Model Of Signal Generation Research Articles

Surface EMG crosstalk between knee extensor muscles: experimental and model results.

Surface electromyographic (EMG) crosstalk between vastus lateralis, vastus medialis, and rectus femoris muscles was evaluated by selective electrical stimulation of one muscle and recording from the stimulated and another muscle with linear surface arrays of eight electrodes. The ratio between the amplitude of the signals recorded over nonstimulated and stimulated muscles and their correlation coefficient were used as indices to quantify crosstalk. Single-differential and double-differential detection systems were used with interelectrode distances in the range 10-40 mm. The multichannel EMG signals clearly showed that crosstalk is largely due to nonpropagating potentials that correspond in time to the end of the propagation of the action potentials generated by the stimulated muscle. The crosstalk signal increased with increasing interelectrode distance and was statistically higher for single- than for double-differential recordings. The correlation-based indices of crosstalk were poorly correlated with the amplitude-based indices. Moreover, the characteristic spectral frequencies of the signals detected over the nonstimulated muscles were statistically higher than those from the stimulated muscles. A mathematical model of signal generation was used to explain the experimental findings. This study clarifies many controversial findings of past investigations and creates the basis for crosstalk interpretation, simulation, and reduction.

Photothermal determination of optical coefficients of tissue phantoms using an optical fibre probe

The absorption and reduced scattering coefficients of turbid tissue phantoms have been determined from photothermal measurements made using an optical fibre probe. The thermal sensor was a thin polymer film positioned at the end of a multimode optical fibre. The film was illuminated by the output of a continuous-wave diode laser and formed the cavity of a low-finesse Fabry–Perot interferometer. Low energy laser pulses, launched into the fibre and passed through the film, produced an abrupt temperature rise in the target tissue, which was placed in contact with the film. The subsequent conduction of heat into the film caused a change in its optical thickness and hence the reflected intensity. The absorption and reduced scattering coefficients of gelatine tissue phantoms of known optical properties were determined from the measurements using a numerical model of photothermal signal generation and maximum a posteriori estimation. The determined optical coefficients were in good agreement with the known values. The results showed that the probe can be used for the determination of optical coefficients provided the thermal coefficients of the target tissue are known with low uncertainty.

The role of induced contrast in images obtained using the environmental scanning electron microscope

Generation of contrast in images obtained using the environmental scanning electron microscope (ESEM) is explained by interpretation of images acquired using the gaseous secondary electron detector (GSED), ion current, and the Everhart-Thornley detector. We present a previously unreported contrast component in GSED and ion current images attributed to signal induction by changes in the concentration of positive ions in the ESEM chamber during image acquisition. Changes in positive ion concentration are caused by changes in electron emission from the sample during image acquisition and by a discrepancy between the drift velocities of negative and positive charge carriers in the imaging gas. The proposed signal generation mechanism is used to explain contrast reversal in images produced using the GSED and ion current signals and accounts for discrepancies in contrast observed, under some conditions, in these types of images. Combined with existing models of signal generation in the ESEM, the proposed model provides a basis for correct interpretation of ESEM images.

TOWARD HELICOPTER GEARBOX DIAGNOSTICS FROM A SMALL NUMBER OF EXAMPLES

The United States Navy funded Westland Helicopters LTD to carry out a series of tests on the CH-46E helicopter transmission for the purpose of generating a database that can be used to evaluate diagnostic tools in general, and neural networks in particular. The database is extensive in the number of accelerometers, load conditions and diversity of faults. However, only 1–4 instances (levels) of each fault were included. Thus, the number of examples that can be extracted to train any classification scheme is limited, and well below the minimum required. We argue that instead the database is best used to develop specific signal generation models for the CH-46E helicopter gearbox. Such models can then be used to suggest meaningful features and derive appropriate signal-processing tools. Specifically, we demonstrate that the meshing vibrations induced by the large collector gear located on the quill shaft are significant and may interact with the vibrations induced by other elements attached to the same shaft. An appropriate model is developed and the effect, termed cross-gear-pair interaction, is studied using different signal-processing tools.

