Quality limitations in residual-excited LP speech coding
James G. Kubina, Douglas D. O’Shaughnessy · The Journal of the Acoustical Society of America · 1981
In this paper, we report on a study of two factors which determine the RELP speech coder overall transmission rate and output quality, namely, the transmitted residual baseband bandwidth and the baseband encoding technique. The first subjective test revealed a linear relationship between uncoded baseband bandwidth and coder quality referenced to the log PCM standard. The optimum baseband encoding method out of a subset (PCM, APCM, ADM, PPADPCM, ATC) was determined by both an objective (SEGSNR) measure and a subjective (preference) measure derived from the results of a second subjective test. The ADPCM based RELP coder maximized both measures and was used thereafter in a final subjective test which sought to establish a relationship between transmission rate and quality. Test results showed a quasi-linear dependance of quality and transmission rate and also showed that toll quality (7-bit log PCM) cannot be presently achieved with RELP coding.