Journal of the Audio Engineering Society

2002 September - Volume 50 Number 9


Using the 10-dB switch of a condenser microphone in an audio frequency circuit produces a change in its linear (reduction in signal output) and nonlinear behavior. Two classical options are available to reduce the input voltage of the field effect transistor (FET) or the vacuum tube by adding a parallel capacitor or by reducing the polarizing voltage. The effects of both are described theoretically and practically.

Many frequency domain audio tests, such as measuring the frequency response, measuring frequency-dependent crosstalk, or making some kind of distortion measurements, can be performed by applying multitone test signals to the audio device under test and analyzing the output signal by means of the discrete Fourier transform. In contrast to standardized single-tone or dual-tone excitation tests, the multitone test offers significant time savings and enables realistic test conditions by approximating the frequency domain characteristics of a typical program material. But multitone testing requires the design of low-crest-factor test signals. Several computation schemes and indirect optimization methods of low-crest-factor phases, are discussed, and for the first time optimum multitone signals designed by general-purpose nonlinear optimization software are presented.

The equipment impairment factor methodology adopted by the ITU-T for describing perceptual quality degradations associated with nonwaveform telephone speech codecs is discussed with regard to other degradations of the transmission channel. A new algorithm allows such factors to be calculated from the results of auditory tests, and first steps toward a derivation by means of instrumental models are presented. Limitations of the current approach for codec tandems are pointed out.

Spatial quality in reproduced sound is a subset of the broad topic of sound quality. In the past it has been studied less rigorously than other aspects of reproduced sound quality, leading to a lack of clarity in standard definitions of subjective attributes. Rigor in the physical measurement of sound signals should be matched by equal rigor in semantics relating to subjective evaluation. A scene-based paradigm for the description and assessment of spatial quality is described, which enables clear distinctions to be made between elements of a reproduced sound scene and will assist in the search for related physical parameters.

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About the 10-dB Switch of a Condenser Microphone in Audio Frequency Circuits

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