optical storage

What is it?

Optical media like CDs and DVDs store data using microscopic variations in the layers of a (mostly) plastic disk.  The variations are stamped ("pressed" or "molded") onto the surface of the disk, or "burned" into it using a laser, in a spiral pattern that runs from the inside to the outside of the disk.  By contrast, "magnetic" media like hard drives store data using microscopic variations in electrical charge on a (mostly) metal disk.

CDs consist of a 1.2-millimeter (mm) clear polycarbonate plastic disk, with a reflective metal coating on one side.  The standard diameter is 120 mm (with an 80-mm "mini" format).  A laser beamed through the plastic bounces off the metal surface.  Differences in reflection caused by the variations in the plastic layer are detected and translated.  A standard 120-mm CD holds about 700 megabytes of data.

DVDs consist of two half-thickness (0.6-mm) CD-like disks glued back to back, with the reflective coating in the middle.  Using a shorter-wavelength laser allows higher data densities, so that a single side of a DVD can hold about  4.7 gigabytes (4700 megabytes).  Recording on both sides of the DVD doubles this.  (Such DVDs must be flipped over, like an old-fashioned record.)

CDs and DVDs come in many formats, with a confusing array of abbreviations.  It's less complicated than it seems at first, though not necessarily simple.

• The most basic form of data-holding CD or DVD is one that can only be read.  Formally, it  is CD-ROM or DVD-ROM.  ROM stands for "read-only memory" which translates also as "non-writeable."  CD-ROMs and DVD-ROMs are permanently pressed/molded when manufactured.
   
• A CD-R, DVD-R or DVD+R may be recorded (written on), but only a single time.  The R stands for "recordable."  This "write-once" process uses a light-sensitive dye recording layer to which information is irreversibly written by means of a laser heating and altering it, to create a pattern of marks mimicking those of a prerecorded (pressed/molded) CD or DVD.  This is why the writing process is sometimes called "burning."
   
• A CD-RW, DVD-RW or DVD+RW may be recorded (written on) many times.  The RW standards for "rewriteable."  The rewriting process employs a metallic "phase-change" recording layer that can be repeatedly altered and restored by the writing laser (approximately 1000 times). CD or DVD drives capable of writing (as opposed to just reading) can write to either R or RW discs.
   
• Finally, DVD-RAM also uses phase-change technology but can be rewritten roughly 100,000 times.  With its hard sectors, random access capabilities and optional cartridge, DVD-RAM more closely resembles traditional hard drive storage media than do DVD-RW and DVD+RW.

So what's the difference between "+" and "-" for DVDs?  Unfortunately, rival industry groups DVD Forum and DVD+RW Alliance could not agree on a data format.  Fortunately, most computers now incorporate "dual-standard" DVD drives, which can handle either format.  You'll see them referenced as "DVD+/-RW" drives.

As noted, DVDs can use two sides to double recording capacities.  An alternative approach to doubling is provided by "dual-layer" DVDs -- DVD-R DL and DVD+R DL, depending on whether the - or + format is supported -- that have two data-holding layers on a single side with a semi-reflective coating between.  The focus of the laser is changed to access the shallow or deep layer.  Such disks can hold about 8 gigabytes on a single side (and 16 gigabytes on double-sided DLs).  CDs come only in the single-layer, single-side variety.

Almost all computers now come with at least an optical drive capable of reading CDs.  Drives capable of writing data to a CD-R or CD-RW disc -- usually called CD-RW or CDRW drives -- are a bit more expensive.  We recommend a CD-RW drive for most systems, given the utility of using CDs for data backup, storage and transfer.  Since data and software are still commonly exchanged on CD, at least a CD-reading drive is essential.

Most systems now come with an optical drive that capable of reading DVDs too.  Since DVDs are increasingly used for data (as well as, ubiquitously, for video), having such a DVD-reading drive is often beneficial.  Commonly a CD-reading/writing and DVD-reading drive is combined into a single "CD-RW/DVD" drive.

