MP3 Compression to Diminish Adversarial Noise in End-to-End Speech Recognition

Abstract

Audio Adversarial Examples (AAE) represent purposefully designed inputs meant to trick Automatic Speech Recognition (ASR) systems into misclassification. The present work proposes MP3 compression as a means to decrease the impact of Adversarial Noise (AN) in audio samples transcribed by ASR systems. To this end, we generated AAEs with a new variant of the Fast Gradient Sign Method for an end-to-end, hybrid CTC-attention ASR system. The MP3’s effectiveness against AN is then validated by two objective indicators: (1) Character Error Rates (CER) that measure the speech decoding performance of four ASR models trained on different audio formats (both uncompressed and MP3-compressed) and (2) Signal-to-Noise Ratio (SNR) estimated for uncompressed and MP3-compressed AAEs that are reconstructed in the time domain by feature inversion. We found that MP3 compression applied to AAEs indeed reduces the CER when compared to uncompressed AAEs. Moreover, feature-inverted (reconstructed) AAEs had significantly higher SNRs after MP3 compression, indicating that AN was reduced. In contrast to AN, MP3 compression applied to utterances augmented with regular noise resulted in more transcription errors, giving further evidence that MP3 encoding is effective in diminishing AN exclusively.