Speaker Identity in Non-Verbal Vocalizations: Conditional Distillation and Mixture of Experts Approach
Abstract
As expressive text-to-speech (TTS) and voice conversion (VC) systems increasingly generate non-verbal vocalizations (NVVs) to enhance naturalness, reliable speaker verification (SV) becomes essential to objectively assess identity consistency across both verbal and non-verbal segments. Yet current SV systems generalize poorly to NVVs, and fine-tuning on NVV data causes catastrophic forgetting of speech performance. We present the first systematic study across 10 NVV types and propose a framework combining frozen Data2Vec self-supervised features with ECAPA-TDNN, enhanced by a Mixture of Experts (MoE) module with learned domain-aware routing. A conditional distillation loss on speech inputs via a pretrained teacher retains speech-to-speech accuracy, while a contrastive loss bridges the speech-NVV domain gap. Our method reduces speech-NVV EER from 38.93% to 22.66% over a pretrained baseline, and improves speech EER from 13.17% to 9.24% via distillation.
The following analysis is generated by AI and may contain errors. It is separate from the authors’ abstract and is not peer review.
Research summary
This paper presents a novel MoE-based speaker verification framework that effectively bridges the domain gap between verbal and non-verbal vocalizations, significantly improving cross-domain verification performance while mitigating catastrophic forgetting through conditional distillation.
Methodology
The paper proposes a Mixture of Experts (MoE) framework integrated with a frozen Data2Vec self-supervised learning (SSL) front-end and an ECAPA-TDNN backend to address speaker verification (SV) across verbal and non-verbal vocalizations (NVVs). The core methodological innovation lies in the combination of three components: (1) an Inter-Layer Residual MoE (IR-MoE) architecture that routes features to specialized experts for speech vs. NVV, governed by learned domain-aware constraints (entropy maximization, KL divergence for intra-event consistency, and cosine-margin for inter-event separation); (2) a conditional knowledge distillation loss using a frozen WavLM teacher to prevent catastrophic forgetting of speech capabilities during NVV adaptation; and (3) a supervised contrastive loss that bridges the domain gap by enforcing shared speaker manifolds across speech and NVV pairs. The approach is technically sound, leveraging established SSL representations while introducing architectural and loss-function modifications to handle the heterogeneity of non-phonemic acoustic signals. The use of MoE for domain separation in SV is a novel application, moving beyond standard fine-tuning paradigms.
Results & evaluation
The experimental evaluation is comprehensive, utilizing the NonverbalTTS dataset with 10 distinct NVV categories. The authors provide a systematic analysis of domain mismatch, demonstrating the significant performance drop of zero-shot SV models on NVVs (EER ~39%). They compare against multiple baselines, including standard SSL models (WavLM, Data2Vec, Voc2Vec) and fusion strategies. The proposed IR-MoE model achieves a substantial reduction in cross-domain EER (NvS) from 38.93% to 22.66% and improves speech EER (SvS) from 13.17% to 9.24% compared to a fine-tuned baseline. The ablation studies effectively isolate the contributions of the MoE architecture and the conditional distillation loss. The results are statistically significant and demonstrate that the proposed method successfully mitigates catastrophic forgetting while improving NVV verification. The inclusion of mDCF metrics adds robustness to the evaluation.
Reproducibility
The paper provides detailed implementation specifications, including the optimizer (Adam), learning rate schedule (cosine annealing), batch composition strategy (speaker-balanced with cross-domain prioritization), and specific loss weights. The use of standard, open-source components (Data2Vec, ECAPA-TDNN, WavLM) enhances reproducibility. The description of the progressive training schedule and the specific constraints for the MoE routing (entropy, KL, cosine-margin) is sufficiently detailed for replication. However, the exact codebase for the MoE integration and the specific preprocessing steps for the NonverbalTTS dataset (beyond MFA alignment) might require careful interpretation, though the overall pipeline is clear.
Limitations
A primary limitation is the reliance on the NonverbalTTS dataset, which, while diverse, may not capture the full spectrum of NVVs found in real-world expressive TTS/VC systems (e.g., complex emotional laughter, gasps, or synthesized artifacts). The MoE architecture introduces additional parameters and complexity compared to standard fine-tuning, which could impact inference latency, although this is not explicitly measured. The "conditional" distillation relies on the model's ability to correctly route inputs during inference; if the router misclassifies an input (e.g., a speech-like NVV), the distillation benefit might not be fully realized or could even be detrimental if the wrong expert is activated. Furthermore, the performance on NVV-NVV (NvN) verification remains relatively high (EER ~27-28%), indicating that while cross-domain generalization is improved, intra-NVV verification is still challenging.
Broader impact
This work has significant implications for the evaluation and development of expressive TTS and Voice Conversion systems, where NVVs are crucial for naturalness. By providing a reliable SV metric for NVVs, it enables better objective evaluation of these systems, potentially accelerating their adoption in virtual assistants, gaming, and accessibility tools. It also contributes to the broader field of multi-modal and multi-domain speaker recognition, offering a template for handling heterogeneous acoustic inputs. The mitigation of catastrophic forgetting is a generalizable technique for continual learning in speech processing. This paper presents a novel MoE-based speaker verification framework that effectively bridges the domain gap between verbal and non-verbal vocalizations, significantly improving cross-domain verification performance while mitigating catastrophic forgetting through conditional distillation.
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