抄録
Even in the steady part of speech or musical tone, small fluctuation is superposed on regular repeated waveforms, and it has been clarified that these irregularities play an important role in the perception of sound. In general, the sound which we usually hear, includes some random fluctuation. These are still unclear points, however, about the perception of such sound. As a step to interpret these unclear points, we determined the discrimination limit of pitch associated with pulse trains as a function of the pulse frequency and of temporal and amplitude jitters. The results obtained are as follows : 1) Jitter in amplitude has little effect on pitch discrimination, while jitter in pulse intervals has large influence on pitch; for the lower frequency the effect of the jitter on pitch discrimination is greater than that for the higher frequency, and even a small amount of jitter makes the discrimination worse in a great deal (Fig. 3). 2) Random fluctuation in amplitude has large effect on detectability of temporal jitter (see Fig. 6). 3) The above results, 1) and 2), can be explained by the power spectrum of a pulse train (see Fig. 4 and Fig. 5). 4) In the case where the band-limited pulse trains including only the fundamental component were used as stimuli, there is little difference in pitch discrimination from the case without band limitation. This result shows that the former contains sufficient information on pitch (see Fig. 7). 5) When the band-limited pulse trains including only the second harmonic were used, pitch discrimination rapidly deteriorates with the increase of jitter in pulse interval. This result is due to the fact that the power of the second harmonic of the pulse trains including jitter, rapidly decreases with an increase of jitter (see Fig. 7). 6) When pulse trains with band limitation within 2 octaves including the 4th and higher harmonics were used, fairly good discrimination of pitch was obtained in low frequency (below several hundred Hz), but in higher frequency the pitch discrimination is scarcely possible. The upper frequency limit capable of pitch discrimination is about 800 Hz, and this frequency is considered to correspond to the upper limit of periodicity pitch perception. This value is regarded as valid in consideration of the psychophysical and physiological evidences reported by many authors (see Fig. 7 and Fig. 8).
