Self-Supervised Speech Representations are More Phonetic than Semantic (2406.08619v1)

Abstract: Self-supervised speech models (S3Ms) have become an effective backbone for speech applications. Various analyses suggest that S3Ms encode linguistic properties. In this work, we seek a more fine-grained analysis of the word-level linguistic properties encoded in S3Ms. Specifically, we curate a novel dataset of near homophone (phonetically similar) and synonym (semantically similar) word pairs and measure the similarities between S3M word representation pairs. Our study reveals that S3M representations consistently and significantly exhibit more phonetic than semantic similarity. Further, we question whether widely used intent classification datasets such as Fluent Speech Commands and Snips Smartlights are adequate for measuring semantic abilities. Our simple baseline, using only the word identity, surpasses S3M-based models. This corroborates our findings and suggests that high scores on these datasets do not necessarily guarantee the presence of semantic content.

• The paper establishes that self-supervised speech models encode phonetic features more robustly than semantic content across various architectures and languages.
• The research contrasts feature slicing, which compresses representation similarity, with audio slicing that preserves distinct phonetic details.
• Different pooling methods were analyzed, showing that center pooling captures word identity effectively while mean and centroid pooling retain richer semantic cues.

Self-Supervised Speech Representations: Phonetic Dominance Over Semantic Encoding

Overview

The paper "Self-Supervised Speech Representations are More Phonetic than Semantic" undertakes a comprehensive analysis of self-supervised speech models (S3Ms) to discern the extent to which these models encapsulate phonetic versus semantic properties. The authors leverage a novel dataset, comprising near homophone and synonym word pairs, to meticulously evaluate the similarities between respective S3M word representations. The paper spans various S3Ms, including HuBERT, wav2vec 2.0, and WavLM, probing their ability to encode linguistic information across different layers.

Key Findings

1. Phonetic Superiority: The core finding of this work is that S3M representations consistently demonstrate more phonetic similarity than semantic similarity. This phenomenon persists across multiple models, pre-training objectives, and languages.
2. Variability in Techniques: Both feature slicing and audio slicing methodologies were employed. Notably, feature slicing tends to squash representations, making them uniformly similar, while audio slicing retains more distinct linguistic features, elucidating the phonetic dominance.
3. Pooling Methods: Different pooling strategies, such as mean, center, and centroid pooling, were explored to derive word-level representations. Results indicated that while mean and centroid pooling retain richer semantic content, center pooling is more effective for capturing word identity.
4. Crosslingual Analysis: The crosslingual evaluation using the Multilingual Spoken Words dataset upheld the primary finding; S3Ms exhibited significant phonetic similarities even across languages, suggesting limited crosslingual semantics encoding capacities.
5. Impact on Downstream Tasks: Requiring semantic understanding, Intent Classification (IC) tasks—using datasets like Fluent Speech Commands and Snips Smartlights—showed that a simple bag-of-words (BoW) approach often outperformed sophisticated S3M-based models. This result questions the semantic capabilities attributed to S3Ms solely based on high task performance.

Implications of Findings

The findings present both theoretical and practical implications. Theoretically, they postulate that self-supervised models, while excelling at phonetic encoding, may not adequately capture semantic content without supplementary context-processing mechanisms. This insight is critical for understanding the limitations of S3Ms and directs future research towards enhancing semantic encoding.

Practically, the results stress the need for improved benchmarks and evaluation methodologies for tasks purported to assess semantic capabilities. The superior performance of a simplistic BoW model in IC tasks questions the semantic validity of these benchmarks, highlighting a necessity for more robust, contextually rich datasets.

Future Directions

The paper opens several avenues for further research:

• Enhanced Contextualization: Future work can focus on models capable of better contextualizing speech, potentially integrating cross-modal inputs, such as text or visual cues, to enrich semantic understanding.
• Layer-wise Analysis: An in-depth layer-wise analysis could provide finer granularity regarding which layers predominantly encode phonetic versus semantic information, guiding model architecture refinements.
• Task-Specific Benchmarks: Development of more nuanced benchmarks that better isolate and test the semantic capabilities of S3Ms, beyond traditional IC tasks, is essential.
• Language Generalization: Extending the analysis to a broader spectrum of languages can help in understanding linguistic properties encoded by S3Ms in low-resource or typologically diverse languages.

Conclusion

This paper provides a critical fine-grained analysis of S3Ms, highlighting their phonetic predisposition. The insights call into question the semantic efficacy of these models when evaluated on certain downstream tasks. By dissecting the limitations and potentials of S3M representations, the authors set a precedent for more rigorous and comprehensive evaluations in future speech model research, ultimately contributing to the broader discourse on enhancing semantic comprehension in self-supervised learning paradigms.