Phase distortion resulting in a just noticeable difference in the perceived quality of speech

• Many enhancement methods only suppress noise in magnitude spectrum, and use noisy or degraded phase in signal reconstruction.
• Degradation of the phase spectrum reduces stimuli quality where SNR is low.
• Where instantaneous-SNR (I-SNR) is greater than 7 dB, distortion in phase does not audibly degrade speech quality.
• For I-SNR lower than 7 dB, distortion is audible, and further processing of phase could improve quality.
• Where magnitude also includes distortion, I-SNR corresponding to a JND in speech quality due to phase distortion is reduced.

Common speech enhancement methods based on the short-time Fourier analysis–modification–synthesis (AMS) framework, modify the magnitude spectrum while keeping the phase spectrum unchanged. This is justified by an assumption that the phase spectrum can be seen as unimportant to speech quality, and hence the noisy phase spectrum can be used as a reasonable estimate of the clean phase spectrum in signal reconstruction. In this work we show, by using an ideal magnitude estimator, that corruption in the phase spectrum can still affect the quality of the resulting speech in low SNR environments. Furthermore, we quantify the distortion in the phase spectrum which can be tolerated before it begins to affect speech quality. This is done through a series of experiments, using both subjective and objective tests, and statistical analysis to evaluate the results. The results show that the phase spectrum computed from noisy speech can be used as an estimate of the phase spectrum of the clean signal without noticeably affecting perceived speech quality, only if the segmental SNR of the noisy speech signal is greater than 7 dB.