Science Interviews


Sun, 27th Feb 2005

How a Cd Player Works

Philippa Law interviews Dr Lucy Green and Dr Julian Alwood

Part of the show Dinosaurs, Ancient Diets and Fossilised Crocs

Phillipa - My mission for today is to find out how a CD player works. I know there's a laser in a CD, but I have no idea what one actually is. Let's ask Lucy Green from Cardiff University. What's the difference between a laser and a normal light?

Lucy - In a laser you have light given off in a beam where all the light waves are moving in exactly the same way and in the same direction. In a normal light, the light comes from all over the place. So a laser isn't a different substance, it's just that the substance is all moving in the same direction. As the light is all lined up in this special way, it allows you to point the light very directly at something. It can be used to point at information in a lecture and even for very precise laser eye surgery.

Phillipa - How does the laser in a CD player work?

Lucy - A laser, which actually stands for Light Amplification by Stimulated Emission of Radiation, it really does what is says on the tin. You have your source of atoms and you excited them by passing a current through them. When atoms are excited, they can lose that energy by giving off light. So in a laser, you have a source of atoms that are all concentrated in a small region and you excited them. All it takes is for one atom to spontaneously move to a lower energy and give off one photon of light. That photon can interact with another atom and cause that atom to give off another photon of light. Now you have two photons. These two photons can go on and excite two more atoms and give off even more light. This is a chain reaction, where one photon causes another atom to give off a photon of light. All that light is being trapped within a cavity, but when the intensity gets high enough it can break through part of the cavity and that's when you get your columnated laser beam.

Phillipa - What was Einstein's contribution to the invention of lasers?

Lucy - Einstein had two ways in which he contributed to the invention of lasers. Firstly, he made us think about light in this totally different way: rather than thinking about light as a wave, he made us think about it is packets of energy, or photons. Secondly, Einstein studied how large collections of atoms worked together. He realised that you could make excited atoms give off a photon of light. He also realised that that photon could force another atom to give off another photon of light, and so on.

Phillipa - So now I understand the physics of lasers, what else do I need to know? Here's Julian Alwood from the engineering department at Cambridge University.

Julian - When any machine records sound then it has to convert the sound wave that the ear experiences into some form that can be recreated. It's very difficult to envisage waves in the air, but when you stand at the end of a pier, you are used to seeing waves and water. Sometimes when you stand under the pier you can see a ruler which shows you the height of the tide. If you imagine a wave coming into the wall of the harbour and you see the place it comes to on the ruler going up and down, then that's a measure of the wave of the sea. When we record sound, we try to do exactly the same thing. We try to record how high the wave is, although in this case it's pressure in the air rather than the movement of water. When Edison made the first sound recording, he literally took the sound waves through a trumpet, which made a diaphragm vibrate and in turn made a little needle vibrate, which he recorded in marks on tin. After a while that turned into the vinyl long-playing record. This has a track in it. The vibrations on the track are a representation of the waves in the air. The problem is that if any dirt gets into the track then you hear that as well. This makes it very difficult to get a pure recording, which is why we now use digital recording most of the time. A digital recording is just like standing by the ruler on the harbour wall and writing down the numbers at regular intervals so that you can recreate the wave.

Phillipa - If you hold a CD up to a bright light you can see that there are all these different colours of light shining on it. Why is this? Let's speak to Dr Sally Day at University College London.

Sally - The CD itself is made of plastic and it has aluminium on it like a mirror. You may have seen at the edge of normal mirrors that you get lots of rainbow colours. This is from the light being split up like it is in a prism. Lines on the CD surface can break up the mirror and create all the colours.

Phillipa - If you look closely at the back of a CD, you can see those lines going round and round. What are they for?

Sally - In each of those lines are lots and lots of little dots where the aluminium is either slightly thicker or thinner in different regions. Those different regions mean that the light reflects slightly differently where there is a dot or no dot. The whole CD is covered in dots and no dots, and that's where all the information is.

Julian - The dots are put on the disc in a long spiral groove that would be three and a half miles long if you unwound it. The little dots are very small indeed. If they are up then it represents and one, and if it's down it represents and zero.

Sally - So a CD has an enormous amount of room for zeros and ones on it, and it can actually store six billion bits of binary data.

Phillipa - This must be where the laser comes in.

Julian - When you put the CD into the CD player, the disc starts spinning round and the laser shines onto the bottom of the CD. The information is stored in very very small regions of the CD with widths one thousandth of the width of a human hair. You can point the laser very precisely at this point. The light is reflected back down onto a little detector. That detector then detects either some light or no light depending on whether it's a nought or a one. The computer that reads the CD is then able to count the ones and zeros, convert them into numbers, which are the height on the ruler of the harbour wall. After 16 numbers, it has built up a number big enough to describe the wave accurately; in effect the height on the ruler. Each one of those represents the height of the wave once every one forty-four thousandth of a second. The circuitry of the computer converts that to a voltage. That voltage is then passed to a loud speaker where it is converted through a magnet into the movement of the loud speaker cone. In effect, the number describes the placement of the loud speaker cone, and that directly recreates the wave that hits your ears.


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