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6.6.1 Advantage of using PESQ instead of PSQM
6.6.2 Explanation of the Measured Parameters
MOS
C H A P T E R
6 :
T E L E P H O N Y
T E S T I N G
One of the major advantages of PESQ over PSQM(+) is, that it contains a real
good time alignment algorithm, which is capable of handling varying delays.
With PSQM, such time alignment was missing in the standard, and it became
the responsibility of the implementers to resolve this issue. As experience
showed, only very few PSQM implementations came with a time alignment
algorithm that was well suited for static delays on real networks, and even fewer
measurement systems were capable of handling varying delays, as they appear
on e.g. packet based networks. One of the best algorithms was probably the
one used in OPERA, which worked quite well for most situations, but as a result
of its design for realtime applications, it also failed from time to time. As the
outcome of the wrong time alignment, two parts of the reference and the test
signal were compared that did not match and
different. This sonic difference unquestionably led to a PSQM score that was too
pessimistic and simply wrong. With PESQ, this shortcoming is finally eliminated
and the user will obtain realistic results for the device under test. There is no
danger any longer, that the tested system is downgraded, only because of a
deficiency of the measurement algorithm.
This paragraph explains the basics of the parameters which are measured by the
PESQ implementation on OPERA, as well as how these parameters are defined.
The most eminent result of PESQ is the MOS. It directly expresses the voice
quality. The PESQ MOS as defined by the ITU recommendation P.862 ranges
from 1.0 (worst) up to 4.5 (best). This may surprise at first glance since the ITU
scale ranges up to 5.0, but the explanation is simple: PESQ simulates a listening
test and is optimized to reproduce the average result of all listeners (remember,
MOS stands for Mean Opinion Score). Statistics however prove that the best
average result one can generally expect from a listening test is not 5.0, instead it
is app. 4.5. It appears the subjects are always hearing distortions, even if there
is no degradation at all of the signal available. OPERA can determine the MOS
for the entire reference and test signal, for active speech parts of the signals only
and for the silent parts of the signals. In the two latter cases, active speech is
detected by using the VAD, which is part of the PESQ time alignment. Knowing
the individual MOS scores is especially useful for optimising e.g. comfort noise
generation or noise reduction systems.
6.1.1.1
P.800 MOS and PESQ-LQ
Listening tests are very difficult to repeat and will never give identical results.
Moreover it is generally required to apply at least a linear transformation to the
results of one test if they shall match the results of a second test (with identical
test material, but performed at a different place or at a different time). The same
holds for the correspondence between a listening tests and the PESQ MOS. If
highest correlation is required, a linear mapping of the PESQ MOS to the scale
which was actually used by the test subjects must be applied. The PESQ MOS
according to P.862 was derived by optimizing a third order polynome to give
highest correlation on a very large set of data. Although this is generally the best
approach, it is of course possible to achieve higher correlations on a smaller set
of data by applying a second polynome. One such approach is the PESQ-LQ
value. It uses the following formula to transform the PESQ score (x) into the
PESQ-LQ value (y):
B A N D
V O I C E
Q U A L I T Y
consequently did sound
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