Ratio Transfer Function Research Articles

The Effects of Load Impedance, Line Length, and Branches in the BPLC—Transmission-Lines Analysis for Medium-Voltage Channel

This paper presents the effects of load impedance, line length and branches on the performance of medium-voltage power-line communication (PLC) network. The power-line network topology adopted here is similar to that of the system in Tanzania. Different investigation with regard to network load impedances, direct line length (from transmitter to receiver), branched line length and number of branches has been investigated. From the frequency response of the transfer function (ratio of the received and transmitted signal), it is seen that position of notches and peaks in the magnitude and phase responses are largely affected in terms of attenuation and dispersion by the above said network parameters/configuration. These are observed in the time domain responses too. The observations presented in the paper could be helpful in suitable design of the PLC systems for a better data transfer and system performance.

Transformer winding defects identification based on a high frequency method

The transformer diagnostic methods are systematically being improved and extended due to growing requirements for reliability of power systems in terms of uninterrupted power supply and avoidance of blackouts. Those methods are also driven by longer lifetime of transformers and demand for reduction of transmission and distribution costs. Hence, the detection of winding faults in transformers, both in exploitation or during transportation is an important aspect of power transformer failure prevention. The frequency response analysis method (FRA), more and more frequently used in electric power engineering, has been applied for investigations and signature analysis based on the admittance and transfer function. The paper presents a novel approach to the identification of typical transformer winding problems such as axial or radial movements or turn-to-turn faults. The proposed transfer function discrimination (TFD) criteria are based on the derived transfer function ratios, manifesting higher sensitivity.

Damage Assessment of Reinforced Concrete Beams by Complex Ratios of Transfer Functions

In this paper, a newly developed approach for the determination of characteristics associated with local defects of a beam is briefly introduced. From the numerical studies carried out in this study, the complex ratio between two transfer functions associated with an imaginary intact beam and a damaged beam can provide such information. Based on the numerical studies, the rules in determining degree of damages and locations of defects were first briefly outlined. These idealized rules were then verified by experimental data obtained from dynamic tests of realistic specimens of simply-supported reinforced concrete beams. The preliminary results indicate that the defined ratio of transfer functions can potentially serve as an exaggerated indication for the degree of changes in certain modal responses. Thus, the proposed ratio of transfer functions can be used to assist system identification, while the nature of high sensitivity also restricts its direct application to certain complicated data associated with field tests. A variety of test setup for both receivers and impact sources were studied and the test results appear to agree with designated conditions of the specimens.

Predicting Patients’ Night Vision Complaints With Wavefront Technology

To evaluate the accuracy of the diagnostic capabilities of optical metrics generated from wavefront measurements in relationship to post-laser-assisted in situ keratomileusis (LASIK) visual complaints as expressed and drawn by patients. Retrospective analysis and observational case series. Patient wavefront data from an investigational device exemption study for wavefront-guided ablations were used to derive normative modulation transfer function (MTF), encircled energy (EE), and Strehl ratio. These optical metrics and their point-spread functions (PSF) were compared with data from five postoperative patients with night vision complaints. Patients were asked to draw their symptoms, which were elicited by testing with a Fenthoff muscle light, while using their best-corrected distance vision. The MTF, EE, and Strehl ratio of most patients were markedly different from those of the averages of 208 normal myopic eyes before and after LASIK surgery. The spatial extent of the PSF correlated positively with the severity of the visual complaints. Wavefront-derived PSFs were markedly similar to the patients' drawings. The results of this study demonstrate the diagnostic capability of the wavefront system in predicting visual symptoms and complaints of patients with high-order aberrations. Objective visual metrics from patients with night vision complaints were different from those of normal myopic eyes that had undergone LASIK procedures.

