mean opinion score (speech encoding) (MOS)
The Mean Opinion Score(MOS) is a subjective evaluation measure for the transmission of speech. It offers the possibility to compare the transmission quality for different speech encodings.
The MOS value is a dimensionless value between one and five that represents speech quality; where the value "one" represents poor speech quality where no communication is possible, whereas the value "five" represents excellent transmission quality that is indistinguishable from the original.
In contrast to the objectively determined R-factor, the MOS value is determined subjectively. The MOS value is determined by playing speech samples to test persons, who rate them. The ratings are weighted and the statistical results are determined from this. The evaluation methods are refined in ITU Recommendations P.830 to P.834.
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The most important quality criteria for the transmission of voice information are delay times, bit error rates, echoes and jitter. Since the ear is sensitive to sound fluctuations and speech interruptions, the delay times should be approximately constant. Speech quality is not affected by the delay during transmission; only the call quality deteriorates. Bit errors, on the other hand, have the effect of crackling noises.
Echoes occur in analog systems at the transition from four- wire to two-wire technology and irritate the speaker due to the speech reflection, which affects intelligibility.