Annual Report 2010 - Fachgruppe Informatik an der RWTH Aachen ...

the most probable sentence. These modules of an automatic speech recognition system (cf.
Figure above) are characterized as follows:
• The acoustic model captures the acoustic properties of speech and provides the probability
of the observed acoustic signal given a hypothesized word sequence. The acoustic model
includes:
• The acoustic analysis which parameterizes the speech input into a sequence of acoustic
vectors.
• Acoustic models for the smallest sub-word units, i.e. phonemes which usually are
modeled in a context dependent way.
• The pronunciation lexicon, which defines the decomposition of the words into the subword
units.
• The language model captures the linguistic properties of the language and provides the a-
priori probability of a word sequence. From an information theoretic point of view,
syntax, semantics, and pragmatics of the language could also be viewed as redundancies.
Statistical methods provide a general framework to model such redundancies robustly.
Therefore state-of-the-art language models usually are based on statistical concepts.
• The search realizes Bayes decision criterion on the basis of the acoustic model and the
language model. This requires the generation and scoring of competing sentence
hypotheses. To obtain the final recognition result, the main objective then is to search for
that sentence hypothesis with the best score, which is done efficiently using dynamic
programming. The efficiency of the search process is increased by pruning unlikely
hypotheses as early as possible during dynamic programming without affecting the
recognition performance.
(a) Speech waveform of the utterance “Sollen wir am Sonntag nach Berlin fahren”, (b) the
corresponding FFT spectrum
At ‘Lehrstuhl für Informatik 6’, the following research directions related to all main areas of
automatic speech recognition (ASR) were pursued in 2008/09:
The generation of the European Parliament Plenary Session (EPPS) corpus for speech
recognition and speech-to-speech translation was continued for the main European
languages. This corpus consists of transcribed speech and parallel texts in the languages
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