Differential Mean Opinion Score (DMOS) measures how much a clip degraded relative to its pristine reference, not how good it looks on its own. It exists to cancel a confound in plain MOS, where a viewer's rating mixes the impairment with how much they liked the content and with their personal harshness; DMOS always measures each viewer against a reference they themselves rated. There are two conventions, and confusing them is a real error source. In ITU-T P.910's ACR-HR method the per-viewer differential is DV = V(PVS) minus V(REF) plus 5, averaged into a DMOS where higher means better. In academic databases like LIVE, a per-subject difference is Z-scored and rescaled, often to 0-100, where higher usually means worse, the opposite direction. Same three letters, inverted meaning, so always state the convention and direction. VMAF was trained on database-style DMOS, which is why a VMAF point predicting a DMOS point depends on knowing which one.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
DMOS measures how much a clip lost against its reference, which is what codec and ladder comparisons usually need rather than absolute MOS. We wrote seven volumes covering the whole video stack, and for DMOS start with Volume 1: subjective scores, rating scales, full-reference metrics and how they map to encoder decisions. Written by Nikolay Sapunov, CEO at Fora Soft.
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