(Continuation of Question 20/12 studied in 1997-2000)

Type of question

Task-oriented leading to a revision of Recommendation G.107 and to new ITU-T Recommendations.

Background

In the last years, Study Group 12 has established a new concept of impairment factors which aim at predicting the perceptive effects of different type of degradations on overall speech communication quality, for network planning purposes. The core algorithm related to this concept is the so-called E-model, a computational model for use in transmission planning. This model has been analyzed in detail, and standardized as Recommendation G.107. It can be applied in network planning of traditional, narrow-band and handset terminated networks. Additionally, a methodology has been set up to derive impairment factors for codecs from subjective listening-only tests.

With respect to the application of the E-model to the planning of modern networks, however, there are still several open questions, which limit the model’s usability. They result from the terminal and transmission characteristics of modern networks, which could not have been taken into account at the time the model was established. In order to guarantee that the achieved concept keeps track with the technological progress in both the transmission system and the terminal equipment areas, it is highly desirable to maintain and update the model. The validity range of the E-model should be extended so that it cannot only be applied to traditional networks, but e.g. to packet-based transmission, wide-band systems, or non-handset terminal equipment, too.

The extension of the E-model is also a prerequisite in order to keep other Recommendations up to date, which deal with the impairment factor concept and with categories of speech transmission quality. These Recommendations include but are not limited to:

• G.108 and G.108.01 on the application of the E-model, and on conversational aspects not covered by the current version,
• G.109 on classes of speech transmission quality,
• G.113 on transmission impairments,
• G.175 on the interconnection of the PSTN with private and/or IP based networks,
• P.562 on the analysis and interpretation of INMD voice-service measurements.

Text of the question

The main question is how the E-model can be extended in order to cover the effects of an impaired digital transmission and of terminal equipment other than handsets.

Does practical experience with the new methodology to derive equipment impairment factors from subjective tests lead to a consistent framework of Ie values? What scaling method should be applied for this purpose?

How can instrumental models currently developed and standardized by Q.J/12 be fruitfully used to derive equipment impairment factors for

• new codecs in single operation,
• codecs in tandem operation, and
• codecs under conditions of random bit errors, frame erasures or packet loss?

Is it useful to derive a table of total Ie values for codec tandems, which do not satisfy the additivity property, assumed by the impairment factor principle? Can new instrumental measures be used to establish such a table?

Can new formulae be derived for codecs under transmission error conditions, in order to keep the model simple and manageable?

Which quality issues have to be taken into account when extending the E-model to terminal equipment other than handset telephones (e.g. HFTs, headsets)? Problems to be resolved may include:

• the determination of new input parameters to the model, e.g. for SLR and OLR, STMR, as well as room noise (send and receive side),
• the coverage of acoustic echo and sidetone,
• the pickup of ambient room noise by this terminal equipment, and
• the perceptual effects of signal-processing equipment which forms part of such terminals, as well as their coverage in instrumentally measurable input parameters to the model (clipping, sound degradation, impact on the conversation, etc.).

How can or should quality dimensions other than "impairment" be covered, e.g.

• ‘speech sound quality’, e.g. due to terminal equipment other than handsets, due to transmission bandwidth other than the normal 3.1 kHz band, e.g. wide-band transmission, or due to frequency distortion or non-linear codecs,
• conversational quality features (cf. draft Rec. G.108.01).

What is the influence of user expectation on the overall quality, e.g. for terminal equipment other than handsets, or for computer operated VoIP services?

What are the psychological dimensions commonly handled by the term ‘expectation’?

How will user expectation develop with time (cf. COM 12-98, COM 12-111, study period 1997-2000)?

Study items

• Analysis and verification of Ie values obtained with the methodology described in draft new Recommendation P.833
• Definition of a new method for deriving equipment impairment factors from instrumental prediction models (e.g. P.86x)
• Analysis of the additivity property of Ie values for codecs in tandem or codecs under transmission error conditions
• Establishment of new formulae describing impairments for codecs under transmission error conditions
• Definition of instrumentally measurable input parameter to the E-model describing non-handset terminal equipment
• Investigation of perceptual effects related to terminal equipment other than handsets, e.g. due to signal processing in such terminals
• Investigation of the so-called ‘speech sound quality’ and its relation to impairment as predicted by the E-model
• Investigation of the effects of wide-band transmission on ‘speech sound quality’ and speech communication quality
• Investigation of conversational quality features currently not covered by the model
• Analysis of dimensions related to user expectation.

Objectives and schedule

• Revision of G.107,
• New Recommendation on the derivation of equipment impairment factors from instrumental measures,
• Potential revision of other related ITU-T Recommendations as indicated above,
• Potentially further new Recommendations as outcome from the study items.

Relationship to other activities:

• With other ITU-T Study Groups To be studied
• With other bodies: ETSI TC STQ, ETSI EP TIPHON, TIA TR-41, T1A1.