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The ITU-T P.801
MOS scale
C H A P T E R
6 :
T E L E P H O N Y
T E S T I N G
Figure 6.17: The diagram type MOS etc.
Model Output Variable (MOV)
Silence
Sev. Distorted
Time Clipped
OMOS-W0
OMOS-W2
OMOS-W4
OMOS+
OMOS-I
Table 6.3: Results calculated by the OPERA™ PSQM version, and their interpretation
Table 6.4 shows the simple interpretation of the MOS (P. 801):
MOS Value
Interpretation
5
= Excellent
4
= Good
3
= Fair
2
= Poor
1
= Bad
Table 6.4: The opinion scale according to P.801
Since subjective tests show
quality (some listeners always hear some distortions...), the MOS scaling of
PSQM ranges from 1.0 to 4.05 (and not up to 5.0). The only exception from this
is the OMOS-I. Since this value represents the behaviour of an individual, ideal
listener, it covers the full range of the ITU scale (1..5).
During a measurement the result values are averaged between the start sample
and the end of either the reference, or the test file. The stop point as defined by
B A N D
V O I C E
Interpretation
The percentage of silent intervals
during a measurement
The percentage of severely distorted
frames during measurement
The percentage of time clipped
frames during measurement
MOS according to P.861,
silence weight = 0.0
MOS according to P.861,
silence weight = 0.2
MOS according to P.861,
silence weight = 0.4
MOS according to PSQM+
MOS of the individual listener
an average between 4.05 and 4.5 for transparent
132
Q U A L I T Y
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