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C H A P T E R
2 :
T E S T
M E T H O D O L O G Y ?
In contrast to the listening test according to ITU-T P.800, an explicit comparison
between the test signal and a reference signal is needed in the case of BS.1116,
since the listener never knows how the original signal sounds.
This method was applied in a variety of international verification tests in the
past. However, keep in mind that because of the scope of BS.1116 it can be
applied to small impairments only, which means a practical limitation to almost
"transparent" studio quality. Another issue which has been discussed among
experts, is the recommendation to use the scale at a resolution of one decimal
place, resulting in 41 (!) discrete steps. There are indications that for some
subjects this is too great a choice, and furthermore the meaning of the
impairment anchors is interpreted differently [KARJ85].
Because of the restrictions to small impairments, there is consensus among
experts that other methods are needed for very low bit rate tests (i.e. of large
impairments). Various approaches have been introduced and work is currently
in progress in several task groups, e.g. the MPEG standardisation work
[GILC96]. The methods according to ITU-T P.800 were adopted for some
assessments to overcome the problem of a "gap" for a useful recommendation
on testing significantly impaired wide band audio signals. While in principle
they seem to be better suited for impaired music signals when compared to the
BS.1116 method, it's exploitation for very low bit rate audio coding applications
still remains questionable, as there are no clearly defined example distortions in
such a case. The scale was derived from telephone speech quality, and is not
well defined when translated to music coding. The achieved results may
therefore significantly depend on the subjective interpretation of the impairment
levels.
At the time of drafting this manual, an advanced listening test procedure has
MUSHRA
been advised by an EBU expert group, known as "MUSHRA". MUSHRA stands
for "Multiple Stimulus With Hidden Reference Anchors". The new method
targets testing significantly impaired audio signals, such as those derived at very
low bit rates. MUSHRA is expected to become adopted as an international
recommendation by the ITU working party 10-11Q. As soon as more
experience has been gained, OPTICOM will provide its customers with more
information at
www.peaq.org
.
2.2 Advanced Audio Measurements
Employing "Perceptual Modeling"
Assessing the quality was a pending issue during the years of the development
of compression schemes. Consequently, the idea of substituting the subjective
tests by objective, computer based methods has been an ongoing focus of
research and development. Early work motivated through the development in
speech coding was reported in [KARJ85]. Since then several methods were
introduced.
The underlying concepts of the proposed algorithms for perceptual
measurement techniques are all quite similar. The common structure of these
algorithms is depicted in Figure 2.1. The process of human perception is
modelled by employing a difference –measurement-technique which compares
both, a reference signal (i.e. the "input" signal to a codec) and a test signal (i.e.
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