Maximum a posteriori deconvolution of ultrasonic signals using multiple transducers

A new method for deconvolution of ultrasonic pulse-echo measurements employing multiple-transducer setup is proposed in the paper. An optimal way of estimating the material reflection sequence for a linear signal generation model using maximum a posteriori estimation is proposed. The method combines the measurements from a number of transducers covering different frequency bands yielding an optimal estimate of the reflection sequence. The main idea of this approach is to complement the information unavailable from one transducer in some frequency bands with the information from the other transducers. The method is based on the assumption that the measurements are performed using transducers with identical apertures and apodization, which are located exactly at the same position relative to the test object during the measurement. An error analysis presented in the paper proves that when the above assumptions are fulfilled, the proposed method, by utilizing more data for estimation, consistently yields more accurate reflection sequence estimates than the classical Wiener filter. Experimental evidence is presented using both simulated and real ultrasonic data as a verification of the correctness of the multiple-transducer model and the estimation scheme. An illustration of the advantages of the method is also given using real ultrasonic data.

Photoacoustic determination of the recombination velocity at the AlGaAs/GaAs heterostructure interface

The surface recombination velocity at the AlGaAs/GaAs single heterostructure interface is determined using the photoacoustic technique. We analyze different heat generation mechanisms for this type of heterostructure with different Al composition and thickness of the AlGaAs layers. A theoretical model of the photoacoustic signal generation in the heat transmission detection configuration was developed. The values of the surface recombination velocity obtained by comparing the theoretical expressions with the experimental results are in good agreement with the literature reported ones. The results were also correlated with those of photoluminescence measurements in the same samples.

Thermophysical characterization of artificially aged papers by means of the photothermal deflection technique

The photothermal deflection technique has been used to determine the thermophysical properties of artificially aged paper samples. The deflection amplitude ratio and the phase difference of the deflection angle between front and rear surface illumination have been measured as a function of the pump beam modulation frequency, in order to avoid the dispersion effects mainly introduced by the probe beam vertical offset with respect to the sample surface. A theoretical model for the photothermal signal generation, taking into account the unavoidable light scattering occurring at the paper fibers–air interfaces, has been developed. A photothermal deflection method has been also proposed to measure the transmittance and the reflectance values of a given paper sheet, thus offering a proper constraint to the whole fitting procedure. In this way it has been possible to estimate the thermal diffusivity, the optical absorption and the scattering coefficient values of all the samples examined. A correlation between these values and the aging state of the paper has been found.

Influence of pole trimming on track-following accuracy

The correlation between the pole shape of a magnetic write head and the track-following accuracy has been investigated. To evaluate the effect of head properties on head positioning, an analytical model for position signal generation was developed. This model clarified the effect of the erase-band width and the read track-width on the track-following accuracy by revealing the position signal linearity. The investigation demonstrated that pole trimming of the write head improves head-positioning performance.

Signal generation in a non-EU country.

One of the most important objects of post-marketing surveillance is the early detection of serious, unknown and unexpected adverse events. As spontaneous reported adverse events vary considerably in their signal-generating value the Swiss Drug Monitoring Center SANZ implemented an early signal detection system in 1991. With a set of screening criteria the individual case reports are automatically checked by the system and read into a signal file. Subsequently these potential signals are processed to quantify the signal generating value of the particular case. According to the screening criteria 2072 (53%) of the reported cases were considered as potential signals. After validating and quantifying their signal value 27% (565) were drug-related and unlabelled, 8% (173) were also rated as serious. On the basis of these cases 114 labelling changes were made. Some of the most interesting cases are discussed. This computer-assisted early signal generation model in a small country with 7 million inhabitants has a high detectability of new, rare, serious and quality of life affecting adverse drug reactions (ADRs). These newly detected ADRs must be interpreted in cognizance of the limitations and restrictions of spontaneous reporting and do not allow pharmacoepidemiologic conclusions.

