M.Ing. (Electrical and Electronic Engineering) / During the past decade the optical disc has become increasingly popular. Write-once optical recording systems will mainly be used in data storage systems in which archival aspects or mass storage requirements prevail. In write-once optical data storage one is faced with an asymmetry between marks and non-marks due to a practical lower limit of the mark size. In some optical fibre communications there is also an asymmetry present in injection lasers and it may be feasible to use asymmetrical codes. In this study information theoretical methods are used to find values of channel capacity for sequences complying with binary asymmetrical runlength constraints. Different coding methods are used to construct encoders and decoders for generating and decoding these sequences with high values of efficiency. The power spectra of maxentropic binary asymmetrical runlength limited sequences complying with different runlength constraints are also investigated.

Thesis (M.E.)--University of Canterbury, 2006. / Typescript (photocopy). "30th January 2006." Includes bibliographical references (p. 127-130). Also available via the World Wide Web.

A large spectrum of new technologies are being explored in the optical disk drive systems. Optics, lasers, media, and servomechanisms are a few examples. This thesis will be directed to the study of a servomechanism used in a majority of the optical disk drives, commonly referred to as the tracking servomechanism. The tracking servomechanism, consisting of a fine and a coarse actuator, is mechanically analyzed by the use of free body diagrams. A transfer function for each actuator is derived. Analog compensators are designed to achieve specific phase and gain margin requirements. A digital compensator is derived from the analog compensator by the use of a mapping technique. Major contributions of this thesis include studies to determine an acceptable sampling rate, number of bits, and computation delay associated with the implementation of a digital servo controller in a tracking servomechanism.

Computer simulations of the optical servo functions of optical disk drives are developed and compared with experimental results. The focus control servo is investigated first, with emphasis on the astigmatic focus detection method. A paraxial ray trace, enhanced to allow tolerance studies of tilted and decentered surfaces, is used to calculate the size and orientation of an astigmatic blur on a quadrant photodetector as a function of focus error. The resulting irradiance distribution is integrated over the detector elements and processed to yield typical focus servo signals. A method for simulating generalized astigmatic focus systems, independent of a particular design, is also shown. The simulation results are used to derive normalized tolerance curves for detector misalignment and spot motion. Alignment diagnostics based on the servo signals are also presented. A wavefront aberration model is also developed and used to investigate the focus servo's performance in the presence of common aberrations. Simulations based on diffraction theory are used to investigate the radial tracking servo. Both scalar and vector diffraction theories are considered. The scalar theory is found to be adequate in most cases, while offering a large advantage in computational efficiency. A model for computing the signals detected by scanning the microscopic features of the disk is developed using the optical cross transfer function that describes the imaging characteristics of partially coherent systems. This model is used to investigate push-pull and three beam tracking. Aberrations, data patterns, detector misalignment, and pregroove profile are all examined for their effects on the servo signals. Crosstalk between the focus and tracking error detection is also briefly considered, and a possible extension of the radial tracking servo model to investigate this phenomenon is suggested.

No description available.

This thesis concerns a study of the application, and performance optimization, of standard lead-lag compensation techniques to improve the closed loop performance of a focus servo system for an optical data storage device. Only with proper application of these compensation techniques will it be possible to meet the sub-micrometer focus error tolerances while maintaining the stability of the closed loop system. The performance indices used in this optimization study are the Integral of the Squared Error (ISE), the Integral of the Absolute Error (IAE), and the Integral of the Time multiplied by the Absolute Error (ITAE) as defined below, with the error function e(t) being the focus error of the closed loop servo system in response to a step input. ISE= ∫OT e²(t) dt IAE= ∫OT / e(t) / dt ITAE= ∫OT t / e(t) / dt

M.A. / CD-ROM products provide access to information by means of different modes of interaction, often on the same database. Although command language is still widely recognized as the interaction mode which retrieves the most relevant references, it is regarded as difficult to use because of its complex structure. More user friendly modes, for example, menu and direct manipulation are viewed as being more accessible to the up-and-coming end-user. The purpose of this study was to determine by means of an empirical study whether the retrieval effectiveness between two modes of interaction on the same database differed significantly. A literature survey pointed out the unique characteristics of existing modes. It was also established that the traditional measures of retrieval effectiveness through relevance and precision could not be applied in this research. A method was devised in which the results of the two modes were compared. The empirical study was done on the command and form fill-in modes of Wilson Business Abstracts. Total results retrieved through each mode were compared, as well as the ease of entering the search by means of appropriate search facilities for each mode. The results of the research revealed that the presence of unique search facilities in a mode results in better retrieval effectiveness. Searches in both modes also require specific ways of input for optimum quality retrieval and thus has implications for intensive training in search methods.

Thesis (Ed. D.)--West Virginia University, 1989. / Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 101-106).