Handwritten address recognition using hidden markov models

In this paper several aspects of a recognition system for cursive handwritten German address words (cities and streets) are described. The recognition system is based on Hidden Markov Models (HMMs), whereat the focus is on two main problems: the changes in the writing style depending on time or regional differences and the difficulty to select the correct (complete) dictionary for address reading. The first problem leads to the examination of three different adaptation techniques: Maximum Likelihood (ML), Maximum Likelihood Linear Regression (MLLR) and Scaled Likelihood Linear Regression (SLLR). To handle the second problem language models based on backoff character n-grams are examined to evaluate the performance of an open vocabulary recognition (without dictionary). For both problems the determination of confidence measures (based on the frame-normalized likelihood, a garbage model, a two-best recognition or an unconstrained character decoding) is important, either for an unsupervised adaptation or the detection of out of vocabulary words (OOV). The databases, which are used for recognition, are provided by Siemens Dematic (SD) within the Adaptive READ project.