Predicting and preventing unmasking incurred in coded audio post-processing

In modern audio compression algorithms, the masking properties of the auditory system are exploited to improve the coding gain, namely, quantization noise is introduced in the signal in time-frequency regions where it will be masked. However, since signal modifications change the characteristics of the masking regions, degradations may result if a decoded audio signal is modified. In this paper, we explain and demonstrate how modifying an audio signal can result in the unmasking of signal components that were imperceptible in the unmodified signal. We consider both pitch-shifting and linear filtering modifications; synthetic and natural audio examples are provided to verify the unmasking phenomenon. We discuss how modification of decoded audio may lead to unmasking of quantization noise, describe conditions for which such unmasking may occur, and propose a method for adjusting the masking threshold employed in the audio coder to make the decoded signal robust to quantization noise unmasking for a given set of signal modifications.