CA1174733A

CA1174733A - Period measurement system

Info

Publication number: CA1174733A
Application number: CA000381973A
Authority: CA
Inventors: Toshinori Hirano; Masakazu Murase
Original assignee: Terumo Corp
Current assignee: Terumo Corp
Priority date: 1980-07-17
Filing date: 1981-07-17
Publication date: 1984-09-18
Also published as: DE3128171A1; FR2495330B1; FR2495330A1; US4463425A

Abstract

ABSTRACT OF THE DISCLOSURE

A period measurement system adapted to sample a biosignal at a predetermined sampling period, find an autocorrelation function for a variable T from the sampled biosignal, and then find an autocorrelation function corresponding to the value of a phase difference variable obtained by changing the variable T along a time axis. An autocorrelation func-tion found in this manner is stored in memory and then compared with a subsequent-found autocorrelation function. The compari-son operation is repeated for successive autocorrelation func-tions, thereby to find a peak of autocorrelation functions to measure the period of the biosignal.

Description

:~ 17~733 BACKGROUND OF THE INVENTION
This invention rela-tes to a period measurement system for measuring the period of a biosignal, particularly of a signal representative of the heartbeat of a fetus.
A conventional system for measuring the period of a biosignal relies upon a correlation system adapted to derive an autocorrelation function of the biosignal and to measure the period of the biosignal on the basis of the auto-correlation function.
A period measurement system that relies upon the correlation system operates by sampling a biosignal over a suitable sampling period, computing -the autocorrelation function of the biosignal from the sampled data, and detecting the peaks of the biosignal from the computed autocorrelation to thereby obtain the period.
The autocorrelation function indicates the similarity between two portions of the biosignal wave form at two different times separated by a certain time interval. In other words, it represents the degree of similarity of the repeating biosignal waveform. In order to ob-tain the autocorrelation function from the biosignal, we may write the autocorrelation function A (T) in terms of the biosignal f(t) which is a function of the time t. Thus, A(T) may be written.

A( T ) = T 2T r f(t) f (t + T ) d-t .............. (1) in which T represents the period of the biosignal and I
represents a time interval between two points in time separated by a given interval, the earlier point in time being a dm:~ll, - 1 -~ 17~733 ,, .
reference time in connection with the biosignal. In other wor~s, is a variable which applies a phase difference to the biosignal f(t) along the time axis.
The conventional period measurement system that relies upon the correlatlon function to measure the period of a biosignal has data obtained by sampling the biosiynal stored in a data memory composed of a plurality of shif-t regis-ters.
As each item of new data enters the data memory, items of data already stored to that poin-t are shifted to the immediately adjacent register, so that data is shifted sequentially from one register to another, with the oldest item of data in the last register being lost as each new input arise. A
multiplier and an adder constitute an autocorrelation function computing circuit which is adapted to compute an autocorrelation function using the data stored in -the data memory. A
correlation memory stores the results of the computation, namely the computed autocorrelation function. By repeating these computation and storage operations for n cycles, data defining the autocorrelation function is stored in the correlation memory. Peaks representing the periodicity of the auto-correlation function stored in the correlation memory are detected by a peak de-tector in order to obtain the period of the biosignal.

dm: - 2 -.1~

~ 17~3~

In the conventional measurement syst2m of the type described, however, the a~rangement is such that the phase difference variable ~ is varied in each single sampling cycle. It is therefore necessary to store in the correlation memory the results of each and every auto-correlation function computation covering the entire body of data spanning the range over which the variable T is varied in each s~mpling cycle. This means that the correlation memory must have a very large storage capacity. In addition, even when measuring a signal having a short period the computations described above are performed over a time interval corresponding to from two to three times the length of the period, so that much of this computation is without substantial meaning. This fact also calls for a correlation memory of a large storage capacity and is also disadvantayeous when viewed in terms of real-time processing owing to the fact that a large number of substantially meaningless computations are performed.
SUMMAR~ OF THE INVENTION
Accordingly, it is an object of the present invention to provide a system for measuring the period of a biosignal, which system is free of the aforementioned defects so-that it may enable period measurement with a correlation memory of a smaller storage capacity and with a computation time period that is shortened to the maximum possible-extent.
Another object of the present invention is to provide a period measurement system that enables correct measurement of the period by detecting true peaks, which correspond to the period of a biosignal, from a plurality of pea~s ob-tained from an autocorrelation function.

