Spectral equalization in binaural signals represented by order-truncated spherical harmonics

Abstract

The synthesis of binaural signals from spherical microphone array recordings has been recently proposed. The limited spatial resolution of the reproduced signal due to order-limited reproduction has been previously investigated perceptually, showing spatial perception ramifications, such as poor source localization and limited externalization. Furthermore, this spatial order limitation also has a detrimental effect on the frequency content of the signal and its perceived timbre, due to the rapid roll-off at high frequencies. In this paper, the underlying causes of this spectral roll-off are described mathematically and investigated numerically. A digital filter that equalizes the frequency spectrum of a low spatial order signal is introduced and evaluated. A comprehensive listening test was conducted to study the influence of the filter on the perception of the reproduced sound. Results indicate that the suggested filter is beneficial for restoring the timbral composition of order-truncated binaural signals, while conserving, and even improving, some spatial properties of the signal.

Improving Binaural Ambisonics Decoding by Spherical Harmonics Domain Tapering and Coloration Compensation

“…As noted in prior work, order truncation of HRTFs results in highfrequency roll-off that an order-dependent compensation filter can equalize [13]. As expected, a higher SH representation order generally results in less residual error, nonetheless, sidelobes and spectral distortion are still noticeable at high orders.…”

“…Assuming a spherical scatterer object of radius r0 in a diffuse sound field, the order dependent frequency response on the sphere can be derived analytically [13]. Observing the spherical scatterer pressure response of wavenumber k = 2πf /c, we expand on this work by introducing a tapering function wN (n) weighting each mode n tō…”

“…This spectral filter equalizes signals of order N to the frequency response of a signal at order N full > kr0 [13]. Extending the design to account for a window function compensates for the spectral effects of applying SH tapering, which in turn suppresses sidelobes.…”

“…Ben-Hur et al showed that decoding SH-encoded audio of limited SH order to binaural signals results in a high-frequency roll-off, mainly due to the order truncation of the head-related transfer functions (HRTFs) [13]. To reduce the resulting coloration of the audio signal, the authors propose to equalize spectral distortions by applying an order-dependent compensation filter to the binaural signals.…”

Influence of the Number of Loudspeakers on the Timbre in Horizontal and Mixed-Order Ambisoncis Reproduction

“…Timbre is an important perceptual quality which contributes more to the overall perceptual quality than the spatial attributes [8]. Although there have been some researches of timbre change on spatial sound reproduction [9,10], these researches were mainly based on psychoacoustics experiments which are usually complicated and time-consuming. For convenience and efficiency, timbre change in spatial sound reproduction can be analyzed by using appropriate loudness model [11].…”