Subjective quality testing of a new source model of LPO vocoders
A. W. F. Huggins, Richard M. Schwartz, R. Viswanathan, J. Makhoul · The Journal of the Acoustical Society of America · 1978
Our source model, reported at an earlier meeting, excites the LPO speech spectrum with a low-frequency band of pulses mixed with a high-frequency band of noise. Pulses are low-pass filtered and noise is high-pass filtered at the same frequency, to yield a flat source spectrum. The cutoff frequency of the filters, a continuous variable, replaces the usual binary voiced/voiceless decision. Thirty-six phoneme-specific test sentences were processed through a single high-quality vocoder (5 kHz bandwidth, 11 poles, no quantization, 100 frames/s), which was excited in turn by both the usual pulse/noise source and by the new source. Subjects rated the resulting speech separately on eight-point buzziness and breathiness scales. The results show that the new source model greatly reduces perceived buzziness, occasionally at a cost of slightly increased breathiness. Any remaining inadequacies can probably be ascribed to the algorithm that extracts the cutoff frequency during analysis, rather than to the model itself. [Work supported by ARPA-IPTO.]