基于双耳时间差的客观指标对Ambisonics回放系统的性能分析

Evaluation of Ambisonics reproduction system with object metric based on interaural time difference

在基于扬声器阵列的声回放技术中,相比于波场合成(Wave field synthesis,WFS)和矢量基幅度平移技术,以球谐分解为理论基础的Ambisonics回放系统拥有编解码相互独立以及可扩展等优势.基本的Ambisonics系统通过采集和回放一阶声场方位信息:全指向性(W)和双指向性(X,Y,Z)成分,即所谓的B-format重构声场.HOA(Higher order Ambisonics)基于声场球谐函数分解将B-format以更高空间分辨率进行了扩展.已有很多学者关注HOA的回放精度和使用限制,但综合考虑人头散射效应和人耳听觉特性效应的研究还十分缺乏.从低频人耳定位的重要因素——双耳时间差(Interaural time difference,ITD)的角度评价了不同阶数Ambisonis系统的最佳听音区域.将水平面各方向入射平面波编码为HOA分量,在此基础上计算了模拟人头(刚性球)在环形阵列内部移动时二维水平面的ITD波动,并通过ITD阈值确定最佳听音区域边界.仿真结果表明基于ITD的客观评价指标可以较好地体现不同阶数Ambisonics系统的声场回放性能:4阶Ambisoics系统能够使最佳听音区域达到20cm×14cm,而1,2阶系统在中心区域尚不能实现精确回放.因此,高阶Ambisoics系统拥有更好的声源定位性能.

Ambisonics has an advantage over the wave field synthesis（WFS）and vector base amplitude panning（VBAP）techniques in scalability and independence between the encoding and decoding procedure.Ambionics renders3 Dsound field from the knowledge of their 1 st order directive information omnidirective（W）and bidirective（X,Y,Z）components known as the B-format.Higher-order Ambisonics extends B-format to higher spatial resolution by spherical harmonic decomposition of the sound field.Many scholars have investigated the accuracy and limitation ofthe HOA（High order Ambisonics）though few of them have taken into account the diffraction of the human head and the features of the auditory system.In this paper,the sweet-zone size of Ambisonics systems with different order is evaluated from the aspect of the interaural time difference（ITD）,which is the dominant localization cue for human auditory system in low frequency domain.Firstly,plane waves in the horizontal plane with different incident directions are encoded into independent HOA components.Then the values of reproduced ITD are calculated with a simulated head（a rigid sphere）moving inside the circular loudspeaker array in the horizontal plane.Finally,the edge of sweet-zone is determined when the deviation of ITD exceeds the perception thresholds of ITD according to the psycho-acoustics theory.Results show that the object metric based on ITD matches well with the reproduction performance of high-order Ambisonics systems.Ambisonics system of order 4 generates a sweet-zone of about 20 cm×14 cm while the 1 st or 2 nd order systems can hardly yield accurate ITD even in the center of the zone.The range of sweet-zone increases markedly with higher order Ambionics system which means HOA has better robustness when human head is located away from the center of the listening zone.As a result,higher order Ambisonics systems are expected to have a better performance in spatial localization of virtual sound image.