kh/~

--`` 1 17~733 ~ o these ends, the present invention provides a period measurement system for measuring the period of a biosignal, the period having a minimum value, comprising:
data memory means for storing sampled input biosignal data and for shifting the stored biosignal data when new biosignal data is entered; autocorrelation function computation means for computing an autocorrelation function A ~T) of the sampled input biosignal data, given by the equation 1 n A(~ f(k) f(k+~) n k=l wherein a phase difference variable I is speci~ied over each computation cycle.'corresponding to k=l to n, said varia~le T
being set to essentially the'minimum value of ~he biosignal period bei'ng mea'sured for an initial computation cycle, and thereafter incremented by a certain value in a~vance of each succes'sive computation cycle of the autocorrelation function;
peak detection means for detecting a peak by comparing an autocorrelation function value'previously comp~ted by the function computation means with the autocorrel~tion function value most.recently computed by the'function c~mputation means;
and period computation means for computing the period o the biosignal based on the'phase differ'ence variable'for which a peak is detected b~ the peak detection means.
The present invention may also be conside.red as providing a method of measuring the'period of ~ biosignal, comprising the steps of: obtaining biosignal data corresponding to a biosignal the period of which is to be measurea; repeatedly computing an autocorrelation function of the biosignal data kh/,~.,.1, `` ~ 174733 including setting a minimum phase difference, deriving pairs of values of the biosignal data by selecting the data according to the minimum phase difference, summing the products of the derived pairs of values thereby pro-viding a computed output, and incrementally advancing the phase difference between the selected data for each successive computing step; dètecting a peak in the computed output provided by the computing step; and determining the period of the biosignal according to the total phase difference between the selected biosignal data for which a peak in the computed output is detected.
BRIEF DES~RIPT l ON OF THE DRAWINGS
Fig. 1 is a biosignal waveform diagram useful in describing measurement of a period by means of an auto-correlation system;

kh/,,~
.-~a- S

Fig. 2 is a block diagram showing, in simplified form, the construction of period measuring apparatus to which the conventional system of period measurement is applied;
Fig. 3, on the same sheet as Fig. 1, is an illustrative view useful in describing the manner in which an autocorrelation function is computed in a period measurement system according to the present invention;
Fig. 4 is a fetal heartbeat signal waveform diagram useful in describing a case where -the period measurement signal of the present invention is applied to measurement of the period between fetal heartbeats;
Fig. 5 is a waveform diagram useful in describing a system adapted to continue autocorrelation function computation for a fixed period of time following detection of a peak for the purpose of confirming whether or not the detected peak is a true peak;
Fig. 6 is a block diagram showing, in simplified form, the construction of a period measurement apparatus to which the period measurement system of the present invention is applied;
Fig. 7 is a block diagram useful in describing the storing of sampling data in a data memory, as well as the reading and later processing of the data;
Fig. 8 is a block diagram showing the detailed construction of a peak detector, peak level checking circuit and peak confir~ation circuit included in the period measure-ment apparatus shown in Fig. 6; and Fig. 9 is a block diagram useful in describing the details of a reference level generator.

-~ 6 ~ :L7~33 DESCRIPTION OF THE PREFERRED EMBODIMENT
The autocorrelation function indicates the similarity between two portions of the biosignal waveform at two different times separated by a certain time interval. In other words, it represents the degree of similarity of the repeating biosignal waveform. This can be better understood from Fig. 1, wherein it is seen that if a portion Ml which repeats at a certain period T is shifted along the time axis by an interval of time which is equal to the period T, the portion Ml will be super-imposed on the immediately succeeding portion M2 with maximumaccuracy.
Reference will now be had to Fig. 2 to describe the conventional period measurement system that relies upon the correlation function to measure the period of a biosignal, specifically a signal representative of the heartbeat of a fetus, which signal will be referred to as a "heartbeat signal" hereafter.
In Fig. 2, a probe 2 is brought into contact with, say, the abdomen of a female subject to extract the fetal heartbeat signal for the purpose of measurement. The heart-beat signal so detected has its waveform suitably processed in a preprocessing circuit 3 and then sampled at a pre-determined sampling period in a sampling circui-t 4. The data obtained by sampling the heartbeat signal is stored in a data memory 6 composed of a plurality of shift registers. As each item of new data enters the data memory 6, items of data already stored up to that point are shifted to the immediately adjacent register, so that data is shifted sequentially from one register to another, with the oldest item of data in the last register being lost as each new input arrives. A multiplier 8 and an adder 10 constitute an auto-correlation function computing circuit which is adapted to conpute an autocorrelation function using the data stored in the data memory 6. A correlation memory 12 stores the results of the computation, namely the computed autocorrelation function.
Thus the autocorrelation function is computed by the multiplier 8 and the adder 10 on the basis of the data stored in the data memory 6. The computation is performed on the basis of single sampling-cycle divisions and, for each item of data Xl, X2, X3 ..., proceeds in the manner Xl.Xs+l+Al~ Al, l s+2 2 A2, ..., Xl.Xs+m+Am ~ Am, the result of each computation being stored sequentially in the correlation memory 12. By repeating these computation and storage operations for n cycles, data defining the autocorrelation function is stored in the correlation memory 12. Peaks representing the periodicity of the autocorrelation function stored in -the correlation memory 12 are detected by a peak detector 14 in order to obtain the period of the biosignal.
Fig. 3 is useful in describing a period measurement system in accordance with -the present invention, and illustrates the system employed in computing the autocorrelation function of a biosignal.
If we let f(k) (where k = 1, 2, 3 ..., n) denote the data obtained by respective sampling operations applied to a biosignal at a fixed sampling period Ts, then the autocorrelation function A(r) of the biosignal will be expressed by equation - 7a -~ 17~733

