Overview
The Teletext character set is a collection of graphical and textual symbols designed for the transmission and display of Teletext services—broadcast information systems that overlay digital text and simple graphics on analog television signals. The set defines the repertoire of characters, control codes, and mosaic graphics that Teletext decoders interpret to render pages of news, weather, subtitles, and other data.
Historical development
Teletext originated in the United Kingdom in the early 1970s, with the BBC’s Ceefax service and the independent ITV teletext service. To ensure interoperability across different broadcasters and receiver manufacturers, an international standard for the character repertoire was established by the International Telecommunication Union (ITU) and the European Broadcasting Union (EBU). The primary specifications are:
- ITU‑R Recommendation BT.470‑5 (formerly CCIR 468) – defines the basic Teletext character set (G0 set).
- ETSI (European Telecommunications Standards Institute) EN 300 706 – outlines the data transmission format and includes supplemental character sets (G2).
- Later extensions incorporate the ISO‑8859‑1 (Latin‑1) character set for enhanced language support.
Technical specification
| Component | Description |
|---|---|
| G0 set | The core 96 printable characters (ASCII‑compatible) plus control codes. Includes the standard Latin alphabet, digits, punctuation, and a set of 12 block‑graphics symbols used for rudimentary images. |
| G2 set | An optional supplemental set containing additional graphical symbols such as hatched mosaics, arrows, and special symbols (e.g., currency signs, the Euro sign). Implementations may enable or disable G2 based on regional requirements. |
| Control codes | Non‑printing codes that manage page layout, colour, flashing, double‑height/width characters, and cursor positioning. Prominent examples are ALPHA, MOSAIC, RED, GREEN, FLASH, and DOUBLE‑HEIGHT. |
| Encoding | Characters are transmitted in a 7‑bit form (bits 0‑6), with the most‑significant bit (bit 7) reserved for parity or other protocol‑specific purposes. The Teletext decoder maps the 7‑bit value to the appropriate glyph according to the active character set (G0 or G2). |
| Mosaic graphics | The set includes 2 × 3 and 2 × 2 block‑mosaic cells that combine to produce simple pictures, icons, and shading effects. These are derived from the original BBC TV graphics and are integral to the visual style of classic Teletext. |
Character groups
- Alphanumeric (ALPHA) set – Standard letters, numbers, and punctuation.
- Mosaic (MOSAIC) set – Block graphic characters enabling 2‑pixel‑wide mosaics.
- Control and formatting codes – Commands for colour (RED, GREEN, YELLOW, BLUE, MAGENTA, CYAN, WHITE), style (BOLD, ITALIC in some extensions), and page handling (CLEAR, HOLD, PAGE‑SYNC).
Usage
- Broadcast subtitles – Teletext subtitles (often labelled “Closed Captions” in the UK) employ the Teletext character set to synchronize text with video.
- Information services – News, weather, TV schedules, and sports scores are transmitted in page‑based formats using the defined characters.
- Data services – Early interactive services (e.g., shopping listings, lottery results) leveraged the set’s control codes for navigation and selection.
Legacy and contemporary relevance
While digital television standards (e.g., DVB, ATSC) have largely replaced analog Teletext, the character set persists in several contexts:
- Digital Teletext (D‑Teletext) – A digital adaptation that retains the original character repertoire for backward compatibility.
- Subtitle formats – Certain subtitle files (e.g., .txt with Teletext control codes) still reference the Teletext set for legacy broadcasting.
- Emulation and preservation – Retro‑gaming and archival projects recreate classic Teletext pages using the original character set to preserve the aesthetic of 1970s–1990s broadcast media.
References
- ITU‑R BT.470‑5: “Teletext – Part 1: Specification of a character set” (adopted 1979, updated 1994).
- ETSI EN 300 706: “Data Service – Teletext” (multiple editions, latest 2018).
- EBU Technical Review – “Teletext: History and Technical Overview” (1995).
Note: The information provided reflects established technical standards and historical documentation related to the Teletext character set.