Teletext System B

A complete Teletext System B (WST) platform: renderer, authoring system, editor, and viewer. Built from scratch in TypeScript based on ETSI EN 300 706.

What is this?

This is a fully functional Teletext system that can:

• Render pixel-perfect Teletext pages through a state machine that implements every Level 1 control code
• Import real broadcast captures from .t42 (DVB) files
• Author new pages with a browser-based visual editor
• Compile pages into exact Teletext packet structures
• Display pages through a CRT shader that simulates scanlines, phosphor persistence, bloom, and aperture grille
• View a complete Teletext service with keypad navigation, fastext links, and subpage carousels

Quick Start

npm install

# Run the homepage (viewer + info)
npm start

# Run the editor
npm run editor

# Run tests
npm test

The Viewer

A standalone Teletext viewer with a CRT shader overlay. Type page numbers on the keypad or keyboard (hex, 100-8FF). Navigate with fastext color buttons. Arrow keys and PgUp/PgDn flip through subpages. Press R to reveal concealed text. Load any .t42 file with the Load button.

The viewer ships with a 36-page Teletext service built from ai-created.com content — products, stories, lab notes, media, and about pages, all formatted as authentic Teletext.

Page map: | Pages | Content | |---|---| | P100 | Main index | | P200 | Products index | | P201-P214 | Individual products | | P300-P304 | 0→1 Stories | | P400-P411 | Lab Notes | | P500 | Media | | P600 | About |

Terminal Viewer

An interactive terminal-based viewer that renders Teletext pages using ANSI escape sequences and Unicode sextant characters.

./teletext.sh <file.t42|file.tti> [page-hex]

Controls:

| Key | Action | |---|---| | 0-9, A-F | Type page number (hex) | | Enter | Navigate to typed page | | Backspace | Delete last digit | | Escape | Clear input | | Left/Right | Previous/next subpage | | R | Toggle reveal | | F1-F4 | Fastext: red, green, yellow, cyan | | Q / Ctrl-C | Quit |

Terminal requirements:

• Unicode 13.0+ font — mosaic graphics use the "Symbols for Legacy Computing" block (U+1FB00–U+1FB3B). Most default terminal fonts don't include these. Iosevka is a good monospace font with full coverage. Verify your font has the glyphs with: fc-list :charset=1FB00 family
• UTF-8 locale — your terminal must be in UTF-8 mode
• Truecolor support — the viewer uses 24-bit color SGR sequences
• Recommended terminal: WezTerm — handles Unicode sextants and truecolor well. Kitty, iTerm2, and Ghostty also work. Classic xterm may struggle with supplementary Unicode planes even with the right font installed.

Known limitation: double-height text renders at normal height. Teletext double-height scales characters vertically without changing width, but the only terminal mechanism for this (VT100 DECDHL) always doubles both height and width. No terminal escape sequence exists for height-only scaling.

The Editor

A browser-based WYSIWYG Teletext page editor. Press ? for the full help overlay.

Tools:

• Text — click to place cursor, type characters. Arrow keys navigate, Enter for new line, Tab advances 4 columns
• Paint Blocks — click/drag to paint individual 2×3 sextant sub-blocks. Right-click erases. Shift+drag constrains to straight horizontal lines. Space places a full block at cursor
• Recolor — flood fill that recolors all connected cells sharing the same fg, bg, and type (text/mosaic)
• Eraser — click/drag to clear cells to default state
• Pick Color — click any cell to grab its foreground and background colors
• Select/Move — drag to select a region, click inside to move it, Delete to clear

Features:

• Undo/redo (Ctrl+Z / Ctrl+Shift+Z)
• Double-height rows
• Foreground and background color selection (8 teletext colors, keys 1-8 / Shift+1-8 in non-text tools)
• Multi-page editing with insert, duplicate, and delete
• Subpage management (add, duplicate, delete, navigate)
• Fastext link editing with separate label and page target fields (renders colored bar on row 24)
• CRT shader overlay toggle
• Full-color bitmap import with configurable position, size, and background color
• T42 import (loads all pages from broadcast captures) and export (saves all pages)
• Smart row compiler that automatically manages teletext control codes, sacrificing cell positions for color/mode transitions as required by the format

The Renderer

The core rendering pipeline:

Raw bytes → State Machine → Cell Grid → Glyph Cache → Render Buffer → RGBA Pixels

• State machine — processes all Level 1 spacing attributes: 8 alpha colors, 8 mosaic colors, flash/steady, double-height, conceal, hold/release graphics, contiguous/separated mosaics, background color, ESC switch
• Glyph system — bitmap font for G0 (text) with national variants, G1 mosaic renderer with contiguous/separated modes
• Render buffer — resolves cells to glyph IDs, renders to 480×240 RGBA pixel buffer with flash phase and reveal support
• CRT shader — 5-pass WebGL pipeline: bloom extraction → Gaussian blur (H+V) → CRT composite → blit. Simulates scanlines, aperture grille, phosphor persistence, halation, RGB convergence, interlace flicker, analog noise

T42 Import/Export

Imports and exports real Teletext broadcast data in .t42 format (DVB teletext data units).