Robust Subspace Analysis and Its Application in Microphone Array for Speech Enhancement

A robust microphone array for speech enhancement and noise suppression is studied in this paper. To overcome target signal cancellation problem of conventional beamformer caused by array imperfections or reverberation effects of acoustic enclosure, the proposed microphone array adopts an arbitrary model of channel transfer function (TF) relating microphone and speech source. Since the estimation of channel TF itself is often intractable, herein, transfer function ratio (TFR) is estimated instead and used to form a suboptimal beamformer. A robust TFR estimation method is proposed based on signal subspace analysis technique against stationary or slowly varying noise. Experiments using simulated signal and actual signal recorded in a real room illustrate that the proposed method has high performance in adverse environment.

Site‐to‐bedrock over 1D transfer function ratio: An indicator of the proportion of edge‐generated surface waves?

We study multi‐dimensional site effects in the small‐size deep sediment filled basin of Grenoble (French Alps). A very dense array composed of 3‐component seismometers over a 1 km aperture is used to investigate propagation parameters of waves propagating across the array. We present results for 6 teleseismic events for which we had a reference rock site. For the frequency range 0.1 to 1 Hz, records are clearly dominated by edge‐generated surface waves. Quantification of energy carried by each identified wave train allows interpreting the difference between site‐to‐bedrock and 1D transfer function estimates in terms of laterally propagating basin‐edge induced waves only above the fundamental resonance frequency of the basin. A simple ratio between site‐to‐bedrock and 1D transfer function curves should thus provide an estimation of the relative contribution of 1D and 2D/3D effects in ground motion on the condition that all the time signal length is taken into account.

The virtual microphone technique in active sound field control systems

Active Sound Field Control (AFC) has been proven very useful in reverberation enhancement applications in large rooms. However, feedback control is required in order to eliminate peaks in the frequency response of the system. The present research closely follows the studies of Shimizu in AFC, in which smoothing of the rooms transfer function is achieved by averaging the impulse responses of multiple microphones. ‘‘The virtual or rotating microphone technique’’ reduces the number of microphones in the aforementioned AFC technology, while still achieving the same acoustical effects in the room. After the impulse responses at previously specified pairs of microphone positions are measured, the ratio of transfer functions for every pair is calculated, thus yielding a constant K. Next, microphones are removed and their impulse responses are reproduced by processing the incoming signal of each pair through a convolver, where the computed K constants have been previously stored. Band limiting, windowing and time variance effects are critical factors, in order to reduce incoherence effects and yield reliable approximations of inverse filters and consequently calculations of K. The project is implemented in a church lacking low frequency reverberation for music and makes use of 2 physical and 2 virtual microphones.

Iterative Learning Control Based on the Gradient Method for Linear Discrete-Time Systems

In this paper, we discuss convergence of iterative learning control based on the gradient method for linear discrete-time systems. First, it is shown that residuals generated by the algorithm converge exponentially and therefore, observation errors or disturbance does not cause divergence. Second, the convergence conditions are expressed as strictly positive realness of the ratio of transfer functions. Conditions for the strictly positive realness is presented when there exists uncertainty of parameters in the transfer function and it is known that they are in given intervals. We also propose a simple sufficient condition for the strictly positive realness, which is based on a special structure of the problem. Finally, we illustrate design procedure based on the results given in this paper. Applications of the results to sampled-data systems are also discussed.

The crustal structure of the western Arabian platform from the spectral analysis of long-period P-wave amplitude ratios

The crustal structure of the western Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations, crustal velocities, densities and the angle of emergence at the lower crust and upper mantle interface. Eleven well-defined earthquakes recorded at the long-period RYD station during the period from 1985 to 1994 were selected for analysis based on the following criteria: focal depths with a range between 7 and 89 km, body-wave magnitudes greater than 4.7, epicentral distances with a range from 8.8° to 26.5°, and back azimuthal coverage from 196° to 340°. Spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical P-wave motion obtained using the Thomson–Haskell matrix formulation for horizontally layered crustal models. The selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross-correlation coefficient with the observed transfer function ratio. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 1 km/s, respectively, of the observed values. The spectral analysis of long-period P-waves can detect a thin layer near the surface of about 1.6 km thick and a velocity contrast of about 10% with that of the underlying layer. A strong velocity gradient of about 0.05 km/s per km was found in the upper crust and 0.02 km/s per km in the lower crust. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The crustal model suggests that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km/s and is about 1.6 km thick. The second layer has a velocity of about 6.2 km/s and is 10.2 km thick. The third layer shows a velocity of 6.6 km/s and is 6.8 km thick. The fourth layer has a velocity of about 6.8 km/s and is 12.3 km thick. The lower crustal layer has a velocity of about 7.5 km/s and is 9.3 km thick. The Mohorovicic discontinuity beneath the western Arabian platform indicates a velocity of 8.2 km/s of the upper mantle and 42 km depth.

