The Double Stimulus Impairment Scale (DSIS) is the name ITU-R BT.500-15 gives to the broadcast-television version of Degradation Category Rating (DCR), the two methods being essentially the same. It is a double-stimulus method: the viewer is shown the pristine source first, knows it is the source, then sees the processed clip and rates how impaired the second is relative to the first. The scale is five impairment categories - 5 Imperceptible, 4 Perceptible but not annoying, 3 Slightly annoying, 2 Annoying, 1 Very annoying. Because the original is in view, the viewer detects small fidelity losses that a single-stimulus test like ACR would miss, which makes DSIS the choice for evaluating fidelity to the source and more sensitive than ACR - in a controlled 1080p H.264 comparison it produced smaller confidence intervals, especially at lower quality. The trade-offs mirror DCR: roughly half the throughput, and a scale that tops out at imperceptible, so it cannot record a clip that looks better than its reference.
