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Speaker 1: Welcome to brain Stuff from how Stuff Works dot com

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Speaker 1: where smart Happens. This podcast is brought to you by

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Speaker 1: Audible dot com, the Internet's leading provider of audio books,

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Speaker 1: types of literature. For brain Stuff listeners, Audible is offering

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Speaker 1: Jobs by Walter Isaacson. This biography is based on many

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Speaker 1: personal interviews with Steve Jobs, his family, his friends, and colleagues.

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Speaker 1: Here the real story of one of the greatest innovators

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Speaker 1: try Audible free today and to get a free audiobook

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Speaker 1: of your choice, get Audible podcast dot com slash brain Stuff.

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Speaker 1: That's Audible podcast dot com slash brain Stuff. Hi Im

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Speaker 1: Marshall Brain with today's question, how do SDR discs work?

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Speaker 1: If you've read the how Stuff Works article describing how

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Speaker 1: c ds work, then you know that the basic idea

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Speaker 1: behind data storage on a normal c D is simple.

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Speaker 1: The surface of the CD contains one long spiral track

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Speaker 1: of data. Along the track, there are flat, reflective areas

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Speaker 1: and non reflective bumps. A flat reflective area represents a

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Speaker 1: binary one, and a non reflective bump represents a binary zero.

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Speaker 1: The CD drive shines a laser at the surface of

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Speaker 1: the c D and can detect the reflective areas and

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Speaker 1: the bumps by the amount of laser light they reflect.

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Speaker 1: Normal c ds can't be modified. There read only devices.

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Speaker 1: A CDR disc needs to allow the drive to write

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Speaker 1: data onto the disk. For a CDR disc to work,

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Speaker 1: there must be a way for a laser to create

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Speaker 1: a non reflective area on the disk. A CDR disc

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Speaker 1: therefore has an extra layer that the laser can modify.

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Speaker 1: This extra layer contains a greenish dye. In a normal

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Speaker 1: c D, you have a plastic substrate covered with a

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Speaker 1: reflective aluminum or gold layer. In a CDR, you have

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Speaker 1: a plastic substrate, a die layer, and a reflective gold layer.

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Speaker 1: When you write data to a CDR, the writing laser,

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Speaker 1: which is much more powerful than the reading laser, heats

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Speaker 1: up that die layer and changes its transparency. The change

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Speaker 1: in the dye creates the equivalent of a non reflective

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Speaker 1: bump on the surface of the disk. This is a

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Speaker 1: permanent change, and both a CD and a CDR drive

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Speaker 1: can read that modified die layer as a bump. Later on,

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Speaker 1: it turns out that the dye is fairly sensitive delight.

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Speaker 1: It has to be in order for a laser to

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Speaker 1: modify it quickly. Therefore, you want to avoid exposing CDR

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Speaker 1: discs to sunlight. This podcast is brought to you by

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Speaker 1: audible dot com, the internet leading provider of audiobooks, with

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Speaker 1: more than downloadable titles across all types of literature and

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Speaker 1: featuring audio versions of many New York Times bestsellers. To

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Speaker 1: try Audible free today and to get a free audio

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Speaker 1: book of your choice, go to audible podcast dot com

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Speaker 1: slash brain Stuff. That's Audible podcast dot com slash brain Stuff.

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Speaker 1: The Howstuff Works iPhone app has arrived. Download it today

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