(2), 1 n A(~ f(k).f(k+T) ~ (2) k=l in which T stands for a variable that applies a phase difference to the biosignal along the time axis, n stands for the -total number of mul-tiplications or additions in one sampling cycle, and k stands for a sampling ordinal number.
Rxpanding equation (2) gives us A(T) = n {f(l)f(l+l) + f(2)f(2+~) + f(3)f(3+~) + ...... + f(n)f(n+l)} .................... (3).
In equation (3), f(l) represents the most recent data.
Equation (3) means that the,autocorrelation function of a biosignal is found by summing the product f(k)f(k+l) a total of n times by changing k, where f(k)f(k+~) is the product of sampled data f(k) and f(k-~T) at two points in time separated by the phase difference variable T along the time axis.
More specifically with reference to Fig. 3, assume that plural items of data are acquired by sampling operations conducted dm~ 7b -, ~

at intervals equal to the sampling period Ts shown along the time axis, and that the phase difference variable T iS given by _. To compute the autocorrelation function A(m~, two items of sampling data displaced from each other by _, such as f(l) and f(m+l), f(2) and f(m+2), f(n) and f(m+n) ..., are multipled to give the products f(l)f(m+l), f(2)f(m+2) ... f(n)f(m+n). These products are then added together for the n sampling operations in the sampling cycle to give the autocorrelation function A(m).
The system adopted in the present invention computes an autocorre-lation function for a certain value of the variable 1, which ap-plies the phase difference to the biosignal on the time axis~ in one sampling cycle of the biosignal, changes the value of the phase difference variable I along the time axis in conformance with the progress of the sampling cycles, and then computes an autocorrelation unction which corresponds to each sampling cycle. The results of the most recent autocorrelation function computation is stored in memory, whereby the signal peaks and signal period can be found.
This will now be described in greater detail taking as an example a case in which the invention is applied to the period measurement of a fetal heartbeat signal.
The period of a fetal heartbeat ranges from approximately 300 to 1,500 milliseconds. Therefore, to compute an autocor-relation function over the range~the entire period of the heart beat signal, it is necessary to find the autocorrelation func-tion by varying the period of measurement from the minimum value of 300 milliseconds to the value of 1,500 milliseconds. In other words, it is necessary to change the phase difference ~ 174~3.~
g Yariable T over the range of 300/TS to 1,5QQ/Ts in equation (2).
Since the autocorrelation function will have a maximum peak with-in this range when the phase difference variable T iS set to the heartbeat signal period T, or to a period of time which is an interval multiple of the period T, the true period of the heart-beat signal can be found if the peak corresponding to the period is detected.
In accordance with the period measurement system of the present invention, the autocorrelation function computation is performed with each sampling cycle serving as a single division.
Ordinarily, the shortest period of a fetal heartbeat signal is approximately 300 milliseconds. As will become clear from the explanation given below, the computation of the autocorrelation function starts from the smallest possible value of the period of measurement, namely 30Q milliseconds, in order to extract the results of measurement over a time interval which is equiva-lent to the period. That is, in the first sampling cycle, the autocorrelation function is first found with regard to the in-terval of 300 milliseconds corresponding to the minimum value of the fetal heartbeat period. In this case the phase difference variable T iS found from T = 300/TS, SO that the variable T will be 60 if we set the sampling period Ts to five milliseconds.
Then, with a sampling period Ts of five milliseconds, the time permitted for a computation concerning the sampled data will be within about five milliseconds. Hence, n sampling operations are carried out under the conditions T = 60 and sampling period Ts = 5 milliseconds, and the autocorrelation function A(60) is found for r = 60. The autocorrelation function A(60) is found 117~733 by the method used to find the autocorrelation function A(T) in Fig. 3.
The foregoing will now be described with reference to Fig. 4 which shows a heartbeat signal. Sampling is conducted up to a total of n times at intervals of five milliseconds, which is equal to the sampling period Ts (i.e., at intervals defined by Ts = 5 milliseconds). Items of data f(l), f(2), f~3), f(4) ...
f(n) obtained by each sampling operation are stored in memory.
Next, two items of data f(k) and f(k+60) obtained at two dif-ferent sampling times displaced from each other by the phase difference variable I = 60 are multiplied together, and a series of these products, such as f(l)f(1+60), f(2)f(2+60) ... are added together to give the sum total of the products. Thus, it is possible to find the autocorrelation function A(60) for the case in which the phase difference variable ~ is set to 60.
The value of A(60) indicates the degree of periodicity in con-nection with I = 60 (i.e., for a period of 300 milliseconds).
The value of A(60) is stored in memory for the purpose of comparison until the autocorrelation function is obtained in the next sampling cycle.
Next, the computation is performed for the second sampllng cycle, wherein the value of the phase difference variable is advanced by one to A(61). In other words, in the second sampl-ing cycle the autocorrelation function is computed for a period of 305 milliseconds~ The computation of the autocorrelation function A(61) is carried out in essentially the same manner as the computation of the autocorrelation function A(60) and is not described again here. The autocorrelation function A(61) ~ ~7~1~33 obtained from the computation for the period of 305 milliseeonds is compaxed with the autocorrelation function A(60~ for the period of 300 milliseconds, as previously computed and stored in memory. Thus, the system adapted herein computes an auto-correlation function for a certain value of the phase difference variable T in one sampling cycle, stores in memory solely the result of this computation, and then compares this result with the result of an autocorrelation function computation for a phase difference variable whose value is advanced by one count in the next sampling cycle. Accordingly, only the result of the autocorrelation function computation in the most recent cycle need be stored in memory. The system of the present in-vention therefore makes it possible to reduce the required me-mory capacity of the correlation memory in comparison with the conventional system which requires that the correlation memory stores the results of each and every autocorrelation function computation covering the entire body of data spanning the range over which the phase difference variable ~ is varied in each sampling cycle.
In order to deteet the signal peaks in accordance with the present invention, the value which has previously been computed and stored for the preceding sampling cycle is com-pared with the value computed for the next sampling cycle.
The signal peaks are then detected by repeating this compari-son process and examining the change in state. When there is a ehange in state from a larger value to a smaller value between two continuous sampling cycles, this indicates the detection of a peak in the first of the two eycles. In effecting the peak ~ 17~733 detection operation, the comparison is made solely with the im-~ , e mediately preceeding computed value, in accordance with~descrip-tion given above. However, it is obviously also possible to store computed values relating to several cycles and to perform a comparison among these values if desired.
