What it checks
Two subtitles in the same place on screen must not be displayed at once: the second one would hide or replace the first one before its end. In formats with display regions (TTML and its profiles, WebVTT), subtitles placed in different areas — one at the top, one at the bottom — may overlap in time; in EBU-STL and SRT, any overlap is reported, since a new subtitle replaces the previous one.
How it is measured
CueNorm compares the start of each subtitle with the latest end time seen so far in the same area (top, middle or bottom; bottom when the subtitle is not placed), so a long subtitle that covers several following ones is caught too.
When it applies
Always, for every delivery profile: it is built in and cannot be removed or relaxed by a profile. It is an error. Mezzanine profiles do not apply it, since overlapping lines are legal in several source formats (ASS, for example).
Example
A subtitle from 00:00:01,000 to 00:00:04,000 followed at the bottom of the screen by one starting at 00:00:03,500 is reported as Overlaps previous subtitle.
How to fix
- Shorten the earlier subtitle or delay the later one in the authoring tool.
- When the destination requires a gap,
ENSURE_MINIMUM_GAPshortens the earlier subtitle's end time if the profile allows that transformation. - If both lines must be read together, merge them into one subtitle or place them in different areas (top and bottom).