The concept of acoustically stable and unstable intervals to structure continuous speech is introduced. We present a method to compute stable intervals efficiently and reliably as a bottom-up approach at an early processing stage. We argue that such intervals stand in close relation to the rhythm of speech as they contribute to the overall temporal organization of the speech production process and the acoustic signal (stable intervals = intervals of reduced movement of certain articulators; unstable intervals = intervals of enhanced movement of certain articulators). To test the relationship of stability intervals with speech rhythm we investigated the between-speaker variability of stable and unstable intervals in the TEVOID corpus. Results revealed that significant between-speaker variability exists. We hypothesize from our findings that the basic segmentation of speech into stable and unstable intervals is a process that might play a role in human perception and processing of speech.

An information retrieval model is presented for the retrieval of speech documents, i.e. audio recordings containing speech. The indexing vocabulary consists of indexing features that have the following characteristics. First, they are easy to recognize by speech recognition methods. Second, the number of different indexing features is small such that a reasonable amount of training data is sufficent to train the hidden Markov models that are used by the speech recognition process. Third, the retrieval method based on such indexing features achieves an acceptable retrieval effectiveness as shown by experiments on text collections. Fourth, these indexing features cannot only be identified in speech documents but also in text documents. From the last characteristic follows that speech documents and text documents can be retrieved simultaneously. Analogously, the queries may contain either speech or text. Thus, we have a simple multimedia retrieval model where two different medias are indexed coherently. We also describe a prototype retrieval system under development.