In the embodiment described above a microprocessor can be employed owing to the reduction in the required storage capacity and the reduction in the number of computations. It therefore becomes possible to effect highly accurate autocorrelation func-tion computations and system control. However, it should be noted that the foregoing operation unfortunately detects not only an intrinsic peak corresponding to the signal period, but other peaks that generally tend to exist in the vicinity of the in-trinsic peak. Therefore, in order to measure the period with a high order of precision, means must be provided to detect the intrinsic or true peak, which corresponds to the signal period, from among the several peaks that may exist.
In order to determine whether a detected peak has the po-tential of being a true peak, two steps are required. First, a level check operation is performed on the basis of a minimum level determined to serve as a threshold value, and second, when a peak has been detected, the autocorrelation function computation is continued for a length of time which corresponds to the smallest period of measurement, to confirm that no peak larger than the detected peak exists in the interval over which the computation has been continued. These two steps enable the detection of a Irue peak.
The level check operation comprises the steps of determining ~ :~7~733 the threshold Yalue of a leyel used in judging whethex a peak has the potential of being a true peak, and then judging whether the level of a peak exceeds the threshold value, whereby it is decided whe-ther the detected peak, which has the potential of being a true peak, should indeed be regarded as a true peak.
In the example of this embodiment, the threshold value is set to ~ one-half the value of a peak employed in an immediately preceding measurement, namely to ~one-half the value of the most recent true peak, and only the peak whose level exceeds the set threshold value is judged to be a peak which has the potential of being a true peak.
The threshold value need not necessarily be set to ~ one-half the value of the most recent true peak, but should be set to the optimum value chosen in accordance with the condition of the signal at that time. In general though the peak value of the true peak that indicates the period of the signal is influenced by the strength and waveform of the signal, noise poses a particular problem. Specifically, the lower the noise the larger and more distinct the true peaks present themselves, whereas the greater the noise the smaller the true peaks appear.
In fact, the value of a true peak in the presence of considerable noise may even be smaller than a false peak in the vicinity of a true peak when there is little noise.
It is for this reason that the threshold value must be set in accordance with the signal conditions that exist during peak detection. In this embodiment, in addition to the level check described above, the autocorrelation function computation is continued for a fixed interval of time following the detection 1 ~ 7~733 of a peak, and a check is performed to determine whether a peak larger than the detected one exists within said fixed interval.
It has been stated above that peaks obtained from an auto-correlation function include, in addition to a true peak that corresponds to the signal period, several peaks located in the vicinity of the true peak. The true peak must be detected among the several peaks in order to measure the period correctly.
Since the peaks in the vicinity of the true peak are generally located quite close to the true peak, it is possible to prevent the former peaks from being detected as the true peak by prolong-ing the autocorrelation function computation for a fixed interval following the detection of a peak and then by checking whether a peak larger than the detected one exists within said fixed in-terval. It should be noted that it is sufficient if the fixed interval is set to an interval of a value corresponding to the minimum period of measurement. Accordingly, in this embodiment, once a peak has been detected the computation of the autocorrela-tion function is prolonged for an interval that corresponds es-sentially to the minimum value of the period of measurement, e c ~ 6/5 namely to 300 }~Y~e~s.
The foregoing will be described in connection with Fig. 5.
If we assume that pea~ Pl is detected at time tll (present time), the computation of the autocorrelation function will be continued for 300 milliseconds after time tll, namely until time tl2. As Fi~. 5 shows, a peak P2 larger than peak Pl is detected at time t21 in the 300-millisecond interval between time tll and time tl2. Under such condition, peak Pl is discarded and the auto-correlation function computation is continued for another 300 1~ 174733 milliseconds starting from the new peak P2, that is, until time t22. Peak P2 is detected as the true peak when no peak larger than P2 is found to exist in the latter 300-millisecond interval.
It will be noted in Fig. 5 that a peak P3, of a smaller ampli-tude than peak P2, is found at a certain time t3l within the 300-millisecond interval between the time tl2 at which P2 is detected, and time t22. However, the peak P3, whose amplitude is smaller than that of peak P2, is not detected as a peak having the potential of being a true peak. Thus, the peak P2 obtained at time t2l is detected as being a true peak indicative of the period when 300 milliseconds have passed starting from time t2l, that is, when time t22 has been reached. At this point in time the autocorrelation function computation ends and the period is calculated. The value of the phase difference variable T of the true peak found in this manner corresponds to the period. Letting Ts be the data sampling period, the period T is found from the computation formula T = T X Ts. The next period measurement again starts from ~ = 60 (corresponding to the period of 300 milliseconds) and proceeds in the same manner.
Thus, the correct period of the biosignal is measured in the manner described above.
In the above, the fact that autocorrelation function starts from 300 milliseconds on the autocorrelation (T) axis and ends at a point equivalent to the biosignal period T ~ 300 milli-seconds, is extremely important in terms of true peak detection and the point in time at which the results of measurement are delivered as an output.
~r~/e ~i~ First, with regard to true peak detection, a~e~ peak ~; 17~733 cannot exist below the shorte$t possible period of the biosignal undergoing measurement, and a true peak also cannot exist in an interval within the shortest period. Therefore, peaks which are confirmed in this manner can be said to be those which have abso-lutely no possibility of indicating peaks of a period which is twice the true period.
In connection wi-th the output timing of the results of measurement, the effect of the arrangement mentioned above is to enable the results-of measurement to be delivered in synchronism with the true period of the biosignal. More specifically, period measurement starts from 300 milliseconds, which is the short pos-sible period. On the other hand, 300 milliseconds, equivalen-t to the shortest possible period, is set as the true peak confirmation interval, so that the results of measurement can consequently be delivered in a time interval which is equivalent to the true period of the biosignal. For example, if the true period is 500 milli-seconds, the results of measurement will be output every 500 milli-seconds. When the period changes the output intarvals change cor-respondingly. This is because the autocorrelation function computa-tion proceeds at real-time on the correlation axis if the auto-correlation function computation interval coincides with the data sampling period, that is, because the correlation computation, for a length of time from the shortest period of the biosignal until a time represented by the sum of the shortest period and the true period, is performed within a time equivalent to the true period of the biosignal.
Fig. 6 shows, in simplified form, the construction of a period measurement apparatus for practicing the period measurement ~ 17 -system described above in connection with Figs. 3 through 5.
With reference now to Fig. 6, a transducer is brought into W