DVD-writing drives can be used for data backup, storage and transfer.  Blank DVD-R/+R discs now cost only about twice as much as CD-Rs, but hold 6-7 times as much data; so having a DVD-RW/+RW drive is something to consider if you need to transfer or backup higher volumes of data.  As noted, most computers now have dual format DVD drives, so it's not necessary for you to pick a + or - format.  (The price differential between blank +/- DVD disks is small.)  However, remember to purchase the right kind of media -- + or - as appropriate -- if you don't have a dual format drive. 

RAM and DL DVD formats are relatively expensive, and not for the typical user.   All optical drive types come in a range of speeds -- denoted by "X" numbers, such as 16X, 32X, 48X -- denoting the speed of data reading and writing.  As you'd expect, faster drives are more expensive.

Optical disks are considered highly stable -- manufacturers claim life-spans ranging from 50 to 200 years for CD-Rs, 20 to 100 years for CD-RWs, and comparable periods for DVD-/+Rs and -/+RWs.  (Obviously this is only an estimate.  CDs were not in commercial use until the mid 1980s and DVDs until the mid 1990s.)  

Unlike magnetic media, optical storage is generally invulnerable to electromagnetic damage.  CDs and DVDs react badly to extreme humidity and temperatures, however.  Heat can cause warping, rendering a disk unusable.  Cold can make the disk (more) fragile.  Mechanical trauma, causing damage to the metal reflective or the plastic data layers, can render all or part of the data on a disk unusable. 

Absent such environmental or physical assaults, the main risk to optical storage comes from their portability.  Anything that can be carried around easily can also be lost or stolen easily. 

Loss or theft of a CD/DVD containing sensitive data presents security issues.  In general, you should not carry around large amounts of sensitive data on any portable storage media, for the same reason you shouldn't carry around a large amount of money.  It's too risky.  Sensitive data on CDs or DVDs should be kept locked away, just like any other valuable possession.

As with folders on hard drives, data can be stored in encrypted form on optical media to protect against unwarranted disclosures.  Encryption software typically requires setting a password (that serves as the encryption "key").  The usual considerations of password safety apply.

Optical disks containing obsolete data can be overwritten for security if they are of the RW or RAM (rewriteable) variety.  But R and ROM (non-rewriteable) disks must be physically destroyed to protect data.  (For more, see our discussion of secure data disposal methods.)

High volume storage where re-writing is not needed, such as for archival backup copies, can cheaply use write-once CD-Rs, DVD-Rs and DVD+Rs.  As noted, the long-term productive life of optical media like CDs and DVDs is claimed to be many decades.  But you should not rely on optical media -- or any other kind of media for that matter -- for your only backup copy of important data.

With each passing year, hard drives are able to deliver ever greater capacity, at an ever cheaper cost per stored byte.  Hard drives can generally transfer data much faster than optical drives, so they make sense for data which must be accessed frequently as well as data which must be changed frequently.  Note, however, that the capacities and costs of solid-state flash drives are also improving each year, and they present a compelling alternative to hard drives for smaller data volumes (less than 1-2 gigabytes).

Needless to say -- at least we hope it is needless to say -- whatever data storage medium you use, be sure to keep it in a physically secure place, as safe as possible from human and environmental threats.

The CDs and DVDs common today use a "red-laser" (infrared) technology.  The newest DVD formats use a higher-frequency (shorter-wavelength) "blue-laser" technology that allows much higher recording densities -- from 15 up to 50 gigabytes per disk.  Unfortunately, the rival "HD DVD" and "Blu-ray" formats, backed by rival industry groups, are incompatible with each other. 

Optical disks (Wikipedia)
An overall description of how they work, with links to technical detail on the various implementations (CDs, DVDs, etc.)

Understanding CD-R and CD-RW (Optical Storage Technology Association)
Just about everything you could ever want to know about CD technology

Understanding Recordable and Rewritable DVD (Optical Storage Technology Association)
A similarly thorough treatment of the technical details of DVDs

Last modified: 23-Apr-2006 [RC]