The correct level alignment of multichannel reproduction systems is critical to the quality of the reproduction. This, the third paper discusses the results of three experiments employing the nine test signals discussed in paper I. Experiments employing six subjects using the method of adjustment have been conducted in a standard listening rooms using a symmetrical loudspeaker set-up. The influence of directivity characteristics of the loudspeakers used for the front and surround channels has been investigated and the influence of high-pass filtering the centre and surround channels has also been examined. The results show that there is no significant influence of directivity characteristics for the front channels. This also applies to the surround channels when using standard loudspeakers that produce both a direct and diffuse sound field component at the listening position. A loudspeaker that only produces a diffuse component will have a significantly different calibration of the surround channel. It is suggested that the effect can be explained by simple changes in the SPL at the listening position. The use of high-pass filtered centre or surround channels does not have a significant influence on the calibration.
The correct level alignment of multichannel reproduction systems is critical to the quality of the reproduction. This, the second paper discusses the results of two experiments employing the nine test signals discussed in paper I. Experiments employing the method of adjustment have been conducted in two standard listening rooms, considering both the symmetrical and asymmetrical loudspeaker set-ups. It is found that the calibration signal does not play a major role in the level alignment of multichannel systems whilst source distance is quite a dominating factor.
This paper reports the latest developments in ongoing research to establish a common lexicon of sound quality attributes agnostic to the application under study. It expands on earlier work with the definition of attributes for the evaluation of spatial sound reproduction. A panel of 12 specialized expert assessors was used to expand the lexicon to incorporate spatial sound characteristics. The discussions were based on the results of a semantic clustering of relevant quality attribute sets proposed in the literature. The updated lexicon is presented, with detailed definitions, scale labels and examples provided for the attributes. Further review and formal validation are required in future.
This paper studies the quality of multimedia content focusing on 360 video and ambisonic spatial audio reproduced using a head-mounted display and a multichannel loudspeaker setup. Encoding parameters following basic video quality test conditions for 360 videos were selected and a low-bitrate codec was used for the audio encoder. Three subjective experiments were performed for the audio, video, and audiovisual respectively. Peak signal-to-noise ratio (PSNR) and its variants for 360 videos were computed to obtain objective quality metrics and subsequently correlated with the subjective video scores. This study shows that a Cross-Format SPSNR-NN has a slightly higher linear and monotonic correlation over all video sequences. Based on the audiovisual model, a power model shows a highest correlation between test data and predicted scores. We concluded that to enable the development of superior predictive model, a high quality, critical, synchronized audiovisual database is required. Furthermore, comprehensive assessor training may be beneficial prior to the testing to improve the assessors' discrimination ability particularly with respect to multichannel audio reproduction. In order to further improve the performance of audiovisual quality models for immersive content, in addition to developing broader and critical audiovisual databases, the subjective testing methodology needs to be evolved to provide greater resolution and robustness.
