Combining Vocal Tract Length Normalization with Linear Transformations in a Bayesian Framework

Combining Vocal Tract Length Normalization with Hierarchical Linear Transformations

Lakshmi Babu Saheer, John David Scott Dines, Philip Neil Garner

Recent research has demonstrated the effectiveness of vocal tract length normalization (VTLN) as a rapid adaptation technique for statistical parametric speech synthesis. VTLN produces speech with naturalness preferable to that of MLLR-based adaptation techniques, being much closer in quality to that generated by the original average voice model. However, with only a single parameter, VTLN captures very few speaker specific characteristics when compared to linear transform based adaptation techniques. This paper shows that the merits of VTLN can be combined with those of linear transform based adaptation in a hierarchical Bayesian framework, where VTLN is used as the prior information. A novel technique for propagating the gender and age information captured by the VTLN transform into constrained structural maximum a posteriori linear regression (CSMAPLR) adaptation is presented. This paper also compares this proposed technique to other combination techniques. Experiments are performed on both matched and mismatched training and test conditions, including gender, age, and recording environments. Text-to-speech (TTS) synthesis experiments show that the resulting transformation produces improved speech quality with better naturalness and intelligibility (similar to VTLN transformation) when compared to the CSMAPLR transformation, especially when the quantity of adaptation data is very limited. With more parameters to capture speaker characteristics, the proposed method performs better in speaker similarity compared to VTLN in mis-matched conditions. Hence, the proposed combination combines the quality and intelligibility of VTLN with the speaker similarity of CSMAPLR especially in the mismatched train and test conditions. Experiments are also performed using the automatic speech recognition (ASR) system in a unified framework as that of synthesis. This is to prove that the techniques developed for TTS can be plugged into ASR in order to improve the performance.

2014

COMBINING VOCAL TRACT LENGTH NORMALIZATION WITH HIERARCHIAL LINEAR TRANSFORMATIONS

Lakshmi Babu Saheer, John David Scott Dines, Philip Neil Garner

IEEE SPS (ICASSP)2012

Unified Framework of Feature Based Adaptation for Statistical Speech Synthesis and Recognition

Lakshmi Babu Saheer

The advent of statistical parametric speech synthesis has paved new ways to a unified framework for hidden Markov model (HMM) based text to speech synthesis (TTS) and automatic speech recognition (ASR). The techniques and advancements made in the field of ASR can now be adopted in the domain of synthesis. Speaker adaptation is a well-advanced topic in the area of ASR, where the adaptation data from a target speaker is used to transform the canonical model parameters to represent a speaker specific model. Feature adaptation techniques like vocal tract length normalization (VTLN) perform the same task by transforming the features ; this can be shown to be equivalent to model transformation. The main advantage of VTLN is that it can demonstrate noticeable improvements in performance with very little adaptation data and can be classified as a rapid adaptation technique. VTLN is a widely used technique in ASR, and can be used in TTS to improve the rapid adaptation performance. In TTS, the task is to synthesize speech that sounds like a particular target speaker. Using VTLN for TTS is found to make the output synthesized speech sound quite similar to the target speaker from his very first utterance. An all-pass filter based bilinear transform was implemented for the mel-generalized cepstral (MGCEP) features of the HMM-based speech synthesis system (HTS). The initial implementation was using a grid search approach that selects the best warping factor for the speech spectrum from a grid of available values using the maximum likelihood criterion. VTLN was shown to give performance improvements in the rapid adaptation framework where the number of adaptation sentences from the target speaker was limited. But, this technique involves huge time and space complexities and the rapid adaptation demands for an efficient implementation of the VTLN technique. To this end, an efficient expectation maximization (EM) based VTLN approach was implemented for HTS using Brent’s search. Unlike the ASR features, MGCEP does not use a filter bank (in order to facilitate the speech reconstruction) and this provides equivalence to the model transformation for the EM implementation. This facilitates the estimation of warping factors to be embedded in the HMM training using the same sufficient statistics as in constrained maximum likelihood linear regression (CMLLR). This work addresses a lot of challenges faced in the process of adopting VTLN for synthesis due to the higher dimensionality of the cepstral features used in the TTS models. The main idea was to unify the theory and practise in the implementation of VTLN for both ASR and TTS. Several techniques have been proposed in this thesis, in order to find the best feasible warping factor estimation procedure. Estimation of the warping factor using the lower order cepstral features representing the spectral envelope is demonstrated to be the best approach. Different evaluations on standard databases are performed in this work to illustrate the performance improvements and perceptual challenges involved in the VTLN adaptation. VTLN has only a single parameter to represent the speaker characteristics and hence, has the limitation of not scaling to the performance of other linear transform based adaptation methods with the availability of large amounts of adaptation data. Several techniques are demonstrated in this work to combine the model based adaptation like constrained structural maximum a posteriori linear regression (CSMAPLR) with VTLN, one such technique being using VTLN as the prior transform at the root node of the tree structure of the CSMAPLR system. Thus, along with rapid adaptation, the performance scales with the availability of more adaptation data. These techniques although developed for TTS, can also be effectively used in ASR. It was also shown to give improvements in ASR especially for scenarios like noisy speech conditions. Other improvements to rapid adaptation including a bias term for VTLN, multiple transform based VTLN using regression classes and VTLN prior for non-structural MAPLR adaptation are also proposed. These techniques also demonstrated both ASR and TTS performance improvements. Also, a few special scenarios, specifically cross-lingual speech, cross-gender speech, child speech and noisy speech evaluations are presented where the rapid adaptation methods presented in this work was shown to be highly beneficial. Most of these methods will be published as extensions to the open-source HTS toolkit.

EPFL2013

Unified Framework of Feature Based Adaptation for Statistical Speech Synthesis and Recognition

Lakshmi Babu Saheer

EPFL2013

Combining Vocal Tract Length Normalization with Hierarchical Linear Transformations

Lakshmi Babu Saheer, John David Scott Dines, Philip Neil Garner

2014