ACR with Hidden Reference (ACR-HR), defined in ITU-T P.910 section 8.6.2, is the bridge between the single- and double-stimulus worlds and the most common method in practice. You run an ordinary single-stimulus ACR test, but quietly slip the pristine source clips into the set as unlabelled items; the viewer rates them without knowing they are references and never realises they are doing a comparison. In analysis you subtract: for each viewer and processed video sequence the differential viewer score is DV = V(PVS) minus V(REF) plus 5, where the plus 5 shifts the result onto a 1-to-5 scale on which 5 means as good as the reference. Averaging the DVs gives the clip's DMOS, where higher means better. This recovers a degradation score while keeping ACR's throughput, and because it measures each viewer against their own reading of the source, it cancels personal harshness and content preference. The catch: P.910 warns it may give wider confidence intervals than ACR, and it only works if the hidden reference is genuinely excellent.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
ACR-HR gives you a DMOS from a fast single-stimulus session, but only if the hidden references and the per-viewer subtraction are set up correctly. We wrote seven volumes covering the whole video stack, and for ACR-HR start with Volume 1: quality metrics, rating scales and how compression damage maps to viewer scores, with Volume 3 on QoE and Volume 6 on AI-based quality models. Written by Nikolay Sapunov, CEO at Fora Soft.
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