Import handles:

• Raw 42-byte and 46-byte block formats
• PES-framed DVB data units
• Auto-detection of bit-reversed captures
• Hamming 8/4 decoding with the standard ETSI lookup table
• Persistent page buffers (rows accumulate over multiple transmissions, like a real decoder)
• Null line filtering
• Page number extraction from Hamming-encoded header bytes

Export produces:

• Standard 42-byte wire-format packets
• Hamming 8/4 encoded address and header bytes
• Odd parity on all data bytes
• Correct page flags, subcode, and language subset encoding
• Fastext link packets (packet 27)

TTI Import/Export

Full .tti interoperability:

• Lexer → parser → AST mapper pipeline
• Control character translation (0x00-0x1F ↔ 0x80-0x9F)
• Round-trip fidelity: AST → TTI → AST preserves control codes, page flags, fastext links
• Unknown record preservation for vendor extensions
• Deterministic export ordering

Packet Compiler

Compiles the canonical page model into exact Teletext packet structures:

• Row normalization to 40 bytes
• Hamming 8/4 encoding for address and header bytes
• Odd parity for data bytes
• Page flag and language subset encoding
• Fastext link packets
• Structured diagnostics (warnings/errors with row/col positions)

Smart Row Compiler

The editor uses a smart row compiler that bridges the gap between the visual editing model (each cell has its own color) and the teletext byte format (colors are set by control codes that consume display positions):

• Automatically inserts color/mode control codes before transitions
• Prefers placing control codes in existing empty space cells
• When no space is available, sacrifices cells at the transition point — this is standard teletext behavior where every color/mode change requires one display position
• Handles background color changes, foreground color changes, text/mosaic mode switches, and contiguous/separated mosaic transitions

Bitmap Import

Converts images into editable Teletext mosaic graphics:

• Monochrome — single foreground color, threshold-based
• Two-color — per-row dominant color analysis
• Full color — per-cell color analysis with automatic color run optimization, minimizes control code overhead while maximizing visual fidelity

Pipeline: image → resize/fit → quantize to 8-color palette → encode 2×3 sextant cells → plan color runs → emit mosaic tokens with control codes.

Architecture

src/
├── state-machine/    # Control code state machine
├── glyph-system/     # G0/G1 fonts, glyph cache, national variants
├── render-ansi/      # Cell grid → ANSI terminal output (Unicode sextants)
├── render-buffer/    # Cell grid → RGBA pixel rendering
├── packet-decoder/   # Hamming 8/4 decode, parity
├── page-assembler/   # Packet stream → page reconstruction
├── timing-engine/    # Flash phase timing
├── crt/              # WebGL CRT shader overlay
├── model/            # Canonical page types, validators, factories
├── tti/              # TTI lexer/parser/mapper/exporter, T42 import/export
├── compile/          # AST → packet compiler, Hamming 8/4 encode
├── bundle/           # Service serialization
├── editor/           # Visual editor types, smart row compiler
└── import/           # Bitmap → mosaic pipeline

scripts/
└── ansi-render.ts    # Interactive terminal viewer

demo/
├── index.html        # Viewer + info panel
├── viewer.html       # Standalone teletext viewer
├── viewer-main.ts    # Viewer logic (T42 load, keypad, subpage carousel)
├── editor.html       # Page editor
├── editor-main.ts    # Editor logic (tools, painting, import/export)
└── ai-created-pages.ts  # 36-page teletext service

tests/                # 400 tests across 27 files

Key Design Decisions

• Internal page model is the source of truth — not TTI, not packets. All formats are derived from or mapped to the canonical AST.
• Exactness over convenience — control codes preserve byte-level meaning. The renderer shows what's in the data, errors and all.
• The compiler doesn't "fix" illegal authoring — it may warn, it may fail, but it won't silently rewrite semantics.
• Editor metadata never affects compiled output — guides, locked regions, and trace images are stripped before compilation.