Mean Opinion Score

The Mean Opinion Score ( MOS) is the arithmetic average of individual ratings and can be in a range from 1 ( worst score ) to 5 ( best).

In telecommunications, the MOS value is mainly used to assess the quality of voice and video transmission services. It is the outcome of a specified scheduling multiple tests, in which the perceived quality of speech or of the images is evaluated by a group of subjects. The result of the test series is arranged in a five-step quality scale.

MOS quality scale

With the MOS quality of different language encodings can be compared with each other (codec ), voice calls, echo cancellers or image compression algorithms.

The tests that are performed to determine MOS scores for voice transmissions, a distinction is made between only- Listening, Conversational and Double Talk and Interview and Survey tests. Are thereby different quality reviews carried out, such as an absolute quality assessment, quality deterioration, assessment of the effects of certain effects. The tests ( for voice communications ) are in the ITU- T in Recommendation P.800 specified.

In the ITU recommendations P.830 P.834 to be subjective assessment methods for telephone band codecs (300 Hz - 3400 Hz), wideband codecs (50 Hz - 7000 Hz), developed for echo cancellers and for hands-free operation.

MOS values ​​but also for quality reviews, and the network planning models are performed on the basis of objective models ( which simulate the human subjective perception of quality ). This often leads to confusion and misunderstandings.

Therefore, the following terminology has been introduced to distinguish between different types of MOS values ​​using the ITU -T recommendation P.800.1:

MOS value distinguishing

LQ = listening quality, CQ = conversational quality, TQ = talking quality

S = subjective, O = objective, E = estimated

Y is a placeholder to identify the considered audio bandwidth and can take the following values:

  • N stands for narrow-band (300 Hz - 3400 Hz)
  • W is for broadband (50 Hz - 7000 Hz)
  • M stands for mixed narrowband and broadband tests ( with broadband as a reference quality )

The following MOS examples can the differences between the different codecs see: