Journal of the Audio Engineering Society

2007 September - Volume 55 Number 9


Perceptual Enhancement of Wavefield Synthesis by Stereophonic Means

Authors: Wittek, Helmut; Rumsey, Francis; Theile, Günther

The properties of wavefield synthesis with regard to localization and sound color reproduction are analyzed by listening tests and compared with the properties of stereophony. A novel technique, optimized phantom source imaging (OPSI), designed to avoid spatial aliasing, is presented and analyzed in theory and practice. Both stereophonic phantom sources as well as OPSI sources were perceived to be less colored than was predicted by coloration predictors based on the spectral alterations of the ear signals. This leads to the hypothesis that a decoloration process exists for stereophonic reproduction, as proposed in the association model of Theile.

A perceptual approach to calculating head-related impulse responses (HRIRs) is presented. In this method the ratio between contralateral and ipsilateral HRIRs is represented by a lower order finite impulse response (FIR) filter on the basis of the interaural transfer functions (IATF). The key concept of the proposed method is to use the ipsilateral response to represent the contralateral response efficiently via a perceptual approach. It is assumed that accurate reproduction of the former processing suffices to mask the high-frequency loss incurred in the latter. The FIR filter is obtained by using the Wiener filter approach. To further improve the computational efficiency, the absolute threshold of human hearing is exploited to eliminate the inaudible components in the HRIRs. Numerical simulations have shown that there is close agreement between the method presented and the HRIRs measured. On the other hand, the proposed method allows for a more efficient implementation, with savings of nearly 40% in terms of computation cost as well as memory storage in the HRIR synthesis. In order to validate the proposed technique, a subjective listening experiment was conducted. The objective difference grade (ODG) was used to assess the psychoacoustic performance. The test results reveal that the proposed technique of HRIR calculations is effective in threedimensional sound generation.

[Engineering Report] Wavetable matching of musical instrument tones using principal-component analysis (PCA) takes advantage of spectral correlation information to find the basis spectra. Although PCA matching is efficient, it usually matches the low-amplitude parts of a tone poorly because of its inherent statistical bias. Weighted PCA methods are described, which normalize the tone prior to PCA to fairly weight its different parts. Matching results for a range of instruments show that the PCA of a frame-weighted spectrum with a roughly constant loudness throughout improves on unweighted PCA by an average of about 4% relative spectral error. Listening test results show that frame-weighted PCA gives some perceptual improvements for most of the instruments.

[Engineering Report] Claims both published and anecdotal are regularly made for audibly superior sound quality for two-channel audio encoded with longer word lengths and/or at higher sampling rates than the 16-bit/44.1-kHz CD standard. The authors report on a series of double-blind tests comparing the analog output of high-resolution players playing high-resolution recordings with the same signal passed through a 16-bit/44.1-kHz “bottleneck.” The tests were conducted for over a year using different systems and a variety of subjects. The systems included expensive professional monitors and one high-end system with electrostatic loudspeakers and expensive components and cables. The subjects included professional recording engineers, students in a university recording program, and dedicated audiophiles. The test results show that the CD-quality A/D/A loop was undetectable at normal-to-loud listening levels, by any of the subjects, on any of the playback systems. The noise of the CD-quality loop was audible only at very elevated levels.

Auditory Displays

Authors: Staff, AES

[Feature] Auditory displays are systems that attempt to display information in an audible form. This might include visual information for helping visually impaired people, warning signals, information about spaces or environments, or perhaps navigational aids. In this article we provide an overview of some of the main issues in the field of auditory display and summarize some recent developments in this interesting field, based on papers presented at the AES 120th Convention last year in Paris.

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