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2.3 International Standardization
ITU-T P.861
C H A P T E R
2 :
W H I C H
the "output" signal of the codec). First, the algorithms process an ear model for
the reference and the test signal, in order to calculate an estimate for the audible
signal components. The result can be imagined as the "internal representation"
inside the human auditory system. Comparing the internal representations of
the reference, and the test signal leads to an estimate of the audible difference.
To derive an overall quality figure, this information, which is a function of time,
must be processed accordingly, like the human brain of a subject would do in a
listening test. The respective part of processing within an algorithm is referred to
as cognitive modelling. A total quality figure will be derived as the final result,
which can be compared to a MOS ("Mean Opinion Score") resulting from a
listening test.
Reference
(=Input)
Representation
Perceptual
Model
Perceptual
Model
Test
)
Representation
(=Output
Figure 2.1: The underlying concept for perceptual measurement
The evaluation of the internal representation is often related to an estimate of
the masked threshold. This estimate is based on data found in a number of
psychoacoustic experiments, such as those conducted by Zwicker [ZWIC67,
ZWIC82]. Most of these experiments model certain isolated effects of the
human auditory system. One way to design a perceptual measurement
algorithm is to generalize these model data and apply them to complex audio
signals. This was for example the approach outlined in the NMR Algorithm in
1987 [BRAN87, BRAN89, BRAN92, GILC96, HERR92a, HERR92b, KEYH93,
KEYH96, KEYH98, SEIT89]. Similar approaches were used for PAQM and
PSQM [BEER95, BEER92, BEER94].
International standardization of perceptual audio measurement techniques was
mainly
driven
by
Telecommunications Union (ITU).
Within the telecommunication sector of the ITU, in 1996 study group 12
finalized recommendation P.861 [ITUT861] for the objective analysis of speech
codecs. After a wide-ranging comparison of proposed methods, the group opted
for the PSQM algorithm. PSQM correlated up to 98 percent with the scores of
subjective listening tests.
M E A S U R E M E N T
Internal
Audible
Difference
Internal
two
expert
groups
16
D O
I
N E E D ?
Total Quality
Cognitive
Model
Figure
within
the
International
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