Phasing of harmonic components to optimize measured signal-to-noise ratios of transfer functions

Much can be learnt about a system by studying how its various transfer functions vary with frequency. The best measured signal-to-noise ratio (SNR), and hence the most precise results, will occur when a sequence of stimuli, each containing only a single frequency, is applied to the system. However, it is often advantageous to shorten the total measurement time, with a consequent decrease in the measured SNR, by applying a stimulus that simultaneously contains several different frequency components. The author considers the various compromises between the measured SNR and the frequency components that are simultaneously present. The author concentrates upon improvements in the measured SNR that result from optimum phasing of the harmonic components. Calculations show that the SNR of a system with a transfer function that is substantially independent of frequency can thus be improved by a factor of approximately k0.4, where k denotes the number of frequencies that are simultaneously present. The measured SNR will thus vary approximately as k-0.6 if optimum waveforms are used.

Adaptive Active Noise Control System with Howling Canceler. 2-Microphone Transfer Function Ratio System.

Adaptive Active Noise Control System with Howling Canceler. 2-Microphone Transfer Function Ratio System.

Effects of aging in retinal image quality

The retinal image quality characterized by the modulation-transfer function of the eye was measured for two groups of subjects aged in the late twenties and mid sixties, respectively. In both groups, we obtained modulation transfer functions by using a double-pass method under the same experimental conditions: 4-mm artificial pupil, paralyzed accommodation, and objective control of the refractive state and centering. Results showed lower values of modulation in the retinal image for older subjects compared with the younger subjects. The modulation transfer function ratio is similar to that previously found for contrast-sensitivity measurements with subjects in the same age groups. These results suggest that a significant fraction of the loss in spatial vision with age has an optical origin. Apart from the well-known increase in intraocular scattering, there also appears to be an increment in ocular aberration that causes an additional reduction in the contrast of retinal images.

Use of auralization to accomplish small elevation shifts of a loudspeaker in the median plane

The use of frequency domain equalization to change the perceived elevation of a loudspeaker over a fairly small range in the frontal plane (±20°) will be described. Because of the application (the center channel speaker in home video theater), several constraints were in effect. Among these were (1) the effect must be applicable over a broad range of people, and over a fairly broad listening area, (2) the equalization applied must not introduce an objectionable amount of ‘‘coloration’’ into the sound, and (3) the resulting circuit must be feasible to implement into a consumer product. The procedure used was to first determine equalizations from ratios of head-related transfer functions measured on an artificial head and on human subjects. Salient features were then determined and implemented in hardware for listening tests, where further refinements took place. Despite a common assertion that there is little directional dependence in the median plane below 4–5 kHz, listening tests indicated a more convincing effect took place when several features below 4 kHz were included in the equalizations. It was also found effective to include a feature at 12.5 kHz, even if source material was limited to male speech. The results of demonstrating a 20° to 0° ‘‘delevator’’ to 125 groups of 12 people will also be discussed. [Work supported by Thomson Consumer Electronics.]

Effect of Howling Canceller in Active Noise Control.

