Fabio P. Freeland

On the quality assessment of sound signals

This paper constitutes an introduction to the field of quality evaluation of sound (speech and audio) signals. The need for such an assessment is inherent to modern communications: VoIP, mobile phone, or teleconference systems require meaningful measures of performance, which may ultimately assure good service or profitable business. A brief survey on subjective and objective evaluation methods is provided. Recent developments as well as new topics to be investigated are also addressed. Experiments are conducted to illustrate how to validate quality assessment methods.

HRTF Interpolation Through Direct Angular Parameterization

Under anechoic conditions, a system that generates 3D binaural sound should simulate the head-related transfer functions (HRTFs) that represent sound changes from the source to the listeners ears. Such a system could directly use HRTFs measured over a sphere around the listener. However, since it is not feasible to perform measurements for every position, practical systems interpolate the necessary functions between measured HRTFs. This paper proposes a generalization of the continuous variable digital delay (CVDD) described in the literature, yielding a two spatial-variable dependent transfer function. It performs a polynomial interpolation whose coefficients are optimized on a region over the sphere, so that the angular coordinates of any point inside the region map directly into the corresponding HRTFs. Performance assessment is done through graphical means. The issue of continuity in moving sound generation is also approached, through the combination of adjacent-region structures

Using inter-positional Transfer Functions in 3D-sound

This paper addresses the interpolation of Head-Related Transfer Functions (HRTFs) for 3D-sound generation through headphones. HRTFs are transfer functions associated with the paths between sound sources and the ears, and are usually measured for a finite set of source locations around the listener. Generation of 3D-sound at any other virtual positions requires interpolation procedures. In this work, the Inter-positional Transfer Function (IPTF) is introduced and an IPTF-based new method for HRTF interpolation is proposed. Comparisons between the IPTF-based and the bilinear interpolation methods are provided, with focus on interpolation accuracy and computational complexity.

An objective method for quality assessment of ultra-wideband speech corrupted by echo

Modern telepresence systems can deliver multimedia signals of unprecedentedly high quality of experience to the user. Setting and maintaining such services call for reliable and automatic tools for multimedia quality probing, in special those targeted at speech data along the transmission path. Most of the objective methods for sound quality assessment (QA) in the literature are intended for either speech signals of 4- to 8-kHz bandwidth or general audio until 24 kHz, but are not specifically designed for speech at high sampling-rates. This work approaches quality evaluation of full-band (24 kHz) high-quality speech corrupted by echo. A simple metric singled out from a standardized double-ended tool for audio QA is proposed as a solution for the problem at hand. Quality measures from a set of speech stimuli corrupted by echo under controlled conditions were obtained via listening tests to allow calibration and evaluation of the proposed method. Experimental results reveal an overall correlation of 0.94 between objective and subjective scores, even in the presence of moderate additive noise.

Quality Evaluation of Reverberation in Audioband Speech Signals

Modern telepresence systems constitute a new challenge for quality assessment of multimedia signals. This paper focuses on the evaluation of the reverberation impairment for audioband speech signals. A review on the reverberation effect is presented, with emphasis given on the mathematical modeling of its components, including early reflections and late reverberation. A subjective test for evaluating the human perception of the reverberation phenomenon is completely described, from its conception to the final results. Analyses are provided comparing the average subjective grades to current quality-evaluation standards for speech and audio signals. It is verified how the reverberation perception can be mapped onto three main system characteristics: reverberation time (associated to the room acoustical properties), source-microphone distance, and room volume. Direct estimation of these parameters from the room impulse response is discussed. One established reverberation measure is then revisited in the audioband speech context, showing high correlation with the subjective grades previously obtained.