Speech signals are the consequence of the deformation of the vocal tract under the effect of the movements of the jaw, lips, tongue, soft palate and larynx to modulate the excitation signal produced by the vocal cords or air turbulence. These deformations are visible on the face (lips, cheeks, jaw) through the coordination of different orofacial muscles and skin deformation induced by the latter. These deformations may also express different emotions. We should note that human speech expresses more than just phonetic content, to be able to communicate effectively. In this project, we address the different aspects related to speech production from the modeling of the vocal tract up to the production of audiovisual speech. On the one hand, we study the relationship from acoustic speech signal to vocal tract, in the context of acoustic-to-articulatory inversion, and from vocal tract to acoustic speech, in the context of articulatory synthesis. On the other hand, we work on expressive audiovisual speech synthesis, where both expressive acoustic speech and visual signals are generated from text. Phonetic contrasts used by the phonological system of any language result from constraints imposed by the nature of the human speech production apparatus. For a given language these contrasts are organized so as to guarantee that human listeners can identify sounds robustly. From the point of view of perception, these contrasts enable efficient processes of categorization in the peripheral and central human auditory system. The study of the categorization of sounds and prosody thus provides a complementary view on speech signals by focusing on the discrimination of sounds by humans, particularly in the context of language learning.
Statistical modeling of speech
This research direction is concerned by investigating complex statistical models for speech data. Acoustic models are used to represent the pronunciation of the sounds or other acoustic events such as noises. Whether they are used for source separation, for speech recognition, for speech transcription, or for speech synthesis, the achieved performance strongly depends on the accuracy of these models, which is a critical aspect that is studied in the project. At the linguistic level, MULTISPEECH investigates models for handling the context (beyond the few preceding words currently handled by the n-gram models) and evolutive lexicons necessary when dealing with diachronic audio documents in order to overcome the limited size of the current static lexicons used, especially with respect to proper names. Statistical approaches are also useful for generating speech signals. Along this direction, MULTISPEECH mainly considers voice transformation techniques, with their application to pathological voices, and statistical speech synthesis applied to expressive multimodal speech synthesis.
Uncertainty estimation and exploitation in speech processing
We focus here on the uncertainty associated to some processing steps. Uncertainty stems from the high variability of speech signals and from imperfect models. For example, enhanced speech signals resulting from source separation are not exactly the clean original speech signals. Words or phonemes resulting from automatic speech recognition contain errors, and the phone boundaries resulting from automatic speech-text alignment are not always correct, especially in acoustically degraded conditions. Hence it is important to know the reliability of the results and/or to estimate the uncertainty on the results.