Voice Quality Measurement – MOS

Mean opinion score (MOS) is a test that has been used for decades in telephony networks to obtain the human user’s view of the quality of the network. In multimedia (audio, voice telephony, or video) especially when codecs are used to compress the bandwidth requirement (for example, of a digitized voice connection from the standard 64 kilobit/second PCM modulation), the MOS provides a numerical indication of the perceived quality from the users’ perspective of received media after compression and/or transmission. The MOS is expressed as a single number in the range 1 to 5, where 1 is lowest perceived audio quality, and 5 is the highest perceived audio quality measurement.

MOS tests for voice are specified by ITU-T recommendation P.800

The MOS is generated by averaging the results of a set of standard, subjective tests where a number of listeners rate the heard audio quality of test sentences read aloud by both male and female speakers over the communications medium being tested. A listener is required to give each sentence a rating using the following rating scheme:

Mean opinion score (MOS)
 MOS    Quality        Impairment
 5    Excellent    Imperceptible
 4    Good        Perceptible but not annoying
 3    Fair        Slightly annoying
 2    Poor        Annoying
 1    Bad        Very annoying


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