Spatial Audio Processing: MPEG Surround and Other Applications

7.1 Introduction

Convolution using head-related transfer functions (HRTFs) is a widely used method to evoke the percept of a virtual sound source at a given spatial position. As described in Chapter 3, the use of HRTFs requires a database of pairs of impulse responses, preferably matched to the anthropometric properties of the user. Because of the large amount of data present in individual HRTF sets that is normally required to generate externalized virtual sound sources, it is desirable to find an efficient representation of HRTFs. For example, attempts have been made to only measure HRTF sets for a limited range of source positions and to interpolate HRTFs for positions in between (based on the magnitude transfers [270], spherical harmonics [81], eigentransfer functions [56], pole-zero approximations [27] or spherical spline methods [54]). Other studies described HRTFs by deriving a small set of basis spectra with individual, position-dependent weights [56, 59, 162]. Although these methods are sound in physical terms, there is a risk that the basis functions that are very important in terms of the least-squares error of the fit are not so relevant in terms of human auditory perception. Another, more psycho-acoustically motivated approach consisted of determining the role of spectral and inter-aural phase cues present in the HRTFs. Wightman and Kistler [274] showed that low-frequency inter-aural time differences dominate in sound localization, while if the low frequencies are ...