Journal of the Audio Engineering Society

2004 September - Volume 52 Number 9


Errors in Real-Time Room Acoustics Dereverberation

Authors: Hatziantoniou, Panagiotis D.; Mourjopoulos, John N.

Significant measurable audible distortions are generated during real-time room acoustics dereverberation, which are far greater in magnitude than those expected from identical simulated (off-line) dereverberation experiments. Long inverse filters worsen this effect. Possible mechanisms for such discrepancy between simulated and real-time tests are examined. It is also shown that dereverberation based on complex smoothing is immune to such errors.

In a previous study it was explored how the perceived naturalness of music and speech signals was affected by various forms of linear filtering. In the present paper a model is introduced to account for the results. The model is based on the assumption that changes in perceived naturalness produced by linear filtering can be characterized in terms of the changes in the excitation pattern produced by the filtering. The model takes into account both the magnitude of the changes in the excitation pattern and the rapidity with which the excitation pattern changes as a function of frequency. It also includes frequency-weighting functions to take into account the fact that naturalness is affected little by changes in amplitude response at very low and very high frequencies. The model accounts very well for the data presented in the earlier study. Two validation experiments were conducted in which naturalness ratings were obtained for speech and music stimuli passed through new sets of linear filters, including filters based on the measured frequency responses of real transducers. The model predicted the results of these experiments well.

Interpositional Transfer Function for 3D-Sound Generation

Authors: Freeland, Fábio P.; Biscainho, Luiz W. P.; Diniz, Paulo S. R.

The interpolation of head-related transfer functions (HRTFs) for 3D-sound generation through headphones is addressed. HRTFs, which represent the paths between sound sources and the ears, are usually measured for a finite set of source locations around the listener. Any other virtual position requires interpolation procedures on these measurements. In this work a definition of the interpositional transfer function (IPTF) and an IPTF-based method for HRTF interpolation, recently proposed by the authors, are reviewed in detail. The formulation of the interpolation weights is generalized to cope with azimuth measurement steps which change with elevation. It is shown how to obtain a set of reduced-order IPTFs using tools such as balanced model reduction and spectral smoothing. Detailed comparisons between the accuracy and the efficiency of the IPTF-based and the bilinear interpolation methods (the latter using the HRTFs directly) are provided. A practical set of IPTFs is built and applied to a computationally efficient interpolation scheme, yielding results perceptually similar to those attainable by the bilinear method. Therefore the proposed method is promising for real-time generation of spatial sound.

A new technique for measuring nonlinear distortion in transducers is presented which considers a priori information from transducer modeling. Transducers are single-input, multiple-output (SIMO) systems where the dominant nonlinearities can be concentrated in a single source, adding nonlinear distortion to the input signal. Assessing the distortion at the source results in more meaningful data describing the large-signal behavior of loudspeakers independent of the linear transfer response. Since the distortion source is not accessible for direct measurements, the equivalent input distortions are calculated from the sound pressure output. This technique makes it possible to predict the distortion in the sound field, to investigate the influence of the acoustical environment (room), and to separate noise and other disturbances not generated by the transducer.

[feature] The tendency in modern surround sound systems is for very low frequency information from all the main channels to be reproduced through a single subwoofer so that the main loudspeakers can be made smaller and more convenient to use. This same low-frequency loudspeaker can also be made to reproduce the content of a separate low-frequency effects (LFE) channel—the .1 channel of 5.1 surround sound. This channel has extra headroom compared with the main channels and can be used for special effects. Bass management can be used to split the LF information between the main loudspeakers and the subwoofer, incorporating the LFE signal as appropriate with its 10-dB gain, as shown in Fig. 1. This is the norm for most consumer systems, where the crossover frequency may be relatively high in some cases, resulting in much of the general low-frequency energy being radiated through a single loudspeaker. In movie theaters, however, the subwoofer is normally connected directly to the LFE channel and radiates only this information.

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