Journal of the Audio Engineering Society

2007 March - Volume 55 Number 3


On the Audibility of All-Pass Phase in Electroacoustical Transfer Functions

Authors: Møller, Henrik; Minnaar, Pauli; Olesen, S. Krarup; Christensen, Flemming; Plogsties, Jan

Audible effects of second-order all-pass sections with center frequencies in the range of 1-12 kHz were studied in headphone listening experiments. All-pass sections give rise to two effects. 1) A perception of “ringing” or “pitchiness,” which is related to an exponentially decaying sinusoid in the impulse response of all-pass sections with high Q factors. The ringing is especially audible for impulsive sounds, whereas it is often masked with everyday sounds such as speech and music. With an impulse signal the ringing was found to be audible when the decay time constant for the sinusoid exceeds approximately 0.8 ms (peak group delay of 1.6 ms), independent of the center frequency within the frequency range studied. 2) A lateral shift of the auditory image, which occurs when an all-pass section is inserted in the signal path to only one ear. The shift is related to the low-frequency phase and group delays of the all-pass section, and it was found to be audible whenever these exceed approximately 35 s, independent of the signal.

A digital artificial reverberation algorithm is presented, based on a full-time domain simulation of plate vibration. As such it may be considered to be a physical model plate reverberation, a popular means of processing audio signals in the days of analog production. A small number of parameters are available to the user, to be used to tune the plate response, in a means analogous to that for the acoustic plate reverberation unit. Such parameters include stiffness, aspect ratio, tension, and two-parameter loss. A variety of other possibilities are opened up, including multichannel input and output, possibly over time-varying locations, and various types of boundary termination. The complete numerical method is presented, along with a discussion of implementation details and computational complexity (which is near real time). Numerical results and sound examples are also presented.

[Engineering Report] The psychoacoustics theory involved in the design of state-of-the-art digital multiband audio processors used for AM and FM broadcasting transmission is described. It is analyzed how the average rms value of the audio signal can be improved without loosing audio quality. It is further demonstrated that audio processing increases the coverage of FM stereo transmission.

Multichannel Audio Processing Using a Unified-Domain Representation

Authors: Short, Kevin M.; Garcia, Ricardo A.; Daniels, Michelle L.

[Engineering Report] The unified-domain representation for synchronized multichannel audio streams is introduced. This lossless and invertible transformation describes multiple streams of audio as a single-frequency-domain magnitude component multiplied by a complex matrix encoding the spatial and phase relationship information for each channel. Unified-domain analysis and signal-processing techniques for applications such as high-resolution frequency analysis, sound source separation, spatial psychoacoustic models, and low-bit-rate audio coding are presented.

[Feature] We review recent AES papers dealing with reverberation, its artificial generation, analysis and enhancement. How can some of the characteristics of reverberation be measured and how can reverberation be removed successfully from other wanted audio signals? What is the role of reverberation in scene analysis and synthesis?

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