The listening talker: A review of human and algorithmic context-induced modifications of speech

International audienceSpeech output technology is finding widespread application, including in scenarios where intelligibility might be compromised - at least for some listeners - by adverse conditions. Unlike most current algorithms, talkers continually adapt their speech patterns as a response to the immediate context of spoken communication, where the type of interlocutor and the environment are the dominant situational factors influencing speech production. Observations of talker behaviour can motivate the design of more robust speech output algorithms. Starting with a listener-oriented categorisation of possible goals for speech modification, this review article summarises the extensive set of behavioural findings related to human speech modification, identifies which factors appear to be beneficial, and goes on to examine previous computational attempts to improve intelligibility in noise. The review concludes by tabulating 46 speech modifications, many of which have yet to be perceptually or algorithmically evaluated. Consequently, the review provides a roadmap for future work in improving the robustness of speech output

Enhanced amplitude modulations contribute to the Lombard intelligibility benefit: Evidence from the Nijmegen Corpus of Lombard Speech

Speakers adjust their voice when talking in noise, which is known as Lombard speech. These acoustic adjustments facilitate speech comprehension in noise relative to plain speech (i.e., speech produced in quiet). However, exactly which characteristics of Lombard speech drive this intelligibility benefit in noise remains unclear. This study assessed the contribution of enhanced amplitude modulations to the Lombard speech intelligibility benefit by demonstrating that (1) native speakers of Dutch in the Nijmegen Corpus of Lombard Speech (NiCLS) produce more pronounced amplitude modulations in noise vs. in quiet; (2) more enhanced amplitude modulations correlate positively with intelligibility in a speech-in-noise perception experiment; (3) transplanting the amplitude modulations from Lombard speech onto plain speech leads to an intelligibility improvement, suggesting that enhanced amplitude modulations in Lombard speech contribute towards intelligibility in noise. Results are discussed in light of recent neurobiological models of speech perception with reference to neural oscillators phase-locking to the amplitude modulations in speech, guiding the processing of speech

Investigating supra-intelligibility aspects of speech

158 p.Synthetic and recorded speech form a great part of oureveryday listening experience, and much of our exposure tothese forms of speech occurs in potentially noisy settings such as on public transport, in the classroom or workplace, while driving, and in our homes. Optimising speech output to ensure that salient information is both correctly and effortlessly received is a main concern for the designers of applications that make use of the speech modality. Most of the focus in adapting speech output to challenging listening conditions has been on intelligibility, and specifically on enhancing intelligibility by modifying speech prior to presentation. However, the quality of the generated speech is not always satisfying for the recipient, which might lead to fatigue, or reluctance in using this communication modality. Consequently, a sole focus on intelligibility enhancement provides an incomplete picture of a listener¿s experience since the effect of modified or synthetic speech on other characteristics risks being ignored. These concerns motivate the study of 'supra-intelligibility' factors such as the additional cognitive demand that modified speech may well impose upon listeners, as well as quality, naturalness, distortion and pleasantness. This thesis reports on an investigation into two supra-intelligibility factors: listening effort and listener preferences. Differences in listening effort across four speech types (plain natural, Lombard, algorithmically-enhanced, and synthetic speech) were measured using existing methods, including pupillometry, subjective judgements, and intelligibility scores. To explore the effects of speech features on listener preferences, a new tool, SpeechAdjuster, was developed. SpeechAdjuster allows the manipulation of virtually any aspect of speech and supports the joint elicitation of listener preferences and intelligibility measures. The tool reverses the roles of listener and experimenter by allowing listeners direct control of speech characteristics in real-time. Several experiments to explore the effects of speech properties on listening preferences and intelligibility using SpeechAdjuster were conducted. Participants were permitted to change a speech feature during an open-ended adjustment phase, followed by a test phase in which they identified speech presented with the feature value selected at the end of the adjustment phase. Experiments with native normal-hearing listeners measured the consequences of allowing listeners to change speech rate, fundamental frequency, and other features which led to spectral energy redistribution. Speech stimuli were presented in both quiet and masked conditions. Results revealed that listeners prefer feature modifications similar to those observed in naturally modified speech in noise (Lombard speech). Further, Lombard speech required the least listening effort compared to either plain natural, algorithmically-enhanced, or synthetic speech. For stationary noise, as noise level increased listeners chose slower speech rates and flatter tilts compared to the original speech. Only the choice of fundamental frequency was not consistent with that observed in Lombard speech. It is possible that features such as fundamental frequency that talkers naturally modify are by-products of the speech type (e.g. hyperarticulated speech) and might not be advantageous for the listener.Findings suggest that listener preferences provide information about the processing of speech over and above that measured by intelligibility. One of the listeners¿ concerns was to maximise intelligibility. In noise, listeners preferred the feature values for which more information survived masking, choosing speech rates that led to a contrast with the modulation rate of the masker, or modifications that led to a shift of spectral energy concentration to higher frequencies compared to those of the masker. For all features being modified by listeners, preferences were evident even when intelligibility was at or close to ceiling levels. Such preferences might result from a desire to reduce the cognitive effort of understanding speech, or from a desire to reproduce the sound of typical speech features experienced in real-world noisy conditions, or to optimise the quality of the modified signal. Investigation of supra-intelligibility aspects of speech promises to improve the quality of speech enhancement algorithms, bringing with it the potential of reducing the effort of understanding artificially-modified or generated forms of speech

