ON THE IMPORTANT ROLE OF CHINESE TONES IN SPEECH INTELLIGIBILITY

Chinese is a tone language. The role of Chinese tones played in the speech perception were investigeted by speech articulation tests. The first part of these tests was carried out under four conditions: I. Speech via low-pass filter, cutoff frequency at 1600 Hz, S/N = 10 dB, II. speech via high-pass filter, cutoff frequency at 1600 Hz, S/N=20 dB, III. speech via low-pass filter with some nonlinear distortions, cutoff frequency at 750 Hz, S/N=25 dB, and IV. speech via high-pass filter with some nonlinear distortions, cutoff frequency at 2500 Hz, S/N = 25dB. It is shown that the tone articulation essentially does not change when the syllable articulation is changed from 77% to 28%, but at the sametime the initial articulation is changed from 87.2% to 46.7% and the final articulation from 89% to 39.5%. So the tones possess strong anti-interference ability.
In order to separate the tones from the speech sounds, a 2400 bits/s digital channel vocoder was used in the second part of the articulation tests under four different conditions of excitation source: 1. the excitation source was only a noise source, 2. the excitation source was only a triangular pulse train source with constant fundamental frequency F₀=127 Hz, 3. the excitation source was only a triangular pulse train source with fundamental frequency controlled by the voice, and 4. both noise source and pulse source were used and the fundamental frequency controlled by the voice. It is shown that I. the rank order of the tone articulation for the four testing conditions is 4, 3, 1, 2; II. the fundamental frequency is the most important cue for tones; III. the intensity variation in noise spectrum （condition 1） with the variation of fundamental frequency of the speech input may provide some acoustical cues for tone perception; IV. the unvoiced sounds may contribute somewhat to the tone perception （the tone articulation in condition 4 is higher than condition 3）.