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C H A P T E R
2 :
2 TEST METHODOLOGY
Essential Knowledge about Perceptual
Measurements and a Guidepost to the Correct
Measurement Method.
T
his chapter describes listening test methods which are modeled by the
OPERA™ system, and a description of the underlying concept of the
proposed algorithms for perceptual measurement and the international
standardization of perceptual audio measurement techniques. In
addition, this chapter will provide a guide to the correct measurement method
for your applications.
2.1 Assessing Quality
Until recently, the only widely accepted assessment procedures for audio or
speech codecs were listening tests, due to the lack of international standards for
measuring the perceived audio quality.
Historically related to the assessment of telephone connections, useful methods
for testing telephone band speech signals were first standardized within the ITU-
1
T
. Recommendation P.800 [ITUT800] defines the absolute category rating test
method (ACR) which has been used for the assessment of speech codecs since
1993. Within the ACR test method, the ITU five grade impairment scale is
applied (see Table 2.1). In the telecommunication environment, testing is
done without a comparison to an undistorted reference. This copes with a
typical situation of a phone call, where the listener has no access to a
comparison with a reference, e.g. the original voice of the other party.
However, it should be noted that the listening test according P.800 could be
regarded as a comparison between a test signal and a reference "in the mind" of
the listener. The reason for this is that the listener is very familiar with the
natural sound of a human voice.
For comparison reasons, and in order to be able to merge the results of different
individuals, it is necessary to adjust the listeners' opinions to an absolute scale.
For this purpose, predefined examples with well defined noise insertions of
fixed modulated noise reference units (MNRU, [ITUT810]) are presented at the
beginning of a test. Each sample represents an example distortion
corresponding to the ITU-T version of the five grade impairment scale.
1
International Telecommunication Union, Geneva, (former CCITT), see also
http://www.itu.org
T E S T
M E T H O D O L O G Y ?
13
2
Chapter
ITU-T P.800
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