Browsing by Author "Asimopoulos, Nikos"

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    Design and implementation of a portable omnifont reading aid for the blind
    Asimopoulos, Nikos (Virginia Tech, 1990-07-11)
    The design and implementation of a handheld scanner that can help sight-impaired or even blind users to manually scan and read text is discussed in this dissertation. A thorough investigation of all the elements involved in such a system is presented and optimal solutions are proposed. A unique velocity compensation technique based solely on optical information obtained by the scanning device is discussed and a real time segmentation technique based on topological properties (Quasi-Topological Codes) of connected segments is presented. A skew detection algorithm is discussed that can trace typed and printed text manual1y scanned with skew up to 15 degrees and can guide blind users to properly scan a document. Real time extraction of quasitopological codes for automatic text recognition and the hardware implementation is also discussed in this work. A hierarchical optical character recognition method is proposed which is based on syntactic and metric analysis of the Quasi-Topological Codes and their position in the scanned image. The proposed method can recognize characters stretched to approximately two times their original width or rotated by a few degrees. Finally, an automated iterative learning process is discussed which includes generalization of the recognition logic and dynamic adaptation of the syntactic and metric recognition rules.
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    Performance of error correcting codes with random and burst errors
    Asimopoulos, Nikos (Virginia Polytechnic Institute and State University, 1983)
    The errors that can occur in a computer system during data reading or recording in the memory can be of different types depending upon the memory organization. They can be random bit errors or burst errors. Therefore, if high reliability is required, the use of an error correcting technique that will be able to handle both types of errors is necessary. In this study the capability of some classes of error correcting codes is analysed and their performance with both types of errors is tested. Reed Solomon and concatenated codes are examined in more detail because they are known to be among the best classes of codes. In order to evaluate the performance of these codes two well known classes of codes are used: BCH codes and Fire codes. The performance of all the codes with regard to random error correction is analysed using a binary symmetric channel model. BCH codes are shown to be more powerful for average codeword length, but as the codeword length increases RS and concatenated codes perform better than BC3 codes of the same rate of transmission. A new model for systems with burst errors is introduced with which a large variety of real systems can be simulated by choosing the appropriate distributions of burst errors. The performance of all these codes at correcting burst errors is simulated using this model. It is shown that RS codes and concatenated codes are very powerful with burst errors and can increase significantly the reliability of a signaling system incorporating these types of errors. An advantage of RS codes compared to concatenated codes is that they can be very easily implemented and can be employed efficiently for systems with any codeword length. Concatenated codes can perform better than RS codes only when very long codewords are required.