contact with the abdomen/of a female subject in order to detect the fetal heartbeat signal. A sampling circuit 24 is connected to the transducer 22 through a preprocessing circuit 23. The heartbeat signal detected by the transducer 22, after having its waveform suitably shaped by the preprocessing circuit 23, is sampled by the sampling circuit 24 at a predetermined sampl-ing period and is subjected to an analog-to-digital conversion (AD conversion) by the sampling circuit. The heartbeat signal therefore emerges from the sampling circuit 24 as a digital signal. A data memory 26 is connected to the sampling circuit 24 and stores the sampled data obtained from the sampling cir-cuit. The data memory 26 is composed of a plurality of shift registers and operates as follows. As each new item of data enters the data memory, items of data already stored up to that point are shifted byte-to-byte, with the oldest item of data be-ing lost as each new input arrives. A multiplier 28 is connected to the data memory 26, and an adder is connected to the multi-plier 28. More specifically, the data memory 26 or shift regis-ter comprises a l-byte ~8-bit) parallel register which is adapted to "shift in" the sampled data in digital form. It is so con-structed that arbitrary positional data specified by signal line ad can be read out therefrom. Included in the data memory 26 are a random access memory (RAM) with a read and write capability, and a controller for the RAM.
The multiplier 28 and an adder 30 constitute a computation circuit for computing the autocorrelation function. This circuit ~ ~7~733 computes the autocorrelation function of a biosignal, namely the fetal heartbeat signal, by performing the computation speci-fied essentially by equation (3) using the data stored in the data memory 26. In other words, the computation of an auto-correlation function is performed in connection with a phase difference variable T of a certain value in each sampling cycle.
To be more specific, two items of data, which represent two posi-tions on the time axis separated from each other by the phase difference variable T ~ are produced by a control circuit 42 in a manner to be described later, and the two items of data are stored at two addresses in the memory section of the data memory 26 (the addresses giving the memory locations, which are indi-cated by the hatch marks in block 26 of Fig. 7). To compute the autocorrelation function, the two items of stored data are c~ ~e rC~
multiplied and the product is plantcd in an accumulator located in the adder 30. The number of multiplication operations for one phase difference variable T iS _ in equation (3), as will readily be understood from the foregoing description, so that the number of additions is n. Completing n additions in effect computes the phase difference variable T as a value which is n times the autocorrelation function. However, since n is con-stant, the data which is computed is proportional to the auto-correlation function in equation (3), so that, in essence, the autocorrelation function is calculated.
A peak detector 32 is connected to the adder 30 and is capable of storing a small quantity of data and of performing a comparison operation. An input to the peak detector 32 is the value of the autocorrelation function calculated by the - ~ ~ 7~733 computation circuit constructed by multiplier 28 and addex 30.
The peak detector 32, as will be described in more detail later, stores the previously computed value of the autocorrelation func-tion for one sampling cycle, and compares this value with the newly arrived computed value of the autocorrelation function for the next sampling cycle. The peak detector then stores the newly arrived computed value if it is larger than the pre-viously stored computed value. Since the peak detector 32 need store only the computed value of the autocorrelation function for the most recent sampling cycle and the value of the phase difference variable T at that time, a small mernory capacity will suffice. Thus, the stored computed value for one sampling cycle is compared with the computed value of the autocorrelation func-tion for the next sampling cycle by means of a comparator, there-by allowing the change in values for the two sampling cycles to be investigated. When the result of the comparison operation shows a transition from a higher to a lower value, this indicates the existence of a peak in the first of the two sampling cycles.
The peak detector 32 performs a comparison between a peak detec-tion signal and a reference level. In order to set the reference level, use may be made of a level ~hich is, for examp1e, one-half the previously measured true peak value, as described earlier.
If the detected peak exceeds the reference level, and it is con-firmed that no peak larger than the detected peak is present with-in a fixed time interval measured from the instant at which the detected peak exceeds the reference level (which fixed time in-terval is 300 milliseconds in this embodiment), then the peak detector 32 judges that the detected peak is a true peak and ~ 17473~

