X-ray medical imaging provides invaluable medical information, while subjecting patients to hazardous ionizing radiation. The dosage that the patient is exposed to may be reduced, at the cost of image resolution. A technology that promises lower dosage for a given resolution is direct conversion digital imaging, typically based on amorphous Selenium semiconductor. Sufficient exposure should be used for the first exposure to avoid subsequent exposures; a challenge is then to reduce the necessary exposure for a suitable image. To quantify how little radiation the detector can reliably discriminate, one needs an analysis of the variance that 1/f and white noise contribute to the signal of such detectors. An important consideration is that the dark current, which varies with time, is subtracted from the photo-current, to reduce the spurious spatial variance in the image. In this thesis, the variance that 1/f noise contributes to integrating detectors is analysed, for a very general integrating detector. Experiments were performed to verify the theoretical results obtained for the 1/f noise variance contribution.

Hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) are used as switching elements in large area active matrix liquid crystal displays and various image sensing devices for radiation detection. The noise inherent in the a-Si:H TFTs contributes to the overall noise figure of such devices and degrades the signal to noise ratio; therefore, the noise is an important factor in the design of the devices. The noise of the a-Si:H TFTs has been studied experimentally, but the origin of the noise is not understood. <p> This work calculates the noise of the a-Si:H TFTs based on a simulation of operation of the TFTs and the hypothesis that the device noise is due to the intrinsic noise of the a-Si:H material. An a-Si:H TFT with an inverted-staggered structure has been simulated by numerically solving the fundamental transport equations for various gate and drain-source voltages. The drain-source curves derived from the simulation agree qualitatively with the experimental results: both the linear and saturated regions are observed. The low frequency noise was calculated based on the charge density distribution in the channel obtained from the simulation and the known dependence of the noise in the a-Si:H on the charge density, Hooges relation. The calculated noise power increases with the drain-source voltage and is inversely proportional to the gate voltage or the effective channel length. The curves agree qualitatively with the experimental results. The calculated noise power agrees quantitatively with the experiments when the scaling parameter in Hooges relation, , is set to . This value agrees with the experimentally determined value for a-Si:H. The results are consistent with the hypothesis that the low frequency noise in the a-Si:H TFTs is due to the material itself.

Hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) are used as switching elements in large area active matrix liquid crystal displays and various image sensing devices for radiation detection. The noise inherent in the a-Si:H TFTs contributes to the overall noise figure of such devices and degrades the signal to noise ratio; therefore, the noise is an important factor in the design of the devices. The noise of the a-Si:H TFTs has been studied experimentally, but the origin of the noise is not understood. <p> This work calculates the noise of the a-Si:H TFTs based on a simulation of operation of the TFTs and the hypothesis that the device noise is due to the intrinsic noise of the a-Si:H material. An a-Si:H TFT with an inverted-staggered structure has been simulated by numerically solving the fundamental transport equations for various gate and drain-source voltages. The drain-source curves derived from the simulation agree qualitatively with the experimental results: both the linear and saturated regions are observed. The low frequency noise was calculated based on the charge density distribution in the channel obtained from the simulation and the known dependence of the noise in the a-Si:H on the charge density, Hooges relation. The calculated noise power increases with the drain-source voltage and is inversely proportional to the gate voltage or the effective channel length. The curves agree qualitatively with the experimental results. The calculated noise power agrees quantitatively with the experiments when the scaling parameter in Hooges relation, , is set to . This value agrees with the experimentally determined value for a-Si:H. The results are consistent with the hypothesis that the low frequency noise in the a-Si:H TFTs is due to the material itself.

X-ray medical imaging provides invaluable medical information, while subjecting patients to hazardous ionizing radiation. The dosage that the patient is exposed to may be reduced, at the cost of image resolution. A technology that promises lower dosage for a given resolution is direct conversion digital imaging, typically based on amorphous Selenium semiconductor. Sufficient exposure should be used for the first exposure to avoid subsequent exposures; a challenge is then to reduce the necessary exposure for a suitable image. To quantify how little radiation the detector can reliably discriminate, one needs an analysis of the variance that 1/f and white noise contribute to the signal of such detectors. An important consideration is that the dark current, which varies with time, is subtracted from the photo-current, to reduce the spurious spatial variance in the image. In this thesis, the variance that 1/f noise contributes to integrating detectors is analysed, for a very general integrating detector. Experiments were performed to verify the theoretical results obtained for the 1/f noise variance contribution.

Research focusing on static scenes with static objects is omitting the time factor from real life examples we are trying to study. Can we say that a lifeguard looking for a drowning man is using the same brain processes that were observed in the laboratory for static scenes? We can conclude that a static scene is a big simplification of the task itself. The aim of this thesis is to prepare a tool which would allow researching dynamic scenes and thus broadening the possibilities of visual detection tasks at hand. Along the tool we also present a couple of simplified examples with which we would like to demonstrate the utilization of the tool. All concluding with a final experiment in which we will try to detect masked patterns in a noisy environment. 1

As the technology is advances the reduced size of hardware gives rise to an additive 1/f baseband noise. This additive 1/f noise is a system noise generated due to miniaturization of hardware and affects the lower frequencies. Though 1/f noise does not show much effect in wide band channels because of its nature to affect only certain frequencies, 1/f noise becomes a prominent in OFDM communication systems where narrow band channels are used. in this thesis, I study the effects of 1/f noise on the OFDM systems and implement algorithms for estimation and suppression of the noise using Kalman filter. Suppression of the noise is achieved by subtracting the estimated noise from the received noise. I show that the performance of the system is considerably improved by applying the 1/f noise suppression.

