旋律了解度の規定因子について
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概要
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1) From the point of view of information theory, perception of melody presupposes the transmission of the melody. As first explicitly expressed by Yuki, the factor of "movement" dominates in the primitive stage of melody perception, but the melody perception has gradually developed into the present form in which position, or the "hereness" and "thereness" of each point of moving tone plays a more important rôle than the factor of movement in building up the perception of melody.These "hereness" and "thereness" can be given by the sequence of discrete and more or less prolonged tones. The sequence of discrete, extended sounds may be decomposed into the following two: the sequence of pitch intervals and the sequence of time intervals during each of which a pitch is extended. As the first step of the information theory approach toward melody perception, only the sequence of pitch intervals is studied in this paper. Several experiments were conducted in which then normal code of melody transmission was distorted or disturbed by a sort of noise, and the effect of deteriorated transmission was measured in terms of melody intelligibility scale constructed analogically to the scale of sentence intelligibility.2) Preliminarily were analysed the frequencies of pitch intervals appearing in the most familiar musical melodies, for example, "Last rose of summer", "Home, sweet home" and the like, with the finding that only ten pitch intervals appear with frequency greater than 1%. (See Table 1.)3) Why the familiar melodies are mainly constructed with so few pitch intervals as ten will be accounted for by the assumption that melodies must be quantized if they are to be communicated exactly. Further, it should be necessary for communication that the rules of quantization must be shared as the common code of information transmission by people under the same historical and social background of music. So it is expected that melody transmission would be deteriorated if the normal pitch intervals as the shared code are distorted, extended or compressed, uniformly. The result of the experiment clearly demonstrates this effect. (Fig. 6)4) It is a common observation that one can often guess the whole melody by listening only a part of it. An experiment was conducted to test how much the melody intelligibility increases as the length of presented melody increases. The result is shown in Fig. 7.5) The third experiment is concerned with how the transmission of a pitch interval is reduced when a disturbing vibrato overlaps the two successive tones by which an interval is defined. The result is shown in Fig. 8.6) Since the third experiment revealed that a disturbing vibrato reduces intelligibility of every elemental pitch interval, the effect of disturbing vibrato upon a melody intelligibility is examined in the last experiment. The result is shown in Fig. 9.
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