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C H A P T E R
2 :
T E S T
Since P.861 was mainly developed for application to isolated speech codecs in
mobile networks, a new measure was required to cope with real networks as
well as packet based transmission. Driven by this demand for a verified test
procedure for VoIP, an expert group within ITU-T SG12 has been working on
an improved speech quality model. After a competitive phase, the new model
"PESQ" has been devised. PESQ stands for "Perceptual Evaluation of Speech
Quality". It combines a further refinement of PSQM and PAMS. Extensive tests
showed PESQ's superior performance especially for VoIP applications. In
February 2001, PESQ was accepted as the ITU-T Rec. P.862. More information
on PESQ can be found at
Within the study period 1994 – 1998, the ITU-R had established task group
10/4 with the scope to recommend an objective, perception based model to
evaluate the quality of wide band audio codecs. After collecting a set of
proposals, including the most popular ideas such as NMR, PAQM, PERCEVAL,
POM and others, the group of model proponents opted for a joint collaboration
to derive an improved model. In 1998, two versions of this new model were
presented: A "Basic" version, featuring a low complexity approach, and an
"Advanced" version for higher accuracy at the trade-off of higher complexity.
After thorough verification, the model was recommended as a measure for the
perceived audio quality ("PEAQ") under recommendation BS.1387 in late
1998.
Both standards, ITU-T P.861, and ITU-R BS.1387, currently represent the state-
of-the-art technique for the objective evaluation of the perceived audio quality
of audio codecs. Both techniques were derived from modelling the
corresponding subjective experiment by an algorithm based approach. Thus it
is essential to understand the scope of the modelled subjective experiment
when trying to interpret the calculated results.
subjective
Data rate
128 kbit/s
ITU-R BS.1116
64 kbit/s
ITU-T P.800
16 kbit/s
Figure 2.2: Overview on subjective and objective recommendations
Figure 2.2 summarizes the subjective test procedures and their
corresponding objective counterpart in the context of typical data rate limits. As
M E T H O D O L O G Y ?
www.pesq.org
.
5.0
Impairment Scale
4.0
triple stimulus,
3.0
double-blind with
2.0
hidden reference
1.0
typ. Music - Codec*
typ. Speech - Codec*
5
Impairment Scale
4
Absolute
3
Category
2
Rating
1
17
objective
ITU-R BS.1387
(PEAQ)
*Only examples,
no fixed
threshold due to
the continous
trend to further
reduce bit rates
ITU-T P.861/862
(PSQM / PESQ)
ITU-T P.862
ITU-R BS.1387
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