D. C. Teas
Krug Life Sci., San Antonio Div., P.O. Box 790644, San Antonio, TX 78279-0644
The equations describing the stimulus configuration for generating the Shepard pitch illusion [R. N. Shepard, J. Acoust. Soc. Am. 36, 2346 (1964)] have been implemented with the computer-assisted programming language, LabVIEW, using a MacIntosh personal computer equipped with digital signal processing capability and digital--analog conversion. Shepard's demonstration used 10 octaves (c=10) and 12 increments (t=12). The intensity distribution of the components ranged from 22 to 56 dB, with the strongest components in the mid-range of frequencies. Qualitative observations of the effects of varying c and t suggest that Shepard's values produce the richest perceptions of pitch. However, even for these original parameters, it is found that the essence of the illusion, i.e., the continuous ascent or descent of the pitch, is lost as the duration of exposure (not specified in the equations) for each t is decreased. The LabVIEW implementation allows the user to vary the number of octaves (c), the number of steps (t), the range of amplitudes for c, or the frequency range over which c is taken. In general, as the values for these parameters vary from optimum, the continuity of the pitch change is lost and the listener hears the cycles (through the t steps) repeat. A modification of the program permits sinusoids to be replaced by narrow bands of noise. At narrow bandwidths, e.g., 0.7(f[sub ti]) and 1.3(f[sub ti]), one can hear the pitch in a background of noise, and the illusion is preserved if the parameters are appropriate. As the bandwidth increases, the strength of the pitch perception decreases. [This research was supported by contract F33615-89-c-0603 to Krug Life Sciences.]