BBC Subtitling Guidelines

Introduction

The British Broadcasting Corporation (BBC) has specific guidelines in place to ensure the quality and accuracy of subtitles and closed captions. They are designed to enhance the accessibility of BBC programming for viewers with hearing disabilities.

This is one of the most detailed sources of caption guidelines. However, they are only mandatory for people who create captions for the BBC. Note that these instructions are also intended for television subtitles (including teletext). Therefore they are not always optimal for online video.

General guidelines for offline captions

Below are some of the technical instructions that, according to the BBC, should be applied when creating closed captions. Since most instructions require the captions to be edited, they are only useful for Video on-Demand.

For live captions, the BBC prefers a word-by-word display with two lines of scrolling text (justified left, not centred) to allow maximum reading time. The formatting instructions for live captions are very limited.

Caption Lines and Line Breaks

• Maximum line length should be 68% of the width of a 16:9 video and 90% of the width of a 4:3 video.
• If possible, a caption should comprise a single complete sentence. There are exceptions to this general recommendation for live subtitling, short and long sentences, speed of speech…
• A maximum subtitle length of two lines is recommended. Three lines may be used if you are confident that no important picture information will be obscured.
• Subtitles and lines should be broken at logical points. The ideal line-break will be at a piece of punctuation like a full stop, comma or dash.
• Good line-breaks make the process of reading and understanding far easier. However, if a good line break can not be combined with well-edited text and/or good timing, the latter should take precedence.

Timing and Gaps

• The recommended subtitle speed is 160-180 words-per-minute (WPM) or 0.33 to 0.375 second per word. Based on this, a caption should be left on screen for a minimum period of around 0.3 seconds per word (e.g. 1.2 seconds for a 4-word subtitle). However, timings are ultimately an editorial decision that depends on other considerations, such as the speed of speech, text editing and shot synchronisation.
• Reasons to vary the timing include shot changes, words that can be lip-read, catchwords, retaining humour, critical information, technical items, several speakers, slow speech …
• Unless there is a specific reasons, the timing of captions should be as consistent as possible.
• If there is a pause between two pieces of speech, you may leave a gap between the subtitles. The minimum gap time should be a second and a half. Anything shorter than this produces a very jerky effect. Try to not squeeze gaps in if the time can be used for text.

Typography

• Generally it is better to use a system font for readability (e.g. Helvetica for iOS and Roboto for Android). Use of non-platform fonts can adversely impact clarity of presented text.
• The final displayed size of closed captions text is determined by multiple factors: the instructions in the subtitle file, the processor and the set of installed fonts available to it, the device screen size and resolution and (on some devices) also user-defined preferences.

Positioning

• The captions should overlay the video image, and may be placed within any black bars present within the video at the top or bottom. For 16:9 video in landscape mode, captions should not be placed outside the central 90% vertically and the central 75% horizontally. Regions can be extended horizontally to allow extra space for line padding.
• Offline captions are normally centre-aligned within a caption region that is horizontally centred relative to the video. Since the BBC assumes scrolling captions that grow in size for live streams, they prefer real-time captions to be left-aligned.

Speech context and sounds for SDH captions

The main purpose of the BBC guidelines is to enhance accessibility for the hearing impaired. They contain a lot of rules that specifically address accessibility and are therefore only applicable to Subtitles for the Deaf and Hard of Hearing (SDH captions). A few examples are listed below. The rest can be found in the BBC guidelines.