Answer Geek: Error Correction Rule CDs

<br> -- Q U E S T I O N: CDs can hold about 650 MB of data, so each individual bit must be a microscopic pit on a CD’s surface. Since dust particles are larger than these individual pits, how does a drive’s laser see the pits buried underneath dust? I’ve played CDs that were covered in dust (and fingerprints, etc.) and yet they still work: how is that possible? — Dave I.

A N S W E R: I don’t know about your house, Dave, but in my house, CDs take a lot of abuse. The problem: a toddler who is old enough to know how to operate the CD player and bossy enough to insist on doing it himself, but too young to understand that you should treat CDs with respect and care.

So the typical CD in my household has probably spent some time scraping along the floor, or served as a Frisbee, or spent time in CD purgatory between the cushions of the couch. It’s definitely not treatment recommended by the manufacturer, and the result is that you can hardly pick a CD up off the floor of my living room that isn’t a welter of scratches, pits, and gouges, with food smears and greasy fingerprints galore. The amazing thing is they all still seem to play just fine.

How is this possible? The answer is one of the minor miracles of the digital age: error correction.

Error Correction: Can You Digit

Without it, we’d still be listening to music on vinyl, or 8-tracks. For that matter, CD-ROMs and virtually any kind of digital storage would be impossible, because in a world where every scratch, scrape, and smudge meant lost data, the odds of actually finding a CD you could play, or a file you could open would be very small indeed.

The key to error correction is something called Cross Interleaved Reed Solomon Code — CIRC for short. (Irving S. Reed and Gustave Solomon were engineers who came up with the idea in 1960 while working at MIT.) CIRC operates on a couple of basic principles. First, extra data is added to the information recorded on the disk. This extra information is called “parity bits.” Next, the data is placed on a disk not in normal linear order, like you might expect, but spread around through a process called interleaving. Let’s start with parity bits.