OPTICOM OPERA - V 3.5 User Manual page 20

ITU-R BS.1116
C H A P T E R 2 : W H I C H
Impairment
Excellent
Good
Fair
Poor
Bad
Table 2.1: The ITU-T five-grade impairment scale
Based on these test conditions a population of typically 20 to 50 test subjects
will be presented with an identical series of speech fragments. Every test subject
will be asked to score each sample by applying the impairment scale. After
statistical processing of the individual results, a Mean Opinion Score (MOS) can
be calculated. With thorough setups, such test results can be reproduced quite
well, even at different locations. It goes without saying that the effort needed in
terms of subjects and time is tremendous. It is clear that such test methods can
not be applied within a practical or field environment in the daily life.
The ITU has also recommended a test procedure to assess wide band audio
codecs on the basis of subjective tests. Subjective assessments of low bit rate
audio codecs in the past always targeted an almost transparent quality. For this
reason, the test method focuses on the comparison of the coded/decoded signal
to the unprocessed original reference. The relevant recommendation is known
as BS.1116, titled "Methods for the Subjective Assessment of small Impairments
in Audio Systems including Multichannel Sound Systems" [ITUR1116] which
was issued by the ITU-R
The test method, which is recommended by BS.1116, is referred to as "double-
blind triple-stimulus with hidden reference". It is extremely sensitive and allows
for the accurate detection of small impairments. The grading scale used should
be treated as continuous with "anchors" derived from the ITU-R five-grade
impairment scale according to ITU-R BS.562 [ITUR562]. It is depicted in Table
2.2.
Impairment
Imperceptible
Perceptible, but not annoying
Slightly annoying
Annoying
Very annoying
Table 2.2: The ITU-R five-grade impairment scale
The analysis of the results from a subjective listening test is generally based on
the Subjective Difference Grade (SDG) and is defined as:
SDG
Provided that the listener correctly assigns the hidden reference signal, the SDG
values will range from 0 to –4, where 0 corresponds to an imperceptible
impairment and –4 to an impairment judged as very annoying. The assignment
of the SDG scale is shown in the last column in Table 2.2.
2
Radiocommunication Sector of the ITU (former CCIR)
M E A S U R E M E N T
Grade
5
4
3
2
1
2
in 1994 and was updated in 1997.
Grade
5.0
4.0
3.0
2.0
1.0
= −
Grade
Signal Under Test
14
D O I N E E D ?
SDG
0.0
-1.0
-2.0
-3.0
-4.0
Grade
Reference Signal