The Double Stimulus Continuous Quality Scale (DSCQS), defined in ITU-R BT.500-15, is the most important of the continuous-scale broadcast methods and the historical gold standard for codec evaluation at high quality. The viewer sees the reference and the test clip - often each shown twice - and rates both on a continuous line marked only with quality adjectives, with the reference hidden by randomizing the order, so the viewer is blind to which is which. The reported score is the difference between the two ratings. Because the viewer marks a continuous line rather than picking a category, DSCQS avoids forcing fine judgements into coarse bins, which makes it the classic choice where the test material and the original are similar - the near-transparent case where even a five-point scale is too coarse. It earns its place through structure - the hidden reference and the differencing - not extra scale resolution, since P.910 notes adding scale points does not improve MOS accuracy. The price is a heavier task and slower running.
