Degradation Category Rating (DCR), defined in ITU-T P.910 section 8.2 and known in the broadcast standard ITU-R BT.500-15 as the Double Stimulus Impairment Scale (DSIS), is the double-stimulus method for the question how faithful is this clip to the original. 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 on a five-point impairment scale: 5 Imperceptible, 4 Perceptible but not annoying, 3 Slightly annoying, 2 Annoying, 1 Very annoying. With the original in view, the viewer spots small fidelity losses that vanish in a single-stimulus test, so P.910 recommends DCR when evaluating fidelity to the source, frequently important for high-quality systems; it is more sensitive than ACR. The costs are two. Throughput roughly halves because each rating consumes two clips. And because 5 means identical to the source, the impairment scale cannot record a clip that looks better than its reference - for that improvement case you need CCR's bidirectional scale.
