A hidden reference is the pristine, unimpaired source clip slipped into a test as if it were an ordinary test item, so the viewer rates it without knowing it is the reference. The trick is the heart of the ACR-HR method in ITU-T P.910: you run an ordinary single-stimulus ACR test in which the references are quietly mixed among the processed clips, and the viewer never realises they are doing a comparison. Two things follow. First, it lets you compute a DMOS while keeping ACR's speed - you subtract each viewer's hidden-reference rating from their rating of the matching processed clip (DV = V(PVS) minus V(REF) plus 5). Because each viewer is measured against their own reading of the source, this cancels personal harshness and content preference; a lenient and a harsh viewer recover the same degradation. Second, a viewer's score on the hidden reference reveals their bias. The method only works if the hidden reference is genuinely excellent, since a flawed one distorts every difference computed from it.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
A hidden reference checks each viewer's bias, because people rate the pristine source without knowing it is the original. We wrote seven volumes covering the whole video stack, and for hidden references start with Volume 1: subjective scores, DMOS and rating scales, and the encoder artifacts viewers are asked to judge. Written by Nikolay Sapunov, CEO at Fora Soft.
Running ACR tests and unsure how to place a hidden reference correctly?
Tell us your clip set and test goal: we design the hidden reference placement and the DMOS analysis so viewer bias does not skew results. 250+ video products shipped since 2005.
