The Double-Stimulus Continuous Quality-Scale method (DSCQS)

DSCQS is useful when it is not possible to provide stimulus test conditions that exhibit the full range of the quality. This method may be of special interest for our method to be used in some kind of networks guaranteeing certain level of QoS, for example, when MOS is greater than 3 on the 5-point quality scale. As in DCR method, the sequences are presented in pairs: the reference one and the impaired one. Subjects are asked to assess the quality of both (and not the impaired one with respect to the reference as in DCR). The unimpaired one is included to serve as a reference, but the observers are not told which is the reference sequence. In the series of tests, the position of the reference is changed randomly. The subjects are asked to assess the overall sequence quality of each presentation by inserting a mark on a vertical scale (see Figure 4.8). The vertical scales are printed in pairs to accommodate the double presentation of each test sequence. The scales are continuous to avoid quantizing errors, but they are divided into five equal lengths, which correspond to the normal ITU five-point quality scale. The associated terms categorizing the different levels are the same as those normally used. Here they are included for general guidance. Figure 4.8 shows a section of a typical score sheet.