OPTICOM OPERA - V 3.5 User Manual page 197

ITU-R
The radio communication sector of the International Telecommunication Union, Geneva, (former CCIR),
see also http://www.itu.int.
ITU-T
The telecommunication sector of the International Telecommunication Union, Geneva, (former CCIR), see
also http://www.itu.int.
LD-CELP
Low-Delay CELP (Code Excited Linear Prediction) Speech Coder. According to standard ITU-T G.726. Bit
rate of 16 kbit/s.
MNRU
Modulated noise reference units
MOS
Mean listening-quality Opinion Score, or simply Mean Opinion Score. The MOS is the mean of the given
scores for a device under test of all test subjects in a subjective listening test.
MOV
The Model Output Variables are intermediate output values of the perceptual measurement method.
These variables are based on basic psycho-acoustical findings and may therefore be used to characterize
the coding artefacts further.
MPLS
Multi-Protocol Label Switching
MUSHRA
Stands for "Multiple Stimulus With Hidden Reference Anchors". A new draft ITU recommendation on
testing very low bit rate audio.
NMR
The measurement scheme NMR (Noise-to-Masked-Ratio) [BRAN87] evaluates the level-difference
between the masked threshold and the noise signal. A DFT with a Hann window of about 20 ms is used to
analyse the frequency content of the signal. The transform coefficients are combined to bands according
to the Bark scale. The masked threshold is estimated for each band. The slope of the masked threshold is
derived using a worst case approach taking into account the fact that the slopes are steeper for weak
signals but run into the absolute threshold at higher levels. The absolute threshold is adapted to the
resolution of the input signal (usually 16 bits), but not to psycho-acoustic demands. Due to these facts
NMR is robust to changes of the reproduction level. The pitch scale resolution is about 1 Bark. Since the
required computational power is low it was possible to implement NMR as a real time system at an early
stage of its development.
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