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Journal of the Audio Engineering Society

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2005 June - Volume 53 Number 6

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Papers

Low Peak Amplitudes for Group Additive Synthesis

Authors: Horner, Andrew; Wun, Simon

The peak amplitude of a waveform for a particular spectrum depends on the phases of its harmonic components, and lower peak amplitudes give better signal-to-noise ratios. Most previous work on peak amplitude reduction has only considered individual spectra, but applications such as wavetable synthesis require multiple wavetables. A recent study compared various phase selection methods for multiple wavetable synthesis and found that genetic algorithm (GA) optimization gave peak amplitude results 10–25% lower than other methods. Various phase selection methods are compared with regard to group additive synthesis, a special case of multiple wavetable synthesis, where each wavetable contains a distinct subset of the harmonics. The results show that GA and simulated annealing optimization are consistently good, and usually outperform random phase selection, Schroeder’s method, van den Bos’s method, and Pumplin’s method with one or two wavetables. The GA runs several times faster than simulated annealing and strikes the best balance between efficiency and effectiveness. With three or more wavetables, the random, van den Bos, Pumplin, GA, and simulated annealing methods give about the same results. Group additive synthesis peak factors are up to 30% worse than their wavetable synthesis counterparts, unless wavetable matching and peak-factor optimization are integrated. While multiple wavetable synthesis allows lower peak factors and relative spectral errors than group additive synthesis, group additive synthesis has the advantage that it allows better and more intuitive control than multiple wavetable synthesis. The GA and simulated annealing low-peak methods help get the best performance out of both multiple wavetable synthesis and group additive synthesis.

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Reconstruction of Recorded Sound from an Edison Cylinder Using Three-Dimensional Noncontact Optical Surface Metrology

Authors: Fadeyev, Vitaliy; Haber, Carl; Maul, Christian; Mcbride, John W.; Golden, Mitchell

[Engineering Report] Audio information stored in the undulations of a groove in a mechanical sound carrier such as a cylinder or disk phonograph record may be reconstructed, without contact, by measuring the groove shape using precision optical metrology methods and digital image processing. The viability of this approach was recently demonstrated on a 78-rpm shellac disk using two-dimensional image acquisition and analysis methods. The first three-dimensional reconstruction of recorded sound from a mechanical carrier is reported. The source material, a celluloid cylinder, was scanned using color-coded confocal microscopy techniques and resulted in a faithful playback of the recorded information.

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Large-Signal Analysis of Class A Vacuum Triode Push–Pull Output Stage

Authors: Abuelma’atti, Muhammad Taher

[Engineering Report] A mathematical model for the input–output characteristic of a class A push–pull output stage using triode vacuum tube amplifiers is presented. The model, basically a sine series function, can easily yield closed-form series expressions for the amplitudes of the output components resulting from multisinusoidal input signals to the output stage. The special case of an equal-amplitude two-tone input signal is considered in detail. The results show that, similar to a transistor-based class A output stage, the vacuum triode class A output stage generates only odd-order harmonic and intermodulation components. The results also show that the amplitudes of these components are strongly dependent on the amount of cathode feedback and the amplitudes of the input tones. For sufficiently small input tone amplitudes, the relative harmonic and intermodulation products are too small and their magnitudes are within or close to the error range of the Fourier series approximation. However, for relatively large input tone amplitudes the relative harmonic and intermodulation components are increasing monotonically with the input tone amplitudes.

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Servo Control of Loudspeaker Cone Motion Using an Optical Linear Displacement Sensor

Authors: Geiger, Wolfgang

[Engineering Report] Loudspeakers normally exhibit the highest harmonic distortion in an audio system. Large signals create nonlinear effects in the motor structure as well as displacement errors, which are caused by the non-linear force factor and stiffness. Both effects can be reduced by an optical linear displacement measurement and closed-loop control of cone motion. The absolute value and the phase of the cone displacement are defined by the controller and not by the loudspeaker. This is particularly important for arrays.

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Automotive Audio Quality

Authors: Staff, AES

[Feature Article] There has been substantial interest at recent Audio Engineering Society conventions in the evaluation of the quality of automotive audio, particularly in relation to new developments such as multichannel audio. This paper reviews relevant material from recent conventions, aiming to summarize some of the most recent findings of the leading researchers in this field.

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