Normal-to-Lombard Adaptation of Speech Synthesis Using Long Short-Term Memory Recurrent Neural Networks

In this article, three adaptation methods are compared based on how well they change the speaking style of a neural network based text-to-speech (TTS) voice. The speaking style conversion adopted here is from normal to Lombard speech. The selected adaptation methods are: auxiliary features (AF), learning hidden unit contribution (LHUC), and fine-tuning (FT). Furthermore, four state-of-the-art TTS vocoders are compared in the same context. The evaluated vocoders are: GlottHMM, GlottDNN, STRAIGHT, and pulse model in log-domain (PML). Objective and subjective evaluations were conducted to study the performance of both the adaptation methods and the vocoders. In the subjective evaluations, speaking style similarity and speech intelligibility were assessed. In addition to acoustic model adaptation, phoneme durations were also adapted from normal to Lombard with the FT adaptation method. In objective evaluations and speaking style similarity tests, we found that the FT method outperformed the other two adaptation methods. In speech intelligibility tests, we found that there were no significant differences between vocoders although the PML vocoder showed slightly better performance compared to the three other vocoders.Peer reviewe

Individual and environment-related acoustic-phonetic strategies for communicating in adverse conditions

In many situations it is necessary to produce speech in ‘adverse conditions’: that is, conditions that make speech communication difficult. Research has demonstrated that speaker strategies, as described by a range of acoustic-phonetic measures, can vary both at the individual level and according to the environment, and are argued to facilitate communication. There has been debate as to the environmental specificity of these adaptations, and their effectiveness in overcoming communication difficulty. Furthermore, the manner and extent to which adaptation strategies differ between individuals is not yet well understood. This thesis presents three studies that explore the acoustic-phonetic adaptations of speakers in noisy and degraded communication conditions and their relationship with intelligibility. Study 1 investigated the effects of temporally fluctuating maskers on global acoustic-phonetic measures associated with speech in noise (Lombard speech). The results replicated findings of increased power in the modulation spectrum in Lombard speech, but showed little evidence of adaptation to masker fluctuations via the temporal envelope. Study 2 collected a larger corpus of semi-spontaneous communicative speech in noise and other degradations perturbing specific acoustic dimensions. Speakers showed different adaptations across the environments that were likely suited to overcome noise (steady and temporally fluctuating), restricted spectral and pitch information by a noise-excited vocoder, and a sensorineural hearing loss simulation. Analyses of inter-speaker variation in both studies 1 and 2 showed behaviour was highly variable and some strategy combinations were identified. Study 3 investigated the intelligibility of strategies ‘tailored’ to specific environments and the relationship between intelligibility and speaker acoustics, finding a benefit of tailored speech adaptations and discussing the potential roles of speaker flexibility, adaptation level, and intrinsic intelligibility. The overall results are discussed in relation to models of communication in adverse conditions and a model accounting for individual variability in these conditions is proposed

Cepstral analysis based on the Glimpse proportion measure for improving the intelligibility of HMM-based synthetic speech in noise

In this paper we introduce a new cepstral coefficient extraction method based on an intelligibility measure for speech in noise, the Glimpse Proportion measure. This new method aims to increase the intelligibility of speech in noise by modifying the clean speech, and has applications in scenarios such as public announcement and car navigation systems. We first explain how the Glimpse Proportion measure operates and further show how we approximated it to integrate it into an existing spectral envelope parameter extraction method commonly used in the HMM-based speech synthesis framework. We then demonstrate how this new method changes the modelled spectrum according to the characteristics of the noise and show results for a listening test with vocoded and HMM-based synthetic speech. The test indicates that the proposed method can significantly improve intelligibility of synthetic speech in speech shaped noise. Index Terms — cepstral coefficient extraction, objective measure for speech intelligibility, Lombard speech, HMM-based speech synthesis 1

Vocal plasticity in harbour seal pups

Vocal plasticity can occur in response to environmental and biological factors, including conspecifics' vocalizations and noise. Pinnipeds are one of the few mammalian groups capable of vocal learning, and are therefore relevant to understanding the evolution of vocal plasticity in humans and other animals. Here, we investigate the vocal plasticity of harbour seals (Phoca vitulina), a species with vocal learning abilities observed in adulthood but not puppyhood. To evaluate early mammalian vocal development, we tested 1–3 weeks-old seal pups. We tailored noise playbacks to this species and age to induce seal pups to shift their fundamental frequency (f0), rather than adapt call amplitude or temporal characteristics. We exposed individual pups to low- and high-intensity bandpass-filtered noise, which spanned—and masked—their typical range of f0; simultaneously, we recorded pups' spontaneous calls. Unlike most mammals, pups modified their vocalizations by lowering their f0 in response to increased noise. This modulation was precise and adapted to the particular experimental manipulation of the noise condition. In addition, higher levels of noise induced less dispersion around the mean f0, suggesting that pups may have actively focused their phonatory efforts to target lower frequencies. Noise did not seem to affect call amplitude. However, one seal showed two characteristics of the Lombard effect known for human speech in noise: significant increase in call amplitude and flattening of spectral tilt. Our relatively low noise levels may have favoured f0 modulation while inhibiting amplitude adjustments. This lowering of f0 is unusual, as most animals commonly display no such f0 shift. Our data represent a relatively rare case in mammalian neonates, and have implications for the evolution of vocal plasticity and vocal learning across species, including humans