issues a true peak detection signal.
Connected to the peak detector 32 is a period computation circuit 38 which, upon receiving the true peak detection signal from a peak detector 32, computes the period on the basis of the value of the phase difference variable in the autocorrelation function at the time that the peak i5 obtained, said value be-ing preserved in a register located within the peak detector.
Connected to the period computation circuit 38 is a heart-beat computation circuit 40 which computes the number of heart-beats on the basis of the period computed by the period computa-tion circuit 38.
The heartbeat computation circuit 40 is connected to a control circuit 42, having a display device 44, such as an arrangement of light-emitting diodes (LED), connected thereto.
The display device 44 displays the number of heartbeats in the heartbeat signal on the basis of the signal obtained from the heartbeat computation circuit 40 through the control circuit 42.
There may be occasions where the signal from the heartbeat com-putation circuit 40 includes a noise component, or where the probe for heartbeat detection slips. The control circuit 42 therefore is adapted to so control the signal from the heartbeat computation circuit 40 as to prevent it from entering the display device 44 on such occasions, thereby assuring that an erroneous heartbeat number will not be displayed.
The control circuit 42 is further adapted to deliver clock pulses to the sampling circuit 24, thereby to control the tim-ing of the sampling operation effected by the sampling circuit.
In addition, the control circuit sends the multiplier 28 a signal, .L 1 7 ~ 7 3 3 indicative of the value of the phase difference variable, upon each sampling operation. The value of the phase difference variable successively advances as the sampling cycles progress, starting from a time which essentially corresponds to the mini-mum value of the heartbeat signal period. The multiplier 28 is adapted to read, from the data memory 26, two items of data separated by the value of the phase difference variable desig-nated by the signal from the control circuit 42, and to find the product of the two items of data. The control circuit 42 sends a timing signal to the adder 30 which, on the basis of the timing signal, adds together the results of the computation operations executed by the multiplier 28. In other words, the multiplier 28 and adder 30, under the control of the control circuit 42, read data from the data memory and compute the auto-correlation function essentially as shown by equation (3).
Connected to the control circuit 42 is a reference level detector 46. The latter, in accordance with a timing signal delivered by the control circuit 42 at a suitable time interval, is adapted to detect the optimum reference level (zero level) for the purpose of attaching a positive (+) or negative (-) sign to the sampled data, and to send a signal indicative of the optimum reference level to the sampling circuit 24 In attaching the signs to the data, the more balanced the polarity of the data, the more reliable will be the periodicity of the autocorrelation function. The reference level detector 46 is provided for the purpose of finding the optimum value for achieving this end. Specifically, the detector 46 finds the optimum value of the reference level by detecting the maximum

