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The Mean Opinion Score (MOS) is the arithmetic mean of individual evaluations and can range from 1 (worst value) to 5 (best value).

In telecommunications, the MOS value is mainly used to assess the quality of voice and image transmission services. It is the result of a defined sequence of several tests in which the subjectively perceived quality of the language or images is assessed by a group of test subjects. The result of the test series is classified in a five-level quality scale. There are already technical measuring methods that are far more objective.


MOS quality scale


Value

Quality

Meaning

5

Excellent

No effort is required to understand the language.

4

Good

Through attentive listening, speech can be heard without effort.

3

Fair

The language can be heard with a slight effort.

2

Poor

It takes a lot of concentration and effort to understand the transmitted language.

1

Bad

Despite great efforts, one cannot communicate.

With the MOS, the quality of different speech codecs, voice connections, echo cancellers or image compression algorithms can be compared with each other.

Measuring method:

 

There are three ways to determine a MOS value.

  1. I only measure the distance from the digitally converted language, via the codec to the PCM file (codec).
  2. I measure from the microphone in the handset via the AD converter, the codec to the PCM file (Send).
  3. I measure from the PCM file via the codec, the AD converter to the loudspeaker in the handset (Receive).

The most common method uses the basically inaccurate "codec" method, because this leaves out the actual hardware components, which lead to further quality losses.


The following MOS examples show the differences between the different codecs.

Codec (Datenrate)Mean Opinion Score (MOS) CodecMOS - SendMOS - Receive
G.711 (64 kbit/s)4,44,23,9
G.722 (64 kbit/s)4,54,44,2
G.726 (32 kbit/s)3,85

G.729 (8 kbit/s)3,92


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