Speech Quality
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Summary of Speech Quality Testing Methods


Diagnostic Acceptability Measure (DAM)

The DAM was developed at Dynastat as a method for measuring the subjective quality or acceptability of voice communications systems or links. It incorporates a number of unique features. First it combines both a direct (isometric) and an indirect (parametric) approach to speech quality evaluation. Thus, the listener has an opportunity to indicate directly not only how acceptable a speech sample is, but to independently indicate to what extent various perceived qualities are present in the sample without regard to how they may affect his evaluation of acceptability. Though two listeners may disagree on their overall acceptability rating of a speech sample with background noise, they would be more likely to agree on how much noise was present in the speech sample.

A second feature is that the DAM requires the listener to make separate ratings of the speech signal itself, the background, and the total effect. A listener makes a total of 21 ratings during the course of a speech sample. Ten ratings are concerned with perceptual qualities of the signal, eight ratings are concerned with the perceptual qualities of the background, and three items are concerned with perceived intelligibility, pleasantness, and overall acceptability. These 21 ratings are combined to produce a CAE for reporting purposes.


Mean Opinion Score (MOS)

The MOS is a test for measuring the acceptability or quality of speech over a communication system. The MOS requires the listener to judge the overall quality of a communication system or link on a five category scale (i.e., excellent, good, fair, poor, and bad) for purposes of telephone communication. These verbal categories are then converted to a five point scale as shown on the scale below.
                    5 - EXCELLENT
                    4 - GOOD
                    3 - FAIR
                    2 - POOR
                    1 - BAD
Speech quality values are reported as a mean MOS score.