Storage Devices - Optical Discs & Drives

Optical discs are a removable storage media that stores information and data on it optically as opposed to magnetically such as the way HDDs do.  The majority of us will have used optical discs on an almost daily basis in the form of compact discs (CDs), digital versatile discs (DVDs) and Blu-Rays.  Common optical discs, such as CDs, DVDs and Blu-rays all have the same 5.25 inch form factor which provides great compatibility, including backwards compatibility, as technology advances.

In some cases, optical discs can be double-sided, the most common of these being double sided DVDs.  Double sided optical discs have a single recordable layer on each side of the disc.  In the case of recordable and re-writable optical discs, a user would write to one side before removing it, turning it over and placing it back into a computer to write to the other side.

Data is written to optical discs  through a laser pressing or 'burning' to create recessed areas (also known as pits) and raised areas (also known as lands) on the reflective surface of the disc.

Types of Optical Disc

There are a number of different types of optical disc on the market, each with their own characteristics and specifications.

• CD-ROM; Compact Disc-Read Only Memory.  These discs are permanently 'burned' with their information at manufacture and cannot be modified.  Data Capacity: 700-860Mb
• CD-RW; Compact Disc-Rewritable. These discs can have data written to them multiple times with a data range of 700-860Mb
• DVD-ROM; Digital Versatile Disc-Read Only Memory. As with the CD-ROM, data is burned permanently onto the disc at manufacture and cannot be modified. Single sided DVD-ROM capacity: 4.7 Gb.  Double-sided capacity: 9.4 Gb
• Single Layer DVD-R & DVD+R; Single Layer Digital Versatile Disc-Recordable and Disc+Recordable.  Data can be written to DVD-R & DVD+R only once and each disc can hold up to 4.7 Gb of data.  Both DVD-R & DVD+R work in exactly the same way, the only difference being that DVD+R is faster than DVD-R.
• Double sided DVD-R & DVD+R; Double sided DVD-R & DVD+R are exactly the same as their single layer variants except that they have one recordable layer on each side of the disc.  As with single layer discs, DVD+R is faster than DVD-R.  Double sided DVD-R & DVD+R can hold up to 8.75 Gb of data.
• Dual Layer DVD-R & DVD+R; Dual Layer Digital Versatile Disc-Recordable & Disc+Recordable.  A single sided DVD that has two separate recordable layers on a single sided disc.  A dual layer DVD-R or DVD+R can hold up to 8.5 Gb of data.  As with their single layer variants, DVD+R is faster than DVD-R.
• Single Layer DVD-RW; Single Layer Digital Versatile Disc-Rewritable.  DVD-RW can be written to multiple times and hold up to 4.7 Gb of data.
• Dual Layer DVD-RW; These discs have two layers of rewritable space, giving them a maximum data storage capacity of 8.7 Gb for single sided discs and 17.08 Gb for double sided discs.  These discs are not widely used due to their cost and the release of Blu-ray technology.
• DVD-RAM; Digital versatile Disc-Random Access Memory; A DVD that can be rewritten or erased a number of times.  it's possible to edit or erase specific sections of the disc without altering or affecting any other part of the disc.  These forms of DVD are often used for data backup storage.
• BD-ROM; Blu-ray Disc Read Only Memory.  As with CD-ROM and DVD-ROM, data is burned onto the disc at manufacture and cannot be modified.  Blu-ray discs are intended for the storage of hi-definition video and data.  BD discs have a maximum capacity of 128 Gb, dependent upon the number of layers that the disc has.  Each layer on the disc has a maximum capacity of 25 Gb and the newer discs can carry up to 4 layers.
• BD-R; Blu-ray Disc Recordable.  As with DVD-R (& DVD+R), data can only be written onto a BD-R once and have a maximum capacity of 128 Gb depending on the number of layers that the disc has.
• BD-RE; Blu-ray Disc Recordable-Erasable.  BD-RE is a form of Blu-ray disc that can be written to and erased multiple times without any reduction in the discs integrity, or the integrity of the data stored on it.  As with other forms of Blu-ray, BD-RE discs can hold up to 128 Gb of data, depending on the number of layers that the disc contains.

Optical Drives are drives that can read or write to optical discs as described above.  They can be internal or external to a computer system and.  Some optical drives only provide read capabilities, whilst others are able to read and write to optical discs.  Due to their subtle differences, CD, DVD and Blu-ray optical drives all have varying characteristics.

A CD Drive optical lense

• CD Optical Drives; use one of two specific file systems known as Compact Disc File System (CDFS) or the Universal Disc Format (UDF).  CD drives must have a data transfer rate of at least 150 Kbps in order to meet the audio CD standard.  The vast majority of CD drives deliver much higher transfer speeds which is at least eight times (8x) or sixteen times (16x) the 150 Kbps transfer rate.  There are some, higher end, CD drives that can offer transfer rates of up to 52x.
• DVD Optical Drives; always make use of the Universal Disc Format (UDF) and are able to access data from anywhere between 600 Kbps to 1.3 Mbps.  DVD optical drives do come in different specifications, the most notable of which is single or dual layer.  In order to be able to utilise dual layer read/write functions, the DVD disc and drive must both be dual layer technology.
• Blu-ray Optical Drives; make use of version 2.5 or the Universal Disc Format (UDF).  Blu-ray gets it's name from the blue laser that's used within the technology, as opposed to the more traditional red lasers used within CD and DVD drives.  This new style of laser makes it possible to 'pack' the data more tightly onto a Blu-ray disc, thereby increasing it's capacity.
• Combination Optical Drives; come in a great variety of designs and specifications.  Earlier combination drives were able to read/write from/to CDs only, but were also able to read from DVDs.  However, todays common optical drives are primarily DVD-RW drives with the ability to also read/write CDs and read Blu-ray discs.   Additionally, modern Blu-ray combination drives are mostly also able to read/write CDs and DVDs but with the huge variations in specifications available, it's always worthwhile ensuring and optical drives' capabilities through the manufacturer.

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