Les travaux de la thèse ont porté sur l'étude, la réalisation, la caractérisation et la modélisation de films 2D à base de nanotubes de carbone. Dans le premier chapitre nous avons présenté des généralités sur les nanotubes de carbone. Ensuite, nous nous sommes intéressés aux jonctions nanotube-nanotube et plus particulièrement à la modélisation du transport dans les différents types de jonction (M/M), (M/SC) et (SC/SC). Avec le deuxième chapitre nous avons entamé l'étude des films 2D à base de nanotubes de carbone. Dans un premier temps nous nous sommes intéressés au transport électrique dans ces structures fortement inhomogènes, en particulier en décrivant les modèles analytiques rendant compte du phénomène de percolation tant au niveau de la conductance que du bruit en 1/f. La seconde partie du chapitre est entièrement consacrée à la fabrication et la caractérisation physico-chimique des films 2D L'objectif principal du troisième chapitre est la modélisation des films 2D de nanotubes de carbone. Par rapport aux autres modèles utilisés dans la littérature, le modèle développé dans cette partie est le seul à prendre en compte la nature physique de chaque jonction tube-tube : (M/M) ou (M/SC) ou (SC/SC). Notre modèle prend ainsi en compte les non linéarités des jonctions. La résolution numérique de ce système est optimisée : i) en utilisant la technique MNA, technique dont le principe consiste à linéariser chaque dipôle du circuit. ii) en parallélisant les calculs sur un cluster informatique d'une centaine de cœurs. Pour le calcul du bruit la même technique est utilisée mais avec, dans ce cas, l'utilisation de la méthode du réseau adjoint. Dans le quatrième chapitre, nous avons, dans un premier temps, présentés et analysés nos résultats expérimentaux concernant la mesure de la conductance et du bruit en 1/f. Quelles que soient les conditions de dépôt nous avons toujours observé un comportement de type percolation au niveau des grandeurs mesurées, conductance et niveau de bruit en 1/f. Nous avons utilisé les paramètres d'ajustement des lois de percolation pour comparer et analyser nos résultats. Il en ressort que l'impact du surfactant sur l'homogénéité de la solution, se retrouve au niveau des résultats électriques des couches déposées, montrant l'avantage d'utiliser du sel biliaire. Quant à l'influence de la densité des tubes, comme attendu, la conductance augmente avec celle-ci. Par contre nous avons remarqué que le bruit en 1/f était beaucoup plus sensible à ce paramètre, avec en particulier un changement significatif au niveau des paramètres de percolation en bruit mis en évidence à forte densité de nanotubes. La deuxième partie de ce chapitre est dédiée à la simulation des paramètres électriques de nos structures expérimentales. Nous avons paramétré l'énergie et la largeur des barrières de potentiel entre chaque jonction. Ces paramètres sont ajustés à partir des résultats expérimentaux et sont fonction de la nature du surfactant. Les résultats de ces simulations concernant la conductance et le niveau de bruit en 1/f s'accordent avec les mesures et dans tous les cas les lois de percolation macroscopique sont respectées, ce qui valide nos modèles ainsi que la possibilité d'intégrer de façon réaliste la différence structurale des surfactants. Pour rendre compte de la déviation de la loi macroscopique de percolation du bruit en 1/f, observée sur les films déposés à partir de solution à forte densité de surfactant, nous avons au niveau des simulations introduit et modulé le nombre d'amas (clusters) de nanotubes en fonction de la densité des couches. Là encore le bon accord observé avec les résultats expérimentaux nous permet de valider la présence d'inhomogénéités dues aux clusters de nanotubes dans nos dépôts. / In this thesis we have focused on the fabrication, the characterization, and the modeling of 2D films based on carbon nanotubes.In the first chapter, we have presented general informations on carbon nanotubes. Then we are interested in the nanotube-nanotube junctions and particularly the modeling of transport in different types of junction (M/M), (M/SC) and (SC/SC).In the second chapter we have presented a study of 2D films based on carbon nanotubes. At first we present the electrical transport in these structures strongly inhomogeneous, especially in describing the analytical models accounting for the percolation phenomenon both in the conductance and 1/f noise. The second part of the chapter is devoted entirely to the manufacture and physico-chemical characterization of 2D films.The main objective of the third chapter is the modeling of 2D films of carbon nanotubes. Compared to other models described in the literature, the model developed in this section is the only one that take into account the physical nature of each tube-tube junction (M/M) or (M/SC) or (SC/SC). Our model takes into account the junction nonlinearity. The numerical solution of the system is optimized: i) using the MNA technique whose principle is to linearize each dipole in the circuit. ii) parallelizing computations on a computer cluster of a hundred core. For the noise simulation, the same technique is used but in this case, we have used the adjoint network method. In the fourth chapter, we have, at first, presented and analyzed our experimental results for conductance and 1/f noise. Whatever the deposition conditions we always observed a percolation-like behavior of our results. We used the fitting parameters of the percolation laws to compare and analyze our results. It appears that the impact of the surfactant on the homogeneity of the solution is found in the electrical measurement results of deposited films. As for the influence of the density of the tubes, as expected, the conductance increases with the increase of nanotubes density. We noticed that the 1/f noise was much more sensitive to this parameter, with in particular a significant change in the noise percolation parameters revealed at high density of nanotubes. The second part of this chapter is dedicated to the simulation of the electrical parameters of our experimental structures. These parameters are adjusted on the basis of experimental results and are based on the nature of the surfactant. The results of these simulations for the conductance and 1/f noise agree with measurements and in all cases the macroscopic percolation laws are respected, which validate our models. To bring to the fore the deviation from the noise percolation law observed in films deposited from solution with a high density of surfactant, we have introduced in our simulated structures a number of clusters of nanotubes according to the density of the deposited layers. Once again we observed a good agreement with the experimental results allowing us to validate the presence of clusters of nanotubes in our deposited films.

