How does a CD Burner work?

The CD burner or more commonly known as the CD writer has become a standard part of the PC today. Its rare to see a PC without the capacity to create customized CD's. Its takes only a few minutes these days to back up your work onto CD or create a customized music from CD's you already own. But how does the CD writer actually work? Well firstly we will need to take a closer look at how the standard CD reader works. This will help us understand the burning process a little better.

The construction of a CD

The CD itself is made up of one continuous track about 0.5 microns wide and around 5km in length. This track is a small groove spiralling round and round the CD from the centre to the edge. The materials used to make a CD are at the top we have the label, then a layer of acrylic, a layer of aluminium ad finally a thicker layer of plastic to protect the CD.

When manufacturing a production CD like what you buy in the shops, A heavy duty stamp is used with microscopic bumps arranged as a single track of data. This is then stamped on a disc of polycarbonate plastic. Then the Aluminium coating is applied for its properties as a reflective surface. Acrylic is then applied for protection, and the label is then placed on. This is obviously a large volume solution and the technique is no good for home use.

Reading the CD

The process of reading the CD is a simple one although it needs to be very precise. When a CD is burned it leaves a pattern of bumps and troughs. This is read as a digital data stream. Each bump is read as a 0 and a trough or flat area is considered to be a 1. (0's and 1's are the basis for digital data or binary code). The bumps are read by an optical sensor, or more accurately the missing bumps are read by the sensor. What actually happens is a low powered laser is projected at the spinning CD, if there is no bump on the CD the light reflects back to the sensor and a binary digit of 1 is recorded, if the laser hits a bump in the CD then the light is reflected away from the sensor and so a 0 is recorded.

The sensor works in conjunction with the motor to work out how fast the CD is spinning and therefore how many times a second it has to send the laser beam to the CD to accurately work out the digital pattern on the CD's surface.

How does the home CD burner differ?

Its obviously impractical to get such a technique as they use on the production line into the home PC. Another solution had to found in order to get CD writing technology into the home. The idea that they came up with was to actually change what the CD was made of. The technology of the laser reflecting off the disc was still there however this time the aluminium layer was totally flat. Instead of covering the bumps in the track like in the production model. What we have on the common CD-R now is a layer of dye. This dye is translucent and so the laser beam will go straight through it and reflect off the aluminium surface and into the optical sensor. Its the same all the way round the disc, this is how you receive your blank CD-R's.

When writing to a CD-R a higher powered laser is used than that of the standard reading laser. This laser is set at a particular frequency and when it comes into contact with the dye its turns it opaque (to a state where the light can't pass through it. To the optical sensor this is the same as the laser beam bouncing off the bump of the production CD as it simply does not get the signal back. The physical shape of the material on the CDR does not change as you can see from the above diagram, but still the effect is permanent which brings us onto the next point. re-writable storage CD's. Now known as CD-RW's

So how does a CD-RW work?

If we now move onto CD-RW's we take a big step forward in the CD storage media's technology. The CD-RW is a lot more complex than the standard CDR (some say a little less reliable, although I personally have not had problems with CD-RW's). The problem obviously comes with the fact that the data on the disc can not be permanent. This means the production style burning and the original home style burning can't be used on a CD-RW. Lets have a quick look at the way a CD-RW is put together.

The CD-RW uses phase shift technology, using a specially created phase shift compound. The idea behind this is that with this compound the laser can heat it up to melting point at which point the compound turn opaque and the read laser would not be able to bounce back a signal. this then turns into an amorphous state (no crystalline form). It will cool this way and be opaque until its reheated. In order to get the compound translucent again it needs to be heated again, this time to a lower temperature. At lower temperatures the phase shift compound will cool in a crystalline form and allow the laser beam to pass through and bounce off the aluminium beneath. To get the amorphous state the laser heats the compound to around 600C where as to get the crystalline state it only requires heating to around 200C.

When you write data to a CD-RW it stays permanent until its written over. However unlike your hard disk it uses an erase function first. To erase the section of the disk the write laser simply heats all the area to a crystalline form and then writes whatever data you need onto the now blank piece of disk.

Jargon