The impairment factor methodology has been adopted by telecommunication experts (ITU-T, ETSI) for describing the relative impact of telephone transmission degradations on the overall quality of transmitted speech. Input parameters to this methodology are mainly instrumentally measurable characteristics of the transmission path, with the exception of low bit-rate codecs, whose perceptual characteristics still have to be determined in auditory tests. In this paper, we describe a new approach for deriving impairment factors for low bit-rate codecs in a purely instrumental way. Using instrumental quality prediction models like PESQ or TOSQA, quality estimations are obtained for the codec under test, as well as for several reference codecs. The reference conditions are subsequently used for anchoring the speech codec of interest, so that the impairments it introduces can be combined with other degradations in order to obtain an overall quality estimation for the whole transmission channel. A comparison with defined values for well-known codecs shows a high correlation of instrumentally derived impairment factors with the corresponding defined values, as well as with auditory test data.
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