3 1~33 value and mini~um value, or the average ~alue, of the data dur-ing sampling.
The peak detector 32 may have the construction shown in Fig. 8. Here a memory 52 comprises two memory units, one for storing the value of the autocorrelation function, and the other for storing the value of the phase difference variable.
More specifically, the memory 52, under the control of a write signal from a comparator 54, stores the value of the auto-correlation function computed by the adder 30, and the value of the phase difference variable obtained from the control-circuit 42. The comparator 54 is adapted to compare the newly computed value of the autocorrelation function obtained from the adder 30 and the most recent, largest computed value of the autocorrela-tion function previously stored in the memory 52, and to deliver the write signal to the memory 52 if the newly computed value of the autocorrelation function is the larger of the two values, whereby the contents of the memory 52 are replaced by the newly computed value of the autocorrelation function and by the value of the phase difference variable obtained from the control cir-cuit 42. When the value of the autocorrelation function changes from an increasing to a decreasing one upon repeating the afore-said comparison operation, the comparator 54 judges that a peak has been detected and therefore issues a signal. The computed en f er~O
value of the autocorrelation function planted in the memory 52 is sent to a comparator 56 for checking the peak level. The comparator 56 compares this value with a reference level received from a reference level generator 58. The latter is set by the output timing of a counter 62 at such time that the preceding 1L 1 ~ 4 7 3 ~

true peak is detected, whereby it stores a leyel equal to, say, one-half the value of the true peak detected by the preceding measurement. It is this level which the reference level genera-tor delivers as the reference level. Obtaining one-half the value of a true peak is accomplished through the technique shown in Fig. 9. Specifically, this is accomplished by shifting the output data from the memory 52 one bit to the LSB (Least Signifi-cant Bit) side, and connecting the data to the comparator 56, which is a magnitude comparator. If the result of the compari-son is such that the computed value of the autocorrelation function stored in the memory 52 is of a level that exceeds the reference level, the comparator 56 issues a signal. An AND
gate 60 takes the logical product of the outputs from the com-parators 54, 56. A positive-going transition in the output of the AND gate 60 resets the counter 62 and sets the value of the phase difference variable 1, which has been stored in the memory 52, in a register 64. When the clock pulses being counted by the counter 62 reach a number which corresponds to a fixed time period, such as 300 milliseconds, the counter issues a signal.
This output signal from the counter 62 indicates that a true peak has been detected, so that the value of ~ which has been set in the register 64 is delivered to the period computation circuit 38. The latter circuit computes the period by taking the product of the variable I and the sampling period arriving from the control circuit 42 on'signal line. By way of`
example, if the sampling period is five milliseconds and T iS
60 milliseconds, the period is computed as being 300 milli-seconds. The obtained period is delivered to the heartbeat ~ ~7~73~

counter circuit 40 where the number of heartbeats for a period one minute is found by dividing 60 x lQ3 (ms~ by the period (ms).
The number of heartbeats found in this manner is then applied to control circuit 42 and displayed on the display device 44 under the con-trol of the control circuit.
Thus, peaks are detected and checked through the foregoing arrangement and operation to assure the extraction of peaks that are true.
In accordance with the present invention as described above, measurement of a biosignal period is performed through the steps of computing an autocorrelation function for a certain value of the phase difference variable T in one sampling cycle of the biosignal, changing the value of the phase difference variable I on the time axis in conformance to the progress of the sampling cycles, computing an autocorrelation function in each sampling cycle, storing solely the result of the auto-correlation function computation for the initial cycle of two consecutive sampling cycles, comparing this result with the result of the autocorrelation function computation for the following cycle, and detecting a peak from the increase and decrease in the result of comparison, whereby the period of the biosignal is measured. Such an arrangement makes it possible to greatly reduce the storage capacity for the results of the autocorrelation function computations, and to eliminate mean-ingless autocorrelation computations for long intervals of time that may be two or three times as long as the actual bio-signal period, thereby allowing data to be processed on an approximately real-time basis.

