A comfort noise addition post-processor for enhancing low bit-rate speech coding in noisy environments

At low bit-rates, speech coders relying on a single source model have difficulties to properly render speech and background noise simultaneously. Difficulties can get even bigger when using speech enhancement techniques within the coding scheme: these have shown to improve quality for clean speech, but introduce unpleasant instabilities under noisy conditions. This paper presents a novel approach named Comfort Noise Addition (CNA) for post-processing noisy speech coding. It continuously injects comfort noise in both active and inactive segments of the decoded speech. Based on an accurate noise estimate, it enhances the reproduction of the background noise while masking artefacts from the coding and the speech enhancement. Listening tests confirm that CNA allows speech coders to compensate for the limitations of speech enhancement and to significantly improve the quality of noisy speech at low bit-rates. CNA was adopted in the recent 3GPP codec for Enhanced Voice Services (EVS).