【正文】 y years before unlimited vocabulary, speakerindependent continuous dictation capability is realized. 3 Future Directions In 1992, the . National Science Foundation sponsored a workshop to identify the key research challenges in the area of human language technology, and the infrastructure needed to support the work. The key research challenges are summarized in. Research in the following areas for speech recognition were identified: Robustness: In a robust system, performance degrades gracefully (rather than catastrophically) as conditions bee more different from those under which it was trained. Differences in channel characteristics and acoustic environment should receive particular attention. Portability: Portability refers to the goal of rapidly designing, developing and deploying systems for new applications. At present, systems tend to suffer significant degradation when moved to a new task. In order to return to peak performance, they must be trained on examples specific to the new task, which is time consuming and expensive. Adaptation: How can systems continuously adapt to changing conditions (new speakers, microphone, task, etc) and improve through use? Such adaptation can occur at many levels in systems, subword models, word pronunciations, language models, etc. Language Modeling: Current systems use statistical language models to help reduce the search space and resolve acoustic ambiguity. As vocabulary size grows and other constraints are relaxed to create more habitable systems, it will be increasingly important to get as much constraint as possible from language models。s performance typically degraded when it was presented with previously unseen data. The recent availability of a large body of data in the public domain, coupled with the specification of evaluation standards, has resulted in uniform documentation of test results, thus contributing to greater reliability in monitoring progress (corpus development activities and evaluation methodologies are summarized in chapters 12 and 13 respectively). Finally, advances in puter technology have also indirectly influenced our progress. The availability of fast puters with inexpensive mass storage capabilities has enabled researchers to run many large scale experiments in a short amount of time. This means that the elapsed time between an idea and its implementation and evaluation is greatly reduced. In fact, speech recognition systems with reasonable performance can now run in real time using highend workstations without additional hardwarea feat unimaginable only a few years ago. One of the most popular, and potentially most useful tasks with low perplexity (PP=11) is the recognition of digits. For American English, speakerindependent recognition of digit strings spoken continuously and restricted to telephone bandwidth can achieve an error rate of % when the string length is known. One of the best known moderateperplexity tasks is the 1,000word socalled Resource 5 Management (RM) task, in which inquiries can be made concerning various naval vessels in the Pacific ocean. The best speakerindependent performance on the RM task is less than 4%, using a wordpair language model that constrains the possible words following a given word (PP=60). More recently, researchers have begun to address the issue of recognizing spontaneously generated speech. For example, in the Air Travel Information Service (ATIS) domain, word error rates of less than 3% has been reported for a vocabulary of nearly 2,000 words and a bigram language model with a perplexity of around 15. High perplexity tasks with a vocabulary of thousands of words are intended primarily for the dictation application. After working on isolatedword, speakerdependent systems for many years, the munity has since 1992 moved towards verylargevocabulary (20,000 words and more), highperplexity (PP≈200), speakerindependent, continuous speech recognition. The best system in 1994 achieved an error rate of % on read sentences drawn from North America business news. With the steady improvements in speech recognition performance, systems are now being deployed within telephone and ce