As a basic study into 3-D audio display systems, this paper reports the conditions of moving sound image velocity and time-step where a discrete moving sound image is perceived as continuous motion. In this study, the discrete moving sound image was presented through headphones and ran along the ear-axis. The experiments tested the continuity of a discrete moving sound image using various conditions of velocity (0.25, 0.5, 0.75, 1, 2, 3, and 4？m/s) and time-step (0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, and 0.14？s). As a result, the following were required in order to present the discrete moving sound image as continuous movement. (1) The 3-D audio display system was required to complete the sound image presentation process, including head tracking and HRTF simulation, in a time shorter than 0.02？s, in order to present sound image movement at all velocities. (2) A processing time longer than 0.1？s was not acceptable. (3) If the 3-D audio display system only presented very slow movement (less than about 0.5？m/s), processing times ranging from 0.04？s to 0.06？s were still acceptable. 1. Introduction 3-D audio display technology is important for virtual reality technologies. Simulation of the head-related transfer function (HRTF) using digital signal processing is the key technology for a 3-D audio display. In principle, sound signals are processed digitally for HRTF simulation and are presented to the listener through headphones. “Head tracking” technologies that control the virtual sound field according to the listener’s head position and orientation are well-known techniques to enhance the sound localization of a virtual sound image [1, 2] and are frequently used with HRTF simulation. Head tracking is based on position and orientation sensing technologies, and various sensors, such as magnetic 6 degrees of freedom (6DOF) sensors [3, 4], a gyro [5], a global positioning system (GPS) [6], and a camera [7], have been used for head tracking with a 3-D audio display. However, these sensors need adequate processing time to obtain the position and orientation. For example, the sampling frequency of GPS is about 5–20？Hz (i.e., the processing time for position and orientation sampling is 0.05–0.2？s). The long processing time makes the 3-D audio display “discrete,” which means that the sound image cannot move continuously and alternates between static and jumping. During the time the system is processing, the sound image cannot move and must remain static, and then after processing, the sound image can change its location. Even if the 3-D audio display processing is
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