FaxClear

How does a fax machine work?
A fax turns your page into 2 million dots, squeezes them 10 to 20 times with a clever code book, and sings them down a phone line. Scan your own drawing, read the real run codes, hear the handshake, and watch line noise smear a page until error correction fixes it.

▶ Play with it  ·  Read the 60-second explainer  ·  Watch the 40-second video

In 60 seconds

1. Three machines in one box. A fax is a scanner, a modem and a printer sharing one phone line. The page you send slides over a contact image sensor bar; the page you receive comes off a roll of thermal paper under a row of tiny heaters.
2. A page becomes dots. The scanner reads one thin line at a time: 1,728 dots across 215 mm, about 204 per inch. Each dot is judged black or white against a threshold. Standard mode takes 3.85 lines per mm (204 × 98 dpi), fine mode 7.7 (204 × 196): about 2 million dots for an A4 page.
3. Runs, not dots. Sending every dot at 9,600 bit/s would take over three minutes. So each line is sent as runs of white and black, and each run length becomes a Modified Huffman code: common runs get codes of 2 to 4 bits, and an all-white line costs 17 bits plus a 12-bit end-of-line. A letter shrinks 10 to 20 times; a halftone photo, all tiny dots, can come out bigger than the raw bits.
4. The fax song. A phone line carries only sound, about 300–3,400 Hz. The caller beeps at 1,100 Hz, the answerer replies at 2,100 Hz, and they agree on settings at 300 bit/s with two warbling tones. After a training burst, the page pours through as a hiss: 2,400 changes a second, 4 bits each for V.29 at 9,600 bit/s, 6 each for V.17 at 14,400.
5. Printing, and fixing errors. The receiver decodes the runs and fires 1,728 heaters, one per dot, against paper that turns black at about 100 °C. A flipped bit throws the codes out of step and smears a whole line. Error correction mode sends data in checked 256-byte frames and resends only the bad ones.
6. Fax today. Over the internet, one lost packet can break the modem sound, so T.38 (1998) sends the fax data in packets with spare copies. Fax survives where a phone number and a delivery report count: clinics, courts, government offices, and in Japan. In 1990s India, STD/PCO booths carried many a business fax.

Words worth knowing

Term	Meaning
Contact image sensor	A page-wide bar of LEDs, lenses and light sensors that reads one line of the page at a time.
Scan line	One thin strip across the page: 1,728 dots in a Group 3 fax.
Resolution	Dots per inch across and down the page: 204 × 98 standard, 204 × 196 fine.
Run-length encoding	Describing a line by how many white and black dots come in a row.
Modified Huffman	The fax code book that gives short codes to common run lengths.
Modem	A modulator-demodulator that turns bits into sound and back.
Handshake	The opening exchange where two fax machines agree on speed and resolution.
Thermal paper	Paper coated with a dye and developer that turn black when heated.
Error correction mode	Sending fax data in checked frames and resending only the damaged ones.
T.38	The standard for sending fax data, rather than fax sound, over the internet.

A short history

180 years of sending pages down a wire, from swinging pendulums to packets on the internet.

• 1843 · A patent for sending pictures by wire (Alexander Bain, London, United Kingdom)
• 1865 · The first public fax service (Giovanni Caselli and the French telegraph service, Paris and Lyon, France)
• 1906 · A photograph by wire (Arthur Korn, Germany)
• 1924 · Wirephoto: 15 photos over the phone network (AT&T, Cleveland to New York, USA)
• 1964 · Xerox LDX (Xerox, USA)
• 1980 · Group 3 and T.4: digital fax (CCITT (now ITU-T), Geneva, Switzerland)
• 1985 · The fax boom (Mostly Japanese makers, Worldwide)
• 1990 · India's STD/PCO booths (Indian telecom policy, Sam Pitroda and C-DOT, India)

The full story, with 25 moments, charts, people and 24 sources: glassbox.how/e/faxclear/history. The data lives in history.json.

Video and slides

Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.

File	What	Size
glassbox/reel.mp4	Reel / Short, with captions and soundtrack	1080×1920
glassbox/video.mp4	YouTube video, with captions and soundtrack	1920×1080
glassbox/slide-1…10.jpg	Instagram carousel	1080×1350
glassbox/thumb.jpg	YouTube thumbnail	1280×720
glassbox/cover.jpg	Share card and repo social preview	1200×630
glassbox/history-reel.mp4	“History in 10 moments” Reel / Short	1080×1920
glassbox/history-slide-*.jpg	History carousel	1080×1350
glassbox/post.json	Post copy and schedule used by the publish kit

Privacy

This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.

It remembers a few things in your own browser only, and never sends them anywhere:

Browser storage key	What it holds
faxclear.v1	Which chapters you have opened, your best quiz scores, and sound on or off.

Exactly what each one sees is at glassbox.how/privacy.

Licences

• Code: MIT. Use it, change it, ship it.
• Explanations, text, images and videos (glassbox.json, glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/faxclear”.
• Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
• The Glassbox name and logo aren't covered by either licence. See the terms.

Found a mistake? Open an issue. Corrections happen in public.

Run it

It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.

python3 -m http.server 8000

Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.

Then open http://localhost:8000.

How it's built

File	What
index.html, css/app.css	The page and its styles
js/app.js, js/stage.js, js/ui.js, js/kit.js	The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director
js/chapters/*.js	One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes
js/fax.js	FaxClear's shared models: the Group 3 page grid, a real Modified Huffman coder and decoder, the T.30 handshake and modem specs, procedural fax sounds (Web Audio), line noise and ECM, and a generic desktop fax in 3D
glassbox.json	Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how
reel in each chapter	The storyboard the Glassbox studio records into short videos
glassbox/	The published video, slides, thumbnail and post copy
fonts/, vendor/three/	Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT)