Comment by Richard Marshall on "How does a computer machine understand 0s and 1s?"

Hello - when you put a CD-ROM into a disc-drive, which I am assuming has data stored on it as 1s and 0s. And the Transistor copies that information to read it. How does it read it? I'm thinking, obviously a laser but how does that laser recognise that it is reading a 0 or a 1. If it acts like a camera, how does the camera know it is recognising a 0 or a 1. I guess, you program it. But how do you program it to read and recognise the information?

Back when computers read things from cards with holes in them, I can understand that because a hole, lets say is 0 and no hole is, lets say 1 (correct me if I'm wrong!) I can understand that because it's a physical action.

Any answers would be greatly appreciated.

Richard Marshall Found my answer: explainthatstuff.com/cdplayers.html

So what's going on in your CD player when the disc spins around?

Artwork showing how a CD player uses a laser beam to read bumps from a compact disc and turn them back into audible sounds.

Inside your CD player, there is a miniature laser beam (called a semiconductor diode laser) and a small photoelectric cell (an electronic light detector). When you press play, an electric motor (not shown in this diagram) makes the disc rotate at high speed (up to 500rpm). The laser beam switches on and scans along a track, with the photocell, from the center of the CD to the outside (in the opposite way to an LP record). The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc (as the track number increases, in other words). Otherwise, as the distance from the center increased, the actual surface of the disk would be moving faster and faster past the laser and photocell, so there would be more and more information to be read in the same amount of time. The laser (red) flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps) and lands (flat areas) on the disc. The lands reflect the laser light straight back, while the pits scatter the light. Every time the light reflects back, the photocell (blue) detects it, realizes it's seen a land, and sends a burst of electric current to an electronic circuit (green) that generates the number one. When the light fails to reflect back, the photocell realizes there is no land there and doesn't register anything, so the electronic circuit generates the number zero. Thus the scanning laser and electronic circuit gradually recreates the pattern of zeros and ones (binary digits) that were originally stored on the disc in the factory. Another electronic circuit in the CD player (called a digital to analog converter or DAC) decodes these binary numbers and converts them back into a changing pattern of electric currents.

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