Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – With auxiliary means to condition stimulus/response signals
Reexamination Certificate
2002-07-02
2004-10-26
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
With auxiliary means to condition stimulus/response signals
C702S189000
Reexamination Certificate
active
06809526
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a signal processing apparatus. More particularly, the present invention relates to an apparatus and method for recovering a transient waveform response from a signal comprised of an additive superposition of responses, such superposition occurring because the length of a single transient response is longer than one or more of the intervals between the stimuli that cause the response.
BACKGROUND OF THE INVENTION
When a system is to be tested, it is common to control the input to the system and then observe the output of the system. In such a case, the input can be called a “stimulus”, and the output called a “response”. It is also common for a response to be sensed and transduced into an electrical signal that can be readily measured and/or converted into numbers (digitized) for subsequent analysis. It is also common for the stimulus timing to be controlled by a digitized number stream that is transduced or converted into a form appropriate to activate the system under test. It is also common to cyclically repeat the stimulus, either to average responses together, or to test whether the system response is affected by the repetition-rate of the stimulation.
A problem arises when the test system response is longer than the interval between stimuli. In such cases the measured electrical signal may be an algebraic summation of the individual responses, superposed in time. Such superposition may obscure features of the individual response that are of interest. Furthermore, if the superposition occurs when the pattern of stimulation is precisely periodic, i.e., when the interval from the start of a stimulus to the start of the next stimulus is always the same, then it is not mathematically possible to compute the individual response from the superposed signal. This is true because multiple solutions will be computed, with no possibility to determine which solution is correct, since the simultaneous equations that describe the waveform have more unknown variables than simultaneous equations.
As a result, it is necessary to test the system by a series of stimuli in which the SI (Stimulus Interval, start-to-start) in the series is not uniform, i.e., by a series of stimuli in which the stimulus repetition-rate “jitters”.
One method to recover the individual response from a superposed signal that uses a non-uniform stimulation sequence is called MLS (Maximum-Length Sequence). The MLS method is described in Thornton U.S. Pat. No. 5,546,956. An MLS is a pseudo-random sequence that has specific mathematical properties that permit easy calculation of a so-called “recovery function” that is cross-correlated to the superposed signal to recover the individual response.
To further discuss MLS and the invention, an SI-ratio is defined by: SI ratio=(SI
max
−SI
min
)/(SI
min
). The SI-ratio with MLS is always equal to, or greater than, unity. In some cases the MLS SI-ratio is more than 4. A major problem arises in the use of MLS if the system has responses that are affected by these SI differences. Thus, MLS works if the system-response is SI-invariant, but fails if the system-response is SI-variant. Furthermore, it may not be possible to know if an error is present: if the tested system has a poor initial signal-to-noise ratio, then any SI-variant response may not be detected, yet can contribute to making the average of the response an inaccurate estimate of the system response. Thus, there is a need for an apparatus and method that can be used to estimate the individual system response from an algebraic summation of superposed individual responses of a system under test, when such individual system response is SI-variant. The present invention fills this need.
Another problem arises if the system response is affected by the stimulus repetition-rate, i.e., is rate-variant. In contrast to MLS, the invention uses a small SI-ratio. A small SI-ratio permits the apparatus and method of the invention to provide a point estimate of the system's response at a given repetition-rate to be obtained for comparison with the response at different repetition-rates. The invention can do this, even if the system is SI-variant, because the invention can use such a small variation in SI that the size of the waveform difference is made sufficiently small so as to be not significant to the user.
A specific application of the invention relates to analysis of sensory-evoked responses at repetition-rates that are above that of stimulus-fusion. Present methods do not permit accurate analysis because the evoked-responses are longer than the time between stimuli when the repetition-rate is high enough to cause perceptual fusion of the stimuli. Clearly, for this use, an apparatus and method are needed that can accurately recover the evoked-response, for purposes of scientific investigation, clinical testing, or screening of children and newborns. The present invention is generally applicable to so-called “Steady-State” responses that occur in several sensory systems (Regan D,
Human Brain Electrophysiology
, (1989), Elsevier, N.Y., at pp. 34-42, 70-126, & 294-295), especially the auditory “40-Hz response” (Regan D,
op. cit
. at pp. 271-275).
SUMMARY OF THE INVENTION
The present invention is an apparatus and method for estimating the individual system response from a system-response signal composed of an algebraic summation of superposed individual responses of a system under test. The invention is especially useful when the individual system response is SI-variant or rate-variant, or both. The invention teaches use of selected stimulation-sequences called q-sequences or quasi-q-sequences. Both q- and quasi-q-sequences have a small variation in stimulus intervals, are pseudo-periodic, have a definitive time pattern, and conform to a rule-set with both time-domain and frequency-domain constraints. The frequency-domain constraints involve the Fourier coefficient magnitude, referred to in the invention as “Q-magnitudes”.
One of the time-domain constraints of q-sequences is a stimulus-interval ratio less than unity but greater than zero. One of the frequency-domain constraints of q-sequences is Q-magnitudes in the bandpass of interest of 0.5 or greater. One of the frequency-domain constraints of quasi-q-sequences is Q-magnitudes in the bandpass of interest less than 0.5 and greater than 0.01. Q-magnitudes can have values between zero and a number equal to the number of stimuli in the sequence.
The q- and quasi-q-sequences are utilized for timing of stimuli in a data-acquisition system that includes capabilities for stimulating the system under test, and for recording the system-response signal in synchrony with the stimulus timing. The data-acquisition system can include additional components, such as: averaging means, filtering means, amplifying means, data-rejection means, data-acquisition stopping means, simultaneous multiple q-sequence data-acquisition means, simultaneous multiple q-sequence data-acquisition including one uniform stimulation-sequence means, data-analysis means, display means, and output means.
The invention teaches data-analysis that utilizes deconvolution, which can be computed by any of a variety of methods. The use of deconvolution and q-sequences is indicated by the acronym for the method of the invention: QSD (q-sequence deconvolution). The deconvolution is carried out on the recorded system-response signal utilizing, in one form of the invention, a recovery sequence adapted from the reciprocal of the set of Q-magnitudes within the bandpass of interest combined with Q-magnitudes at the limit of the computer's floating point numbers in bandreject regions. If averaging is included in the data analysis, the deconvolution can occur before or after averaging. The data-analysis system can include additional components, such as: input means, averaging means, filtering means, amplifying means, waveform-analysis means, noise estimation means, sweep rejection means, data rejection means, adjusted Q-magnitude means, decimation by freque
Abratech Corporation
Le N.
Meyer, Esq. Virginia H.
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