Analytical estimation of signal transition activity from word-level statistics

Presented in this paper is a novel methodology to determine the average number of transitions in a signal from its word-level statistical description. The proposed methodology employs: (1) high-level signal statistics, (2) a statistical signal generation model, and (3) the signal encoding (or number representation) to estimate the transition activity for that signal. In particular, the signal statistics employed are mean (/spl mu/), variance (/spl sigma//sup 2/), and autocorrelation (/spl rho/). The signal generation models considered are autoregressive moving-average (ARMA) models. The signal encoding includes unsigned, one's complement, two's complement, and sign-magnitude representations. First, the foilowing exact relation between the transition activity (t/sub i/), bit-level probability (p/sub i/), and the bit-level autocorrelation (/spl rho//sub i/) for a single bit signal b/sub i/ is derived: t/sub i/=2p/sub i/(1-p/sub i/)(1-/spl rho//sub i/) (1). Next, two techniques are presented which employ the word-level signal statistics, the signal generation model, and the signal encoding to determine /spl rho//sub i/ (i=0, /spl middot//spl middot//spl middot/, B-1) in (1) for a B-bit signal. The word-level transition activity T is obtained as a summation over t/sub i/ (i=0,/spl middot//spl middot//spl middot/, B-1); where t/sub i/ is obtained from (1). Simulation results for 16-bit signals generated via ARMA models indicate that an error in T of less than 2% can be achieved. Employing AR(1) and MA(10) models for audio and video signals, the proposed method results in errors of less than 10%. Both analysis and simulations indicate the sign-magnitude representation to have lower transition activity than unsigned, ones' complement, or two's complement. Finally, the proposed method is employed in estimation of transition activity in digital signal processing (DSP) hardware. Signal statistics are propagated through various DSP operators such as adders, multipliers, multiplexers, and delays, and then the transition activity T is calculated. Simulation results with ARMA inputs show that errors less than 4% are achievable in the estimation of the total transition activity in the filters. Furthermore, the transpose form structure is shown to have fewer signal transitions as compared to the direct form structure for the same input.

Photothermal lens spectrometry of homogeneous fluids with incoherent white-light excitation using a cylindrical sample cell

A model for photothermal lens signal generation in a cylindri- cal sample cell under constant irradiance excitation is described and tested. The model is developed with and without the assumption that the sample cell does not change temperature over the irradiation time. In both cases, the photothermal lens is predicted to be parabolic in form with a strength that is independent of sample cell radius. The predicted irradiance independence suggests that incoherent illumination can be used to perform photothermal lens spectroscopy in low-volume cells. Experimental evidence is obtained using a Xe arc lamp to perform pho- tothermal lens spectroscopy in a 6 mL cylindrical spectrophotometric cell. Optical filters are used to reduce the power at IR and UV wavelengths of the Xe lamp emission spectrum. This pseudo-white-light source enables indirect optical absorbance measurement independent of the absorption spectrum of the analyte. The preliminary data reported show that photo- thermal lens signals can be obtained using wide-spectral-bandwidth, in- coherent excitation sources. Although the theoretical enhancement fac- tor is found to be only ;0.01 for these experiments, limits of detection of the order of 30 to 300 pM pseudoisocyanine dye in ethanol solution are found. This corresponds to spectral integrated absorption detection limit from 10 24 to 10 26 au in the centimeter path length cell. These low de- tection limits are found even with low enhancement factors because the factors that affect the noise in the photothermal lens and conventional transmission spectroscopy signals are not the same in these experi- ments. The major sources of uncertainty in these detection limit esti- mates are knowledge of the excitation source spectrum and periodic chaotic behavior of the diode laser used as a probe of the photothermal lens. Examination of the response time of the signal reveals that thermal conductivity of the sample cell influences the characteristic signal rise and decay time constants. The radiative heat transfer model is applied to interpret measured time constants in terms of the cell thermal conduc- tivity and thickness of the sample cell walls. The sample cell thermal conductivity determined by this method is consistent with ferrous mate- rials. © 1997 Society of Photo-Optical Instrumentation Engineers.

