Objective and Subjective Performance of Tandem Connections of Waveform Coders with an LPC Vocoder

1.1 Background Recent plans for United States government digital communication networks have focused attention on the compatibility of 2.4-kb/s (narrowband) vocoder systems and 16-kb/s (wideband) waveform coding schemes. An important question arises in the implementation of such a system: If both narrowband and wideband systems are designed to provide adequate speech communication individually, will a tandem connection of them also function adequately? 601 A recent study of this question,1,2 using signal-to-noise ratio (s/n) and a spectral distance measure as criteria of merit, has cast doubt on the viability of circuits containing a 2.4-kb/s LPC (linear predictive coding) vocoder and a 16-kb/s CVSD (continuously variable slope delta modulation) waveform coder. In that study, it appeared that CVSD was the weak link in these tandem connections. However, the conclusions could only be regarded as tentative because the speech material included in the study was very limited and because the relationship of the objective performance measures to the quality of communication experienced by human users was by no means evident. 1.2 Aims