In a visual search experiment, the subject must find a target item hidden in a display of other items, and their performance is measured by their reaction time (RT). Here I look at how visual search reaction times are correlated with past reaction times. Target-absent RTs (i.e. RTs to displays that have no target) are strongly correlated with past target-absent RTs and, treated as a time series, have a 1/f power spectrum. Target-present RTs, on the other hand, are effectively uncorrelated with past RTs. A model for visual search is presented which generates search RTs with this pattern of correlations and power spectra. In the model, search is conducted by matching search items up with ¿categorizers,¿ which take a certain time to categorize each item as target or distractor; the RT is the sum of categorization times. The categorizers are drawn at random from a pool of active categorizers. After each search, some of the categorizers in the active pool are replaced with categorizers drawn from a larger population of unused categorizers. The categorizers that are not replaced are responsible for the RT correlations and the 1/f power spectrum.

The aim of this research study is to investigate different practical ways in which fractal geometry and self-similarity can be applied to art music, with reference to music composition and analysis. This specific topic was chosen because there are many misconceptions in the field of fractal and self-similar music. Analyses of previous research as well as the music analysis of several compositions from different composers in different genres were the main methods for conducting the research. Although the dissertation restates much of the existing research on the topic, it is (to the researcher‟s knowledge) one of the first academic works that summarises the many different facets of fractal geometry and music. Fractal and self-similar shapes are evident in nature and art dating back to the 16th century, despite the fact that the mathematics behind fractals was only defined in 1975 by the French mathematician, Benoit B. Mandelbrot. Mathematics has been a source of inspiration to composers and musicologists for many centuries and fractal geometry has also infiltrated the works of composers in the past 30 years. The search for fractal and self-similar structures in music composed prior to 1975 may lead to a different perspective on the way in which music is analysed. Basic concepts and prerequisites of fractals were deliberately simplified in this research in order to collect useful information that musicians can use in composition and analysis. These include subjects such as self-similarity, fractal dimensionality and scaling. Fractal shapes with their defining properties were also illustrated because their structures have been likened to those in some music compositions. This research may enable musicians to incorporate mathematical properties of fractal geometry and self-similarity into original compositions. It may also provide new ways to view the use of motifs and themes in the structural analysis of music. / Dissertation (MMus)--University of Pretoria, 2014. / lk2014 / Music / MMus / Unrestricted

The thesis deals issue of the silicon solar cells non-destructive testing. The manufacturing technology of solar cells currently features a very high level of perfection. Its further development appears to be limited by amongst other issues imperfect diagnostic methods. The objective of presented research consists in non-destructive studies of processes that influence specimen life and reliability. To this end, I will employ mainly noise based analytical methods in connection with observation of defect optical activities, capacitance measurement etc. These methods are closely related to some specimen bulk imperfections, crystal-lattice defect induced traps, local-stress-subjected regions and, finally, breakdowns, which might bring about specimen destruction. Based on a detailed study and understanding of transport processes, regions in which noise is generated can be identified and appropriate technological measures can be proposed and adopted. Presented research focuses, first of all, on the real solar cell structures, which are inhomogeneous in their nature and are difficult to diagnose. The significant part of this study is attend to the random n-level (in most case just two-level) impulse noise, usually referred to as microplasma noise. This noise is a consequence of local breakdowns in micro-sized regions and brings about reduction of lifetime or destruction of the pn junction. The micro-sized regions have been studied separately by electrical and optical methods and defect properties have been put forward. Nevertheless, no less significant part of the thesis is devoted to the fluctuation modeling of the bulk imperfections in the semi-analytical form.