Howling is one of the most important problems to be solved in active noise control. The authors have shown that a 2-microphone system utilizing the time delay corresponding to the distance between the two microphones should have an acoustical condition wherein no reflection wave should exist from the opposite side to the targeted opening end. In this paper, another solution called 2-microphone transfer function ratio system is introduced to eliminate the signal from the secondary source. An echo canceller technique is also studied in this paper. The signal convoluted by the transfer function from the secondary source to the detection microphone is subtracted from the detection microphone signal. The acoustical condition is studied theoretically, taking into consideration the reflection wave from both sides. The effect of eliminating the signal from the secondary source was confirmed by the experiment. The coherence function between the source signal and the howling canceller output was measured. The correlation was recovered by using these systems.

On the location of frequencies of maximum acoustic-to-seismic coupling

Measurements of the acoustic-to-seismic transfer function (ratio of the normal soil particle velocity at a depth d to the acoustic pressure at the surface) for outdoor ground surfaces quite typically reveal a series of maxima and minima. In a publication [Sabatier et al., J. Acoust. Soc. Am. 80, 646–649 (1986)], the location and magnitude of these maxima are measured and predicted for several outdoor ground surfaces using a layered poroelastic model of the ground surface. In this paper, the seismic transfer function for a desert site is compared to the seismic transfer function for holes dug in the desert floor which were filled with pumice (volcanic rock). The hole geometry was rectangular and the hole depths varied from 0.25–2.0 m. The p- and s-wave speeds, densities, porosities, and flow resistivities for the desert floor and pumice were all measured. By varying the hole depth and the fill material, the maxima in the seismic transfer function can be shifted in frequency and the locations of the maxima compare reasonably with that of a hard-backed layer calculation. The area or extent of the acoustic-to-seismic coupling for pumice was determined to be less than 1 m2.

Acoustically induced seismic waves

When an airborne acoustic wave is incident at the ground surface, energy is coupled into the ground as seismic motion. In a previous publication [Sabatier et al., J. Acoust. Soc. Am. 78, 1345–1352 (1986)] the ground surface was modeled as an air-filled poroelastic layer overlying a semi-infinite, nonporous elastic substrate. In this work, the model is extended to include calculations of the normal seismic transfer function (ratio of the normal soil particle velocity at a depth d to the acoustic pressure at the surface). Measurements of the seismic transfer function for three sites are considered and compared to the predicted values. Generally good agreement between theory and experiment is achieved by best fits assuming the soil or seismic attenuation. This is accomplished by specifying the ratio of the imaginary to real part of the measured seismic p- and s-wave speeds. The seismic transfer functions quite typically exhibit minima and maxima which are associated with the seismic layering of the ground surface. Typical layer depths are 1–2 m. An analytical expression predicting the location of these maxima is offered based on hard substrate and the experimental and theoretical comparisons are reasonable.

Reliability of auditory function tests in severely hearing-impaired and deaf subjects.

The test-retest variability of a series of auditory functions has been investigated in a group of severely hearing-impaired and deaf subjects (64 ears, median Fletcher index: 80 dB) and in a group of 10 controls with normal hearing. An adaptive forced-choice procedure was used for both groups. The functions were: tone audiogram, difference limen for intensity, difference limen for frequency, modulation transfer function and critical ratio. In spite of sometimes strongly deviating function values within the hearing-impaired group, the test-retest variability of the two groups was found to be of the same order of magnitude, except for the tone audiogram where the variability in the hearing-impaired group was twice that for the control group.

A comment on the use of spectral characteristics of teleseismic body waves to evaluate fine crustal structure

An investigation of the crustal response in the time and frequency domains is presented to show that for models with similar crustal transit times of P and S waves, any important changes in the internal configuration of the crust such as layering, layer thickness and velocity contrast only influence the amplitudes of the transfer function ratio. Frequency positions of peaks and troughs remain unperturbed unless the total transit time of P and/or S waves through the crust are varied. This would imply that fine crustal structure derived from a matching of peak positions of observed spectral ratios with theoretical transfer function ratios, as is commonly practiced, is not very meaningful. This limitation in the application of the spectral ratio method for detailed crustal structure is demonstrated using two recent applications of this technique.