Subtitles speak in letters

CPS, STL, SDH, IMSC: a small craft with a large alphabet. Here they are in full words.

Closed captions

On-screen text that includes speech plus non-speech sound and speaker identification, so deaf and hard-of-hearing viewers can follow the programme. SDH is the subtitle form of that same job.

Undertitle is built for professional subtitles, not as a captioning tool. Closed layout still uses tighter reading limits when a delivery needs that profile.

Transcription

A written record of the spoken words. It is not a subtitle file: it has no reading speed, line breaks or broadcast timing.

Undertitle transcribes as a first step, then formats professional subtitles for editorial review.

Open and closed subtitles

Open subtitles are always visible, often with a wider line. Closed subtitles can be switched on by the viewer and usually allow fewer characters per line and a slower reading speed.

Undertitle has both layout modes: open at 42 characters per line and 15 CPS, closed at 37 characters per line and 13 CPS.

Timing

Placing each cue on the clock so it follows speech, stays readable, and respects the gap before the next subtitle.

Undertitle times the file, then the quality report flags cues that sit outside the agreed duration and reading speed.

Reading speed

How fast a viewer must read a cue, in characters per second (CPS) or words per minute (WPM). If the speed is too high, the subtitle disappears before the sentence is finished.

Undertitle measures characters per second in the quality report and flags cues that sit above the agreed comfort speed.

WER

Word error rate counts deletions, insertions, and substitutions in recognized speech against a human reference transcript. Reading speed, line length, line breaks, and timing to the picture sit outside that score.

Use it to compare speech engines. Judge a subtitle file on reading speed, line length, and timing.

Segmentation

How spoken language is split into subtitle cues. Each cue should be a readable unit, not a cut in the middle of a thought.

Undertitle formats segmentation against professional house rules before the editor reviews the file.

Line breaks

The split between the two lines of a subtitle. A linguistic break follows grammar and meaning; a visual wrap can split a phrase in a way that is harder to read.

Undertitle checks line breaks in the quality report together with line length.

Terminology

The agreed names, spellings and house words that must stay consistent in every subtitle file.

Undertitle uses screenplays and dialogue lists as context for those names.

Quality report

A delivery document that checks the subtitle file against agreed standards such as reading speed, timing, line length and segmentation. Exceptions that need editorial judgement are marked.

Undertitle sends the report with the subtitle files so editors can start where the file needs attention.

Cue duration

How long a subtitle stays on screen, plus the short gap before the next cue. A cue that is too short cannot be read; a cue that is too long lingers after the moment has passed.

Undertitle uses a one-second minimum, a six-second maximum, and a four-frame gap at 25 fps.

Shot change

A cut in the picture. Subtitle in- and out-times often follow major shot changes so the text does not float across a new image.

Undertitle still follows the speech for minor shot changes, so a readable sentence is not cut only to match a small picture edit.

SRT

SubRip is a plain-text subtitle file. Each cue has a number, a start time, an end time and one or two lines of text.

Undertitle delivers validated SRT today.

EBU-STL

A binary subtitle format defined by the European Broadcasting Union and used in many broadcast playout chains.

Undertitle delivers EBU-STL today.

EBU-TT

EBU Timed Text is an XML subtitle format from the European Broadcasting Union for styled, timed text in broadcast delivery.

EBU-STL is the binary subtitle format used in many playout chains.

PAC

A professional subtitle format used in European localization workflows, often as an exchange file between tools.

Undertitle delivers PAC today.

TTML / IMSC

Timed Text Markup Language (TTML) is an XML timed-text format used by streamers and broadcasters. IMSC, the Internet Media Subtitles and Captions profile, is the constrained TTML profile used for professional delivery.

Undertitle delivers TTML today. IMSC is not part of the standard export.

SCC

Scenarist Closed Caption is a file format for CEA-608 captions, still used in some North American broadcast and disc workflows.

Undertitle can deliver SCC on request.

WebVTT

WebVTT is a W3C timed-text format for subtitles and captions on the web, commonly used with HTML video. The short form is VTT.

Undertitle delivers WebVTT today.

Cue

One timed subtitle block: an in-time, an out-time and one or two lines of text that the viewer reads as a unit.

Undertitle measures and formats cues, then reports outliers in the quality report.

Timecode

The clock values that mark when a cue appears and disappears. In subtitles these are usually hours, minutes, seconds and milliseconds or frames.

Undertitle keeps cue times aligned with speech, then adjusts them for reading comfort and the minimum gap.

Characters per line

The maximum number of characters, including spaces and punctuation, allowed on one subtitle line. The short form is CPL.

Undertitle uses 42 characters for open subtitles and 37 characters for closed subtitles.

Confidence score

A single rating of the delivery so editors can see the overall quality at a glance before they open the detailed report.

Undertitle sends this score with the quality report. Editors still make the final call.

Editorial exceptions

House rules that override a general subtitle standard, such as a required spelling, a kept English name, or a tone decision.

Undertitle can apply those exceptions alongside the professional standards.

Speaker labels

A mark that shows who is speaking, used more often in captions than in standard subtitles.

Undertitle preserves speaker turns from transcription where the delivery needs them.

EAA

The European Accessibility Act, Directive (EU) 2019/882, asks that subtitles arrive in full, display accurately, stay in sync with sound and picture, and stay under the viewer's control.

The Act sets no reading-speed limit, no accuracy score, and no required subtitle format. National rules decide how many programmes need subtitles.

AVMSD

The Audiovisual Media Services Directive, Directive 2010/13/EU, asks each country to make television and on-demand services more accessible over time, through proportionate measures.

Each country sets its own share of subtitled programmes. The European Accessibility Act then sets how those subtitles reach the viewer.