I18N Subtitle

When to use

Use this when producing subtitles or captions from a transcript or translation — turning dialogue into time-coded cues that stay on screen long enough to read and never overflow the frame. Applies to translated subtitles, same-language captions (SDH), and accessibility captions with sound cues.

Not for: untimed prose translation (use i18n-translate), locale formatting of documents (use i18n-localize), glossaries (use i18n-glossary), or tone-only passes (use i18n-tone-adapt).

Method

1. Segment by sense, not by sentence length. Break at natural pauses/clauses so each cue is a readable unit.
2. Enforce reading speed. Target ≤17 chars/sec for adults (≤12 for children). Decision point: if text exceeds the limit for its duration, condense the wording rather than shorten the display time below ~1s.
3. Limit layout: max 2 lines per cue, ~42 characters per line. Balance line lengths; break before conjunctions/prepositions, never mid-word or mid-name.
4. Set timing: minimum 1s, maximum ~7s on screen; leave ≥2 frames (83ms) gap between consecutive cues; sync in-points to speech onset.
5. Decision point: for captions (SDH), add speaker labels and bracketed sound cues [door slams]; for plain translation subtitles, omit them.
6. Validate: no overlaps, no negative durations, monotonically increasing timecodes, and every cue passes the CPS limit.

Example

Line to caption: "I told you yesterday that the shipment would arrive on Monday morning." (~68 chars, spoken over 3.0s → ~23 CPS, too fast).

Split into two cues:

1
00:00:04,000 --> 00:00:06,000
I told you yesterday

2
00:00:06,100 --> 00:00:08,600
the shipment arrives Monday morning.

Each cue now sits under 17 CPS, 2-frame gap between them, condensed "would arrive on" → "arrives".

Pitfalls

• Reading speed too fast. Cramming full sentences into short cues; viewers can't finish reading — condense instead.
• Bad line breaks. Splitting "New / York" or breaking after an article strands words and slows reading.
• Overlapping or gapless timecodes. Cues that collide or touch flicker; keep a minimum gap and no overlaps.
• Under-minimum flashes. Cues under ~1s register as a flash even if the text is short.

Output format

SubRip (.srt): sequential index, HH:MM:SS,mmm --> HH:MM:SS,mmm, then 1–2 text lines, blank line between cues.

<n>
HH:MM:SS,mmm --> HH:MM:SS,mmm
<line 1 (≤42 chars)>
<line 2 optional (≤42 chars)>