Journal of the Audio Engineering Society

2007 October - Volume 55 Number 10


Crossovers are often described in terms of symmetrical pairs of high- and low-pass filters with a common denominator, usually Butterworth, double Butterworth (Linkwitz-Riley), or notched. The native response of the usual closed-back tweeter is second-order high-pass, but its cone excursion goes to a compliance-limited maximum at frequencies below its cutoff. It therefore needs further high-pass filtering to prevent excessive power dissipation and cone excursion produced by components of the program signal at frequencies lower than its passband. Thus the overall high-pass transfer function must be of at least third order, and preferably higher. In the low-pass channel, on the other hand, such high-order filtering is often unnecessary, so the cost and complexity of the crossover can be reduced significantly by using an asymmetrical crossover. Various possibilities are explored, with comments on their advantages and disadvantages compared with symmetrical systems.

To store high-resolution audio signals efficiently prior to DSD or LPCM delivery the audio data format should have a dynamic range and bandwidth that are substantially greater than the final release form. Simple LPCM would dictate an excessive bit rate. Consequently coding is required that takes account of the normally low ultrasonic content of audio and the inherent high sampling rates. Several strategies are presented, each capable of enhanced resolution, which are benchmarked in terms of bandwidth and spectral noise performance against a 24-bit 88.2-kHz LPCM reference. Candidates include multistage lossless differential coding and sigma-delta modulation (SDM) employing multilevel quantization and parametric noise shaping stabilized using the step-back algorithm. Multilevel SDM and 1-bit SDM are then combined to form a new class of archival format where the output code carries simultaneously both extended resolution data and an exact embedded copy of the 1-bit DSD code. Such a format class is considered a candidate for archiving audio data because not only does it carry audio data with resolution and bandwidth substantially in excess of the LPCM reference but it also retains the DSD release-format data.

The roll out of surround sound for broadcasting and packaged media, and the consequent addition of a center loudspeaker for sound accompanying video have the potential to reduce the impact of acoustical crosstalk and so improve the intelligibility of material presented with speech in the center channel. A series of listening tests were carried out assessing any potential intelligibility benefits associated with presenting speech using a central loudspeaker, as found in 5.1 surround sound systems, compared to using a central stereo image. Twenty subjects with normal hearing were presented with a series of sentences containing identifying keywords in a background of multitalker babble over a wide stereo image. Half of the sentences were played as a central source and half as a central stereo image. The tests showed significant improvements in word recognition using a separate center loudspeaker when compared with a phantom center image between a pair of loudspeakers. A theoretical transfer function showing the difference in frequency response between the two conditions is calculated and compared with actual measurements made using a dummy head. The impact of acoustical crosstalk on the intelligibility of speech is assessed.

Ambisonic Synthesis of Complex Sources

Authors: Menzies, Dylan; Al-akaidi, Marwan

Exterior expansions of complex sound sources are presented as flexible objects for producing Ambisonic sound-field encodings. The sources can be synthesized or recorded directly, rotated, and positioned in space. Related techniques can also be used to efficiently add high-quality reverberation, depending on the orientation and location of the source and listener.

[Feature] In this article we summarize some of the papers that were presented at the 30th AES International Conference on Intelligent Audio Environments. Future audio environments are bound to include a variety of devices and applications working together to create an integral audio experience to a user. For this purpose, the applications need to become aware of the surroundings and to unite with each other and with the acoustical environment. Means for controlling and rendering various sound events and acoustical environments are also needed.

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