~ ~7~733 Furthermore, in accordance with another feature of the invention, the correct period can be measured through the steps of beginning -the autocorrelation function computation essentially from the minimum value of the period of biosignal measurement, continuing the autocorrelation computation for an interval corresponding to said minimum value following the detection of a peak, and confixming that there is no peak larger than the initial peak in said interval corresponding to the minimum value measured from the point of initial peak detec-tion, thereby to detect that the initial peak is a true peak.
Thus it is possible to reliably detect solely a true peak which indicates the intrinsic period of the biosignal, thereby enabl-ing measurement of the correct period. Moreover, since the range of autocorrelation function computation is restricted to an area from substantially the minimum value mentioned above to a range of values represented by the sum of the true bio-signal period and confirmation interval (such as said minimum value), the invention has the effect of eliminating meaningless computations and of permitting real-time processing. In addi-tion, the results of measurements can be delivered at a time interval which is equivalent to the period of the signal under-going measurement.
As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that ~he invention is not limited to the specific embodiment thereof except as defined in the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A period measurement system for measuring the period of a biosignal, said period having a minimum value, comprising:
data memory means for storing sampled input biosignal data and for shifting the stored biosignal data when new biosignal data is entered;
autocorrelation function computation means for computing an autocorrelation function A (T) of the sampled input biosignal data, given by the equation wherein a phase difference variable T is specified over each computation cycle corresponding to k=l to n, said variable T being set to essentially the minimum value of the biosignal period being measured for an initial computa-tion cycle, and thereafter incremented by a certain value in advance of each successive computation cycle of the auto-correlation function;
peak detection means for detecting a peak by comparing an autocorrelation function value previously com-puted by said function computation means with the auto-correlation function value most recently computed by said function computation means; and period computation means for computing the period of the biosignal based on the phase difference variable for which a peak is detected by said peak detection means.

2. A period measurement system according to claim 1, including means for continuing the computation of the autocorrelation function for a fixed period of time following the detection by said peak detection means of a certain peak, and means for confirming that no peak larger than said certain peak exists in said fixed period of time, wherein said certain peak is determined to be a true peak.

3. A period measurement system according to claim 2, in which said fixed period of time is set by said continuing means to essentially the minimum value of the period of the biosignal being measured wherein said period computation means computes the period of the biosignal within a time period which is substantially equivalent to the period of the biosignal.

4. A period measurement system for measuring the period of a biosignal, said period having a minimum value, comprising:
means for extracting a biosignal;
autocorrelation function computation means coupled to said extracting means for computing an autocorrelation function of the biosignal wherein said autocorrelation function varies according to position along an autocorrelation axis;
peak detection means coupled to said autocorrelation function computation means for detecting a peak from the auto-correlation function;
period computation means coupled to said peak detection means for computing a period of the biosignal from that position on the correlation axis at which a peak is detected by said peak detection means;
means for continuing the computation of the autocorrelation function for an interval corresponding essentially to the minimum value of the period of the measured signal, which interval begins with detection of a certain peak by said peak detection means; and means for confirming that no peak larger than said certain peak exists in said interval which corresponds to said minimum value and which begins with the detection of said certain peak, wherein said certain peak is determined to be a true peak.

5. A period measurement system according to claim 4, including means for setting a threshold value for detection of a peak by said peak detection means wherein the value of a determined true peak serves as a reference, said peak detection means detecting as peaks only those peaks that exceed the threshold value.

6. A method of measuring the period of a biosignal, comprising the steps of:
obtaining biosignal data corresponding to a bio-signal the period of which is to be measured;
repeatedly computing an autocorrelation function of the biosignal data including setting a minimum phase difference, deriving pairs of values of the biosignal data by selecting the data according to the minimum phase difference, summing the products of the derived pairs of values thereby providing a computed output, and incrementally advancing the phase difference between the selected data for each successive computing step;

detecting a peak in the computed output provided by said computing step; and determining the period of the biosignal according to the total phase difference between the selected biosignal data for which a peak in the computed output is detected.

7. The method of claim 6, including continuing said computing step for a continued interval corresponding essentially to a minimum period of the biosignal being measured, starting said continuing step upon the detection of a peak in the computed output provided by said computing step, confirming that no peak larger than the detected peak exists in the continued interval thus determining that the detected peak is a true peak.

8. The method of claim 7, including establishing a threshold value for the detection of a peak in said detecting step, and basing the threshold value on the value of the determined true peak.