolic
interval is outside of the allowable range and decreased in proportion to
the systolic interval when the systolic interval is within the allowable
range.
As will be readily appreciated, the disclosed invention also is directed to
the methods employed by the apparatus described above and in its broadest
formulation includes the steps of identifying a first portion of the
signal, during which the sign of the signal's slope changes from positive
to negative, and locating the point along the first portion of the signal
having the largest amplitude. Then a second portion of the signal is
identified during which the sign of the signal's slope changes from
negative to positive and the point along the second portion of the signal
having the smallest amplitude is located. In this manner, positive and
negative peaks are defined having a pulse amplitude defined therebetween.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can best be understood by reference to the following portion
of the specification, taken in conjunction with the accompanying drawings
in which:
FIG. 1 is a block diagram of an oximeter including a sensor, input/output
(I/O) circuit, microcomputer, alarm, displays, power supply, and keyboard;
FIG. 2 is a block diagram illustrating the transmission of light through an
absorptive medium;
FIG. 3 is a block diagram illustrating the transmission of light through
the absorptive medium of FIG. 2, wherein the medium is broken up into
elemental components;
FIG. 4 is a graphical comparison of the incident light intensity to the
emergent light intensity as modeled in FIGS. 2;
FIG. 5 is a graphical comparison of the specific absorption coefficients
for oxygenated hemoglobin and deoxygenated hemoglobin as a function of the
wavelength of light transmitted therethrough;
FIG. 6 is a block diagram illustrating the transmission of light through a
block model of the components of a finger;
FIG. 7 is a graphical comparison of empirically derived oxygen saturation
measurements related to a variable that can be measured by the oximeter;
FIG. 8 is a schematic illustration of the transmission of light at two
wavelengths through a finger in accordance with the invention;
FIG. 9 is a graphical plot as a function of time of the transmission of
light at the red wavelength through the finger;
FIG. 10 is a graphical plot as a function of time of the transmission of
infrared light through the finger;
FIG. 11 is a more detailed schematic of the I/O circuit illustrated in the
system of FIG. 1;
FIG. 12 is a schematic diagram of a conventional current-to-voltage
amplifier circuit;
FIG. 13 is a schematic diagram of a differential current-to-voltage
preamplifier circuit included in the I/O circuit of FIG. 1;
FIG. 14 is a graphical representation of the possible ranges of the I/O
circuit output showing the desired response of the I/O circuit and
microcomputer at each of the various possible ranges;
FIG. 15 is a block diagram of a portion of the signal processing software
included in the microcomputer illustrated in FIG. 1, showing
band-limiting, data buffer, autonormalized convolution derivative (ANCD)
processor, peak processor, waveform template comparator, template adaptor,
and template descriptor blocks;
FIG. 16 is a more detailed block diagram of the band-limiting block shown
in FIG. 15;
FIG. 17 is a graphical illustration of the signal received by the ANCD
processor block of FIG. 15 as a function of time;
FIG. 18 is a table illustrating the output S(n) of the ANCD processor block
at a plurality of sample times;
FIG. 19 is a graph representative of the output of the ANCD processor block
over an interval corresponding to one pulse;
FIG. 20 is a more detailed block diagram of the ANCD processor block shown
in FIG. 15;
FIGS. 21 through 25 are more detailed block diagrams of the peak processor
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