Enhancing Noise Robustness for Neural Speech Codecs through Resource-Efficient Progressive Quantization Perturbation Simulation

Abstract: Noise robustness remains a critical challenge for deploying neural speech codecs in real-world acoustic scenarios where background noise is often inevitable. A key observation we make is that even slight input noise perturbations can cause unintended shifts in quantized codewords, thereby degrading the quality of reconstructed speech. Motivated by this finding, we propose a novel and resource-efficient training strategy to enhance the noise robustness of speech codecs by simulating such perturbations directly at the quantization level. Our approach introduces two core mechanisms: (1) a distance-weighted probabilistic top-K sampling strategy that replaces the conventional deterministic nearest-neighbor selection in residual vector quantization (RVQ); and (2) a progressive training scheme that introduces perturbations from the last to the first quantizer in a controlled manner. Crucially, our method is trained exclusively on clean speech, eliminating the need for any paired noisy-clean data. Experiments on two advanced neural speech codecs, Encodec and WavTokenizer, demonstrate that the proposed strategy substantially improves robustness under noisy conditions-for example, boosting UTMOS from 3.475 to 3.586 at 15 dB SNR on Encodec-while also enhancing coding quality for clean speech.