6.7.1 Overview

26.4453GPPCodec for Enhanced Voice Services (EVS)Detailed algorithmic descriptionRelease 15TS

When the decoder is in the CNG operation, the first bit in the SID frame is decoded to point to the CNG mode in which the decoder will be operating. In the LP-CNG mode, excitation(s) and synthesis filter(s) are calculated from the decoded CN parameters, and the comfort noise is synthesized by a LP synthesis approach.

In the FD-CNG mode, a spectral envelope is generated with the help of the energy level information decoded from the SID data. The spectral envelope is refined by a noise estimator running during active frames. The resulting envelope determines the actual comfort noise which is rendered inside different frequency domains, such as MDCT, FFT or CLDFB and finally transformed to time-domain.

Subsequent subclauses describe the respect LP-CNG decoding and FD-CNG decoding in details.