Mean Option Score

Mean Option Score

In order to evaluate the performance of speech transmission under real conditions, a
measurement process for speech quality had to be defined. This would make it possible to
allocate a rating and thus compare the different codecs. In a first approach, only a coarse
assessment of the speech codecs' quality and efficiency was defined. The human ear and
brain can compensate for poor speech quality to a certain extent - two facts of humen
physiological that can not be reproduced easily using technical means. Thus, only subjective
measurements for speech quality can be used.

ITU procedures provide a test scenario where a mixed audience was exposed to various
speech samples under defined conditions. Speech quality was rated taking different aspects
into account such as the speaker's understandability, a dialogue test, subjective loudness
perception and the effort needed to understand a minimum level of information. Together, the
results give an index that represents the test candidates' opinion of the codec type been
tested. This index is called the Mean Opinion Score or MOS. The MOS can range from 1.0
(very bad) to 5.0 (excellent). Legacy circuit-switched ISDN telephony has an MOS of 4.4.