FACT: a CD track is five kilometers long

I’d like to start off by saying that a CD (compact disc) works very much like a vinyl record, but I’m afraid it would be almost as current to say a CD works much like a telegraph machine. Still, I’ll soldier on with my record analogy, since they both store music (data) in a very similar fashion.

The music on a CD is recorded onto a path (a “groove” in vinyl-speak) that spirals around the disc for about five kilometers. Opposite to a record, the spiral starts in the center of the disc and winds it’s way out. In order to keep things moving at a steady pace, the disc spins at 500 RPM when it’s reading near the center, and slowly decreases to 200 RPM as it gets to the outer edge. The stream of data on the path remains constant.

The groove on a vinyl record is analog; a series of detailed bumps wherein the larger the bump the louder the noise. On a CD, the digital data is recorded in a binary form, meaning a series of zeros and ones, either off or on. A laser beam is focused onto the track, which reflects back from the shiny aluminum surface of the disc. Reflecting back is a one… “on”. To record data, a stronger laser has burned a little pit onto the shiny surface. When the reading laser passes that pit, it does not reflect back, indicating a zero or “off”.

So the factually correct response to “Hey what do you think of this new Vanilla Ice album?” was indeed “It’s the pits“.

If you speed the process up to 44,100 of these reflection tests per second, we can start to hear the music. The binary signal of zeros and ones is read and translated back into analog music for our speakers to rock out.

Another interesting fact for those wanting to take good care of your CDs and DVDs… the shiny side of the disc is actually better protected than the label side. There’s more plastic protecting the shinyand fragile aluminum surface. So you’re better off to set the discs shiny-side down.

• Source: http://www.copy-cd.biz/technical-area/compact-discs-explained.jsp

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