|Publication Date:||1 June 2019|
This Recommendation describes the wideband (WB) version of a computational model, known as the E-model, that has proven useful as a transmission planning tool for assessing the combined effects of variations in several transmission parameters that affect conversational1 quality. This computational model can be used, for example, by transmission planners to help ensure that users will be satisfied with end-to-end transmission performance whilst avoiding over-engineering of networks. It must be emphasized that the primary output from the model is the "rating factor" R, but this can be transformed to give estimates of customer opinion. Such estimates are only made for transmission planning purposes and not for actual customer opinion prediction (for which there is no agreed-upon model recommended by the ITU-T).
This version is an adapted version of the NB (300-3400 Hz) E-model, typically referred to as "the E-model", which is described in [ITU-T G.107]. The WB version addresses scenarios which include WB (50-7000 Hz) transmission. It does not replace the narrowband (NB) E-model. Instead, it describes a separate WB-version of the model that uses, within limits, similar concepts and input parameters as the NB E-model. The current version captures the effects of talker echo, absolute delay, WB speech coding, and voice-over-IP packet-loss. Degradations which are covered but have not yet been studied in detail are listener echo. Degradations which are not yet covered are loudness loss, non-optimum sidetone levels, background noise at the sending and receiving side, circuit noise and quantizing distortions.
For many parameter combinations of high importance to transmission planners, ITU-T G.107.1 can be used with confidence (e.g., loudness loss, coding distortions), but for some parameter combinations of high importance (e.g., the effects of delay in conjunction with other impairments), WB E-model predictions have been questioned and are currently under study.
Regarding the interpretation of the WB E-model ratings, note that the current versions of [b-ITU-T G.108], [b-ITU-T G.108.1] and [b-ITU-T G.109] do not refer to the WB version described here, but only to the NB version of the E-model described in [ITU-T G.107].
1 Conversational quality in this context refers to transmission characteristics, e.g., long transmission times, effects of talker echoes, etc. However, the E-model, as described in this Recommendation, is not intended to model transmission impairments during double talk situations.