Analysis of Postfilters for Low Bit Rate Speech Coders in Tandem Connections

"In this paper we analyse postfiltering techniques for very low bit rate speech coders in tandem connections. A mixed multiband excitation (MMBE) linear predictive coding (LPC) algorithm, that encodes voiced frames at 1.75 kb/s and unvoiced frames at 0.4 kb/s, is employed to assess the performance of different postfilters in tandem connections. We perform a comparative analysis of the well known adaptive spectral enhancement (ASE) technique with a recently reported approach, called spectral envelope restoration combined with noise reduction (SERNR) postfilter, using the same MMBE platform. Subjective listening tests in tandem connections show that the SERNR technique is clearly superior to the ASE postfilter."