Unequal saccades produced by aniseikonic patterns: a model approach

This study addresses a possible mechanism for fast disconjugate adaptation of binocular horizontal saccades. Disconjugacy of binocular saccades was elicited by two dichoptically presented, identical but aniseikonic, random checkerboard patterns. Adaptation was achieved with the patterns at far distance (144 cm). In this condition, which requires a relatively small (8%) size difference of the saccades, a short learning period was mandatory for the binocular saccades to become disconjugate. The saccadic modifications were superimposed on an idiosyncratic pattern of intra-saccadic yoking. A model of saccadic signal generation is described, that has been used to separate the contributions on saccadic disconjugacy provided by modification of visual inputs processing, which alters the motor-system inputs, and by modification of the control system: the adaptation. We identified three major components of the saccadic command (two phasic and one tonic) that contribute and in a specific way to the saccadic yoking and disconjugacy. The model analysis proposes that separate control mechanisms exist operating on these phasic and tonic signals. We show that the saccadic system can generate the vergence component shown by our aniseikonic saccades. We discuss a distributed-parallel implementation of the saccadic system able to provide both the conjugate and disconjugate components of control.

Photoacoustic diagnostics of laser-induced processes in reaction centers of Rhodobacter sphaeroides

A theoretical model of photoacoustic signal generation in a suspension of bacterial reaction centers (RCs) is suggested. The existing technique of separation of the two contributions to photoinduced volume changes (thermal expansion and structural rearrangement), based on their different temperature dependence, is extended to take into account the fact that they may also have different time dependence. In the practical implementation of the technique, it was found that electron transfer in the RC isolated from Rhodobacter sphaeroides is associated with a fast (< 100 ns) contraction, 32 ± 1 A ̊ 3 which has the same value within the accuracy of our measurements for wild-type and R-26 preparations. The enthalpy of the P +Q − A state deduced from our measurements is 0.56 ± 0.04 eV. The quantum yield of photochemistry, φ was measured in a broad range of background light intensities. Its value for dark-adapted RCs, φ ≈ 1.0, decreased continuously to φ ≈ 0.3 at background light intensity ≈ 1 photon/(RC · s). Reduced values of φ observed after long background light illumination were attributed to reversible structural changes, which relax in about 15–20 min after charge recombination. In the photochemically ‘closed’ reaction center, a deactivation process, following laser excitation, involves a long-lived intermediate state characterized by an exponential decay with a lifetime of 620 ± 80 ns and an amplitude of 0.14 ± 0.04.

Reduction of ocular artefacts in source current density brain mappings by ARX2 filtering

Sweep by sweep analysis of event-related potentials (ERP) of the human scalp represents a reliable tool for both the diagnosis of neurologic diseases and the study of the central nervous system during cognitive tasks. The off-line procedure based on stochastic parametric identification and filtering herewith described, allows an accurate analysis of single-sweep ERP and a drastic reduction of ocular artefacts variously propagating through the skull. Moreover, the spatial distribution of the recorded ERP in bidimensional form was enhanced by using the Laplacian operator in order to get an estimate of the source current density (SCD) flow from the skull into the scalp. Complete single-trial signals were filtered according to an autoregressive model of signal generation with 2 exogenous inputs (ARX2). The ARX2 procedure models the recorded signal as the sum of three signals: (a) the background EEG activity, modelled as an autoregressive process driven by a white noise; (b) a filtered version of a reference signal carrying the average information contained in each sweep; (c) a signal due to the ocular artefact propagation. The evaluation of the effect of artefact suppression on those channels close to the eyes was compared with standard ordinary least squares method (OLS) based on a linear model of the influence of EOG on ERP. Finally, the better results obtainable through ARX filtering on sweep-by-sweep brain mappings are also presented.

