Speech Quality Measurement

MOS - Mean Opinion Score

The mean opinion score (MOS, specified by ITU-T Rec. P.800) measures voice quality used in telephony networks to obtain the human user's view of the quality of the network. MOS is a subjective rating of user satisfaction and based on a large sample of listeners scoring a set of voice samples on a scale of 5. where 1 is Bad and 5 is Excellent.

A MOS value of 4 or higher represents acceptable satisfaction and denotes toll quality, a level typically associated with circuit-switched networks. The following table shows relation between MOS value (range) and the subjective perception.

Relation between MOS value (subjective perception range) and R-value (objective measurement for quality perception) is described below.

E-model and R-state

The result of the E-model calculation is a transmission rating factor R-value, which is a measure to express the end-to-end speech transmission quality. For successful speech transmission quality in MX-ONE, any connections with R-values below 70 should be avoided as explained in the following sections.

R-value components

The rating factor R is composed of R = Ro - Is - Id - Ie + A where:

• Ro represents the basic signal-to-noise ratio, including noise sources such as circuit noise and room noise.
• Is represents a combination of all impairments which occur more or less simultaneously with the voice signal (e.g. non-optimum side tone, quantizing distortion, overall loudness and other impairments).
• Id represents the impairments caused by talker/listener echo and delay (even with perfect echo canceling),
• Ie represents the effective equipment impairment (e.g. low bit rate codecs). It also includes impairment due to packet-losses of random distribution.
• A, the advantage factor, allows for compensation of impairment factors when there are other advantages of access to the user. For instance, conventional wired equipment has A-factor value 0 while mobile equipment have A-factor varying from 5 - 20 depending on geographical coverage. G.107 also provides the default values for all input parameters used in the algorithm of the E-model.

Relations between R-value and MOS value

The R factor is related to MOS as follows:

Equation for conversion - R-value to MOS value

For R < 0: MOS = 1

For 0 R 100: MOS = 1 + 0.035 x R + 7 x 10-6 x R x (R-60) x (100-R)

For R > 100: MOS = 4.5

The following table shows R-value and MOS value range with regard to user satisfaction.