Contribution to CCITT SG XII Experts Group on Speech Quality, Document Number SQ 13.92, February 1992.

Observations on the T-Reference Condition for Speech Coder Evaluation

Stephen D. Voran

Abstract: In a Study Group XII Contribution dated September 1991, John Rosenberger and Bill Cotton of Bellcore introduced an algorithm for generating temporally correlated distortion on 8 KHz sampled speech data. This distortion is parameterized by a single integer value T, and is referred to as Temporally Correlated Noise or the T-Reference Condition (T-Ref). The T-Ref is a precisely defined, repeatable distortion process, that can generate a wide range of distortion levels, ranging from virtually no distortion (T=256), to a distortion that renders speech unintelligible (< ~ 4). This distortion tends to sound more like a low bit rate speech coder than the modulated noise reference unit (MNRU). In fact, subjective similarity tests at Bellcore revealed when both the T-Ref and the MNRU are available for matching the sound of low bit rate coders, listeners overwhelmingly selected the T-Ref over the MNRU. This similarity of sound is a highly desirable property when using a reference condition to evaluate speech coders. The properties mentioned above make the T-Ref a candidate to replace the MNRU in some tests. This potential utility makes the T-Ref an interesting subject for further study to determine exactly how and why it works as it does. In this contribution, we offer some observations from our study of the T-Ref. First we provide the definition of the process and note several properties. Next we provide time and frequency domain demonstrations of the effects of the T-Ref on sinusoids. We then show its frequency domain response to speech data and compare that response to voice coders and the MNRU. Finally, we suggest a moving average digital filter representation for the T-Ref.

For technical information concerning this report, contact:

Stephen D. Voran
Institute for Telecommunication Sciences
(303) 497-3839

To request a reprint of this report, contact:

Lilli Segre, Publications Officer
Institute for Telecommunication Sciences
(303) 497-3572

Disclaimer: Certain commercial equipment, components, and software may be identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.

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