DIFCARS: A versatile model of CARS signal generation

A versatile model of coherent anti-Stokes Raman scattering (CARS) signal generation is presented. The model combines the novel treatment of CARS signal generation given by Teets with the realistic electric field distributions produced by the model of focused laser beams given by Evans and Grey-Morgan. The model provides a detailed picture of local contributions to the total CARS signal, including interference effects, as well as the total CARS power and the CARS spectrum. Illustrative applications of the model are given for each of the common CARS phase-matching configurations, collinear CARS, USED CARS and folded BOXCARS. A particular feature of the model is that the input data derives from readily accessible features of the pump and Stokes input beams arriving at the focusing lens and of the lens itself.

Influence of the configuration of the optical setup on the characteristics of Doppler signals in a dual-beam laser velocimeter: evolution of signal visibility

A method is presented to compute signal characteristics in a dual-beam laser Doppler velocimeter of any configuration and, as a first application, the influence of the optical configuration on signal visibility is also investigated. A simplified model of signal generation is also introduced by which the behavior of signal properties can be physically explained. This model shows that signal visibility is due to a coupling effect between the spatial period of the scattered interference pattern and the angle subtended by the detection aperture. Closed analytical expressions are derived for this period for the scattering components of reflection and refraction without internal reflections. A similar study concerning the evolution of signal phase shift is now in progress.

Micrometer Resolved Photothermal Displacement Inspection of Optical Coatings

Abstract Two-dimensional c.w. photothermal surface displacement (PTD) scans with high spatial resolution provide a new quality for thin-film characterization. This is demonstrated for optical single-layer films of ZrO2 and MgF2 on various substrates. Inhomogeneities of the films were detected with a lateral resolution < 2 µm. Variation of the modulated frequency was employed for depth-profiling. The resolving power has been investigated experimentally and a model for signal generation by absorbing inhomogeneities is presented. A calculation of the photothermal thin-film response for thermally thick coatings is carried out to obtain a better understanding of the PTD images with respect to film absorption and thermophysical sample properties.

Piezoelectric detection of the photoacoustic signals of n-type GaAs single crystals

Piezoelectric photoacoustic (PA) measurements on liquid-encapsulated-Czochralski-grown n-GaAs were carried out at room temperature. A continuous broad band below 1.35 eV and a peak at 1.383 eV were observed in the PA amplitude spectra. By comparing with the optical-absorption spectra, it is concluded that the broad band is due to the electron transition involving the EL2 deep-lying defect levels. For the observed peak at 1.383 eV, the origin is considered to be dislocation related. The possibility that this peak is an apparent one expected from the proposed models for the PA signal generation is denied.

Photoacoustic signal generation in leaves: are O 2-consuming processes involved?

The photoacoustic signal from a green leaf is the vectorial sum of photothermal (heat emitted by non-radiative deexcitation of molecules) and photobaric (photosynthetic O 2 exchanges) contributions. With sugar maple leaf, we have observed a decline of the O 2 signal in the course of time, once the leaf attained a steady-state photosynthesis. This decline of the O 2 signal was not due to a decrease in photochemical activity as was evident by a stable fluorescence signal and constant energy storage. The rate and the extent of decline were larger at low modulation frequency and low modulated light intensity. The decline continued even in the presence of background far-red light (λ>715 nm). The above observations lead us to conclude that the photoacoustic O 2 signal from leaves is influenced by O 2 -consuming processes. Data are discussed in relation to a possible occurrence of an equilibrium between oxygen-evolving and oxygen-consuming processes to account for the O 2 signal in the plant cell. Moreover, the oxygen-uptake component may have to be included in the vectorial model of photoacoustic signal generation in leaves.

A versatile model of CARS signal generation: optimum beam diameter ratios for different phase-matching geometries

A versatile model of coherent anti-Stokes Raman scattering (CARS) signal generation is presented. It combines the detailed picture of the CARS signal generation volume given by Teets with the realistic field distributions, and use of readily accessible beam parameters, which are features of the model of focused laser beams given by Evans and Grey-Morgan. Applications of the model are exemplified by a study of the optimum diameter ratios of the Stokes and pump beams necessary for efficient CARS signal generation for some major phase-matchng geometries, namely, collinear CARS, USED CARS, and folded BOXCARS.