# Auditory Localization in Low-Bitrate Compressed Ambisonic Scenes
## 2 Background
### Auditory Localization in Ambisonics  
Auditory localization in spatial audio systems can be evaluated by measuring human performance in sound source localization tasks. Such indirect tests use various response techniques, e.g. perceived direction reporting, visual mapping, physical pointing, and acoustic pointer adjustment. Egocentric pointing methods employ localization judgment reported in a coordinate system centered on the listener's body. The position of the sound source is indicated by the listener's hand or head pointed towards the sound source [26]. Another type of egocentric method is _proximal pointing_, where the listener points in the apparent direction of the sound source in the proximal region of the head [27]. In exocentric methods, the listener indicates the perceived direction of the sound source using an external device, e.g. pointing with a stylus on a solid sphere placed in front of their body [28]. Another method of localization performance evaluation employs the use of a real or virtual acoustic pointer. The practical implementation of this method has been described in Section 3.  
The relation between Ambisonic order and auditory localization error has been researched in several experiments. Previous studies used synthesized Ambisonic scenes as well as recorded with Ambisonic microphones [29; 30] reproduced using loudspeaker arrays [31; 32] and binaural rendering [33]. It has been shown that localization error depends on the Ambisonic order as well as the incidence of the virtual sound source. Furthermore, binaural reproduction produces more front-back confusion errors in comparison to loudspeaker-based reproduction [33]. Both loudspeaker and headphone reproduction methods have not been directly compared using HOA and individual HRTF-based rendering.  
Figure 2: Block diagram illustrating audio signal chain for Ambisonic rendering over loudspeakers and headphones.  
There is currently a limited amount of research published on the quality of compressed spatial audio, and particularly, on the compression of First and HOA. The recent version of the Opus codec ([[https://people.xiph.org/~jm/opus/opus-1.3](https://people.xiph.org/~jm/opus/opus-1.3)]([https://people.xiph.org/~jm/opus/opus-1.3](https://people.xiph.org/~jm/opus/opus-1.3))) implements Channel Mapping Family 3 which allows for Ambisonic signal coupling [34]. Previous work by [PERSON] et al. includes subjective evaluation of Ambisonics compressed with Opus 1.2 codec with Channel Mapping Family 2 implementation [35] and the development of a reference objective spatial audio quality metric [36]. They use a MUSHRA paradigm to assess the localization degradation and demonstrate quality degradation between equivalent bitrates at different orders. The absolute extent of localization precision is not shown. These studies also focus on static and generic HRTF binaural listening conditions. The localization performance within Ambisonic scenes compressed with Opus Channel Mapping Family 3 has been researched using loudspeaker-based reproduction exclusively [32]. The work presented in this paper extends this research by binaural evaluation.