Digital Speech: Coding for Low Bit Rate Communication Systems, 2nd Edition

11

Speech Enhancement

11.1 Introduction

In voice communications, speech signals can be contaminated by environmental noise and, as a result, the communication quality can be affected making the speech less intelligible. Furthermore, compression of the noisy speech with a low bit-rate vocoder may result in considerable quality degradation due to frequent estimation errors of speech production model parameters required by the vocoder. This problem can be reduced significantly by speech enhancement (or noise cancellation), which may enable more pleasant voice communication by suppressing the noise components in input signals.

Generally, it is assumed that the noisy speech signal is formed additively by speech and noise signals in which the noise is generated by environmental sources such as vehicles, street noise, babble, etc. Therefore, in real environments, complete noise cancellation is not feasible as it is not possible to completely track varying noise types and characteristics that change with time. However, by assuming that the noise characteristics change slowly in comparison with speech, it is possible to achieve significant reduction in the background noise levels producing more pleasant and intelligible speech quality. Speech enhancement techniques can help the speech model parameter extraction process used in low bit-rate vocoders and hence they are becoming an integral part of low bit-rate speech coding systems.

Speech enhancement techniques can be classified, depending ...