CD-ROM and CD Writer defined

CD-ROM - Compact Disc Read Only Memory
A CD-ROM disc, or data CD as it is sometimes referred to, is exactly the same type of media as an audio CD except it stores data as opposed to music (although technically music is data too).

A typical data CD can store up to 700MB of data and is a very reliable storage media.
The devices that utilise this form of media, referred to as optical drives, come as two basic types, a CD-ROM drive and a CD Writer. A CD-ROM drive can only read the data from a CD whereas a CD Writer can also write (burn) data to a disk.

A CD Writer uses a special type of writable CD of which there are two types, CD-Rs and CD-RWs. A CD-R (CD Recordable) disk can be written to only once but a CD-RW (CD ReWritable) disk can be overwritten many times.

Although external hard drives and USB thumb drives are a very convenient way to backup your data, a CD offers that extra bit of reliability. For example, they are not susceptible to magnetic interference.

You will often see CD Writers advertised as 48X-12X-50X. This means it will burn (write) at 48X, ReWrite (write again over an existing rewritable CD) at 12X, and read at 50X the normal CD speed.

For instance, a 48X CD Writer will write a CD 48 times faster than normal (700MB in just under 2 mins).

Although CD-R disks are capable of storing audio, you can also purchase blank audio CDs which provide slightly better quality.

How optical drives work
The CD itself is made up of several independant layers, one of which is a reflective material such as aluminum or silver. There is a very important difference between commercially purchased CDs, such as software, and the blank CDs (CD-Rs and CD-RWs) that you use to store your own data.

In commercially produced CDs the main layer is pressed to form "pits", or "bumps" depending which way you are looking at the CD. These pits or bumps form a spiral which begins at the inner edge and works its way to the outer edge of the disk. The presence of these imperfections cause the reflective layer to scatter light instead of reflecting it directly.

Writable CDs do not have these bumps, instead they either have a layer of organic dye (write-once disks) or a reactive chemical layer which is transparent but turns opaque when subjected to certain conditions (rewritable disks).

When you store data on a CD-R (write once) disk a special laser "burns" (hence the term CD Burner) blemishes in the organic dye layer and therefore changing its reflectiveness in certain spots. When you write data to a CD-RW (rewritable) disk the laser uses a certain signal strength to force the reactive layer to turn opaque. When you delete data from a CD-RW disk the laser uses a different strength to return the reactive layer back to transparent.

To read the data from the CD the drives use a laser to bounce light off the surface. If the light is reflected back into a special light sensitive device, known as a photodiode, then the drive interprets this as the binary digit 1 (one), if the light is not reflected then it is interpreted as the binary digit 0 (zero). These binary values are the building blocks of the digital data.

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