Wirebook › Glossary › Error correction

What is error correction (and how does a scratched QR code still scan)?

In one sentence: Spare data worked out from a message so that missing or damaged pieces can be rebuilt, like the extra squares that let a scratched QR code still scan.

A little more

Error correction adds extra numbers to a message, worked out from the message itself with some clever maths (QR codes use Reed-Solomon codes). If some pieces are lost or misread, the receiver can work backwards from the extras and rebuild them exactly.

QR codes have four levels: L can rebuild about 7% of the message, M about 15%, Q about 25% and H about 30%. More spare data means more squares, so the same message makes a bigger grid. CDs, phone signals and spacecraft radios use the same idea. It cannot help if the damage is too big, or if the corner squares a scanner looks for are missing.

It's a bit like… Doing a jigsaw with a photo of the finished picture on the box: lose a few pieces and you can still tell exactly what belonged in the gaps.

See it in action

BonusCan a sticker change where a QR code takes you?Build a QR code and colour its parts (corner finder squares, timing lines, alignment square, format notes, message), scratch it, tear it and stick a price label on it while a real decoder tries to read it at four levels of spare data, then be the sticker detective: preview where five pretend posters really go, compare with the printed address and spot the fake.8 min · interactive

Do it for real

Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.

Related words

← All 146 words