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Section: Research Program

Statistical modeling of speech

Whereas the first research direction deals with the physical aspects of speech and its explicit modeling, this second 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.

Acoustic modeling

Acoustic modeling is a key issue for automatic speech recognition. Despite progress made for many years, acoustic modeling is still far from perfect, and current speech recognition applications rely on strong constraints (limited vocabulary, speaker adaptation, restricted syntax...) to achieve acceptable performance. As the acoustic models represent the acoustic realization of the sounds, they have to account for many variability sources, such as speaker characteristics, microphones, noises, etc. Extension of the HMM formalism based on the Dynamic Bayesian Networks (DBN) formalism are investigated further for handling such variability sources; as well as other approaches to dynamically constrain the search space according to known or estimated characteristics of the utterance being processed. Deep Neural Networks (DNN) based approaches will also be investigated as means of making speech recognition systems more accurate and robust. Speaker dependent modeling and speaker adaptation will also be investigated in relation with HMM-based speech synthesis and statistical voice conversion.

State-of-the-art speech recognition systems are still very sensitive to the quality of speech signals they have to deal with; their performance degrades rapidly when they deal with noisy signals. Accurate signal enhancement techniques are therefore essential to increase the robustness of both automatic speech recognition and speech-text alignment systems to noise and non-speech events. In MULTISPEECH, focus is set on Bayesian source separation techniques using multiple microphones and/or models of non-speech events. Some of the challenges include building a non-parametric model of the sources in the time-frequency-channel domain, linking the parameters of this model to the cepstral representation used in speech processing, modeling the temporal structure of environmental noise, and exploiting large audio data sets to automatically discover new models. Beyond the definition of such complex models, the difficulty is to design scalable estimation algorithms robust to overfitting, that will be integrated in the FASST [6] framework that was recently developed.

Linguistic modeling

MULTISPEECH investigates lexical and language models in speech recognition with a focus on improving the processing of proper names and the processing of spontaneous speech. Collaborations are ongoing with the SMarT team on linguistic modeling aspects.

Proper names are relevant keys in information indexing, but are a real problem in transcribing many diachronic spoken documents (such as radio or TV shows) which refer to data, especially proper names, that evolve over the time. This leads to the challenge of dynamically adjusting lexicons and language models through the use of the context of the documents or of some relevant external information possibly collected over the web. Random Indexing (RI) and Latent Dirichlet Allocation (LDA) are two possible approaches to be used for this purpose. Also, to overcome the limitations of current n-gram based language models, we investigate language models defined on a continuous space in order to achieve a better generalization on unseen data, and to model long-term dependencies. This is achieved through neural network based approaches. We also want to introduce into these new models additional relevant information such as linguistic features, semantic relation, topic or user-dependent information.

Spontaneous speech utterances are often ill-formed and frequently contain disfluencies (hesitations, repetitions...) that degrade speech recognition performance. This is partly due to the fact that disfluencies are not properly represented in linguistic models estimated from clean text data (coming from newspapers for example); hence a particular effort will be set for improving the modeling of these events.

Attention will also be set on pronunciation lexicons in particular with respect to non-native speech and foreign names. Non-native pronunciation variants have to take into account frequent miss-pronunciations due to differences between mother tongue and target language phoneme inventories. Proper name pronunciation variants are a similar problem where difficulties are mainly observed for names of foreign origin that can be pronounced either in a French way or kept close to foreign origin native pronunciation. Automatic grapheme-to-phoneme state-of-the-art approaches, based for example on Joint Multigram Models (JMM) or Conditional Random Fields (CRF) will be further investigated and combined.

Speech generation by statistical methods

Voice conversion consists in building a function that transforms a given voice into another one. MULTISPEECH applies voice conversion techniques to enhance pathological voices that result from vocal folds problems, especially esophageal voice or pathological whispered voice. Voice conversion techniques are also of interest for text-to-speech synthesis systems as they aim at making possible the generation of new voice corpora (other kind of voice, or same voice with different kind of emotion).

In addition to the statistical aspects of the voice conversion approaches, signal processing is critical for good quality speech output. Information on the fundamental frequency is chaotic in the case of esophageal speech or non-existent in the case of the whispered voice. So after applying voice conversion techniques for enhancing pathological voices, the excitation spectrum must be predicted or corrected. That is the challenge that is addressed in the project. Also, in the context of acoustic feedback in foreign language learning, voice modification approaches (either statistical or not) will be investigated to modify the learner’s (or teacher’s) voice in order to emphasize the difference between the learner’s acoustic realization and the expected realization.

Over the last few years statistical speech synthesis has emerged as an alternative to corpus-based speech synthesis. Speaker-dependent HMM modeling constitute the basis of such an approach. The announced advantages of the statistical speech synthesis are the possibility to deal with small amounts of speech resources and the flexibility for adapting models (for new emotions or new speaker), however, the quality is not as good as that of the concatenation-based speech synthesis. The reasons are twofold: first, parameters (F0, spectrum, duration...) are modeled independently and the models, even when taking into account dynamics, do not manage to generate parameters with a good precision. Second, the HMM generates sequences of feature vectors from which the actual speech signals are reconstructed, and this impacts on its quality. MULTISPEECH will focus on an hybrid approach, combining corpus-based synthesis, for its high-quality speech signal output, and HMM-based speech synthesis for its flexibility to drive selection, and the main challenge will be on its application to producing expressive audio-visual speech. One secondary objective will be to unify the HMM-based and the concatenation-based approaches.