Towards explainable spoofed speech attribution and detection: A probabilistic approach for characterizing speech synthesizer components

Abstract

We propose an explainable probabilistic framework for characterizing spoofed speech by decomposing it into probabilistic attribute embeddings. Unlike raw high-dimensional countermeasure embeddings, which lack interpretability, the proposed probabilistic attribute embeddings aim to detect specific speech synthesizer components, represented through high-level attributes and their corresponding values. We use these probabilistic embeddings with four classifier back-ends to address two downstream tasks: spoofing detection and spoofing attack attribution. The former is the well-known bonafide-spoof detection task, whereas the latter seeks to identify the source method (generator) of a spoofed utterance. We additionally use Shapley values, a widely used technique in machine learning, to quantify the relative contribution of each attribute value to the decision-making process in each task. Results on the ASVspoof2019 dataset demonstrate the substantial role of waveform generator, conversion model outputs, and inputs in spoofing detection; and inputs, speaker, and duration modeling in spoofing attack attribution. In the detection task, the probabilistic attribute embeddings achieve 99.7% balanced accuracy and 0.22% equal error rate (EER), closely matching the performance of raw embeddings (99.9% balanced accuracy and 0.22% EER). Similarly, in the attribution task, our embeddings achieve 90.23% balanced accuracy and 2.07% EER, compared to 90.16% and 2.11% with raw embeddings. These results demonstrate that the proposed framework is both inherently explainable by design and capable of achieving performance comparable to raw CM embeddings.