Abstract
Spatial audio is essential for creating a sense of immersion in virtual environments. Efficient encoding methods are required to deliver spatial audio over networks without compromising Quality of Service (QoS). Streaming service providers such as YouTube typically transcode content into various bit rates and need a perceptually relevant audio quality metric to monitor users’ perceived quality and spatial localization accuracy. The aim of the paper is two-fold. First, it is to investigate the effect of Opus codec compression on the quality of spatial audio as perceived by listeners using subjective listening tests. Secondly, it is to introduce AMBIQUAL, a full reference objective metric for spatial audio quality, which derives both listening quality and localization accuracy metrics directly from the B-format Ambisonic audio. We compare AMBIQUAL quality predictions with subjective quality assessments across a variety of audio samples which have been compressed using the Opus 1.2 codec at various bit rates. Listening quality and localization accuracy of first and third-order Ambisonics were evaluated. Several fixed and dynamic audio sources (single and multiple) were used to evaluate localization accuracy. Results show good correlation regarding listening quality and localization accuracy between objective quality scores using AMBIQUAL and subjective scores obtained during listening tests.
Highlights
Ambisonics is a 3D spatial audio format which allows sound sources to be placed above, below and behind the listener in addition to the horizontal plane supported by 2D audio formats
To demonstrate the impact of spatial audio compression on perceived audio quality, a set of subjective listening tests were carried out using a double-blind multi-stimulus test method with a hidden reference and hidden anchor (MUSHRA) following the ITU-R BS.1534-3
Multiple point audio sources are denoted in this paper as concatenations of fixed and dynamic audio source labels
Summary
Ambisonics is a 3D spatial audio format which allows sound sources to be placed above, below and behind the listener in addition to the horizontal plane supported by 2D audio formats. Ambisonics is a popular audio format for VR and AR use cases given that it allows full rotation of the soundfield in three dimensions [6]. The most popular format today, known as first-order Ambisonics (FOA), uses 4 spherical harmonics denoted as W, X, Y and Z. The B-format representation can be extended from FOA to second and third-order Ambisonics (SOA and 3OA) to get higher localization accuracy. Second-order and third-order Ambisonics contain 9 and 16 channels, respectively. It was found in [7] that third-order Ambisonics (3OA) gives a significantly better Quality of Experience (QoE). This is further supported in [3]. It should be noted that the number of channels increases according to (n + 1) where n is the order
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