• Question: Could you please explain how a CD can play sound when in a CD player.

    Asked by A Swann to Anna, George, Jodi, Rob, Stefan on 17 Nov 2014.
    • Photo: Stefan Lines

      Stefan Lines answered on 17 Nov 2014:

      Hi A Swann. I believe information is read off the CD using a laser. The data is read in as binary (so zeros and ones). From recollection, a CD and DVD are both covered in bumps and dips. The laser shines on the disk, and depending on whether it scans a bump or dip, the light either bounces back or not. If it does bounce back, then it reads a 1 and if not then a 0 (or vice versa!). It’s these 1s and 0s that are then processed by the device playing the CD to turn it into a sound. It does this by converting the 0s and 1s (digital signal) into an analogue signal using a digital to analogue converter. The signal then is just a series of different currents, that powers the speaker.

    • Photo: Jodi Schneider

      Jodi Schneider answered on 17 Nov 2014:

      Computer technology, THIS I can help with! 😀

      Have you ever seen an old-fashioned record? You can see grooves: where each song starts and ends. Like this:
      http://commons.wikimedia.org/wiki/File:Vinyl_record_LP_10inch.JPG
      And the song is SCRATCHED into the record.

      A CD is a like a record, just a lot smaller. Instead of grooves the CD has tiny dots. The CD player has a laser that reads whether a dot is raised or not — and sends it to a sound processor. (On a record player, there’s a needle not a laser.) (“Raised or not” is the same as 1 or 0 — this is why we call it “digital”.)

      Like records, CD’s can get scratched and skip or repeat. But fortunately, CDs are pretty durable — in part because they use redundancy to correct errors. What do I mean by redundancy? If I told you today was Monday November 18th, you’d know something was wrong, because the date and the day of the week don’t match.

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