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Jan's Computer Basics:
Storage: Optical Discs
An entirely different method of recording data is used for optical discs. These include the various kinds of CD, DVD, Blu-ray, and M discs.
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You may guess from the word "optical" that it has to do with light. You'd be exactly right! Laser light, in fact.
Optical discs come in several varieties which are made in somewhat different ways for different purposes. Most disc players these days are also able to write discs. Most players are also backwards-compatible for reading discs in earlier formats but are not always able to write in those formats.
The new M discs (Millenial disc
) in Blu-ray and DVD versions are more expensive but hold more data and should be stable over many more years than other discs.
Disc vs. Drive: Know what your optical drive can use BEFORE you buy discs. And, read those labels in the store carefully! It is entirely too easy to grab a movie or a box of writable discs that won't work in your device.
All DVD and Blu-ray drives can also read an M-disc. Only some optical drives can write M-discs. Writing M-discs requires a more powerful laser than some optical drives have. Read the box carefully or ask questions before buying!
How optical discs are similar
- Formed of layers
- Data in a spiral groove starting from the center of the disc
- Digital data (1's and 0's)
- 1's and 0's are formed by how the disc absorbs or reflects light from a tiny laser.
The different types of optical discs use different materials, different colors of laser light, and different methods to absorb and reflect the light.
How It Works (a simple version)
An optical disc is made mainly of polycarbonate (a plastic). The data is stored on a layer inside the polycarbonate. The data layer reflects the laser light back to a sensor.
To read the data on a disc, laser light shines through the polycarbonate and hits the data layer. How the laser light is reflected or absorbed is read as a 1 or a 0 by the computer.
In a CD or Blu-ray disc, the data layer is near the top of the disc, the label side.
In a DVD the data layer is in the middle of the disc. A DVD can actually have data in two layers. It can access the data from one side or from both sides. This is how a double-sided, double-layered DVD can hold 4 times the data that a single-sided, single-layered DVD can.
Blu-ray discs can have 3 or even 4 layers of data.
Materials
The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical discs.
| Type | Data Layer | Metal Layer |
|---|---|---|
| Read Only (commercial discs) |
Molded | Aluminum (Also silicon, silver, or gold in double-layered DVDs) |
| Recordable (once!) | Organic dye (except M discs) |
Silver, gold, silver alloy |
| Rewritable (write, erase, write again) |
Phase-changing metal alloy film | Aluminum |
Read Only:
CD-ROM stands for Compact Disc - Read Only Memory. It looks just like an audio CD but the recording format is quite different. CD-ROM discs are used for computer software.
DVD used to stand for Digital Video Device
or Digital Versatile Device,
but now it doesn't really stand for anything at all! DVDs are used for
recording movies and large amounts of data or large programs.
The discs that are commercially produced are of the Write Once Read Many (WORM)
variety. They can't be changed once they are created. The data layer is physically molded into the polycarbonate. Pits (depressions) and lands (surfaces) form the digital data. A metal coating (usually
aluminum) reflects the laser light back to the sensor.
Commercial CD-ROM and DVD-ROM discs should be readable for many, many years (100? 200?), but only if you treat them with respect.
Lost data: Oxygen can seep
into the disc through the edges or scratches, especially in high temperatures and high humidity. This
corrodes the aluminum, making it too dull to reflect the laser
correctly. You will have errors or complete failure when trying to read the disc.
Write Once:
The optical discs that you can write to from your own computer are CD-R, DVD-R, DVD+R, BD-R, and M discs, called writable or recordable discs.
The metal and organic data layers are separate for CD, DVD, and Blu-ray writable discs. The metal layer can be gold, silver, or a silver alloy. M discs use a single inorganic layer instead.
The data layer is changed by the writing laser and cannot be changed again. Write Once!
A writable disc is useful as a backup medium when you need long-term storage of your data. It is less efficient for data that changes often since you must make a new recording each time you save your data. Pricing of the discs will be important to your decision to use writable discs.
Lost data: Ultraviolet light and heat can degrade the organic dye. Sulfur dioxide from the air can seep in and corrode silver
over time.
Old drive: The laser in your device loses strength over time. It won't make as deep a pit when recording. So a disc burned near the end of the laser's life will lose data more easily.
Go for the Gold: Gold layers are best because gold does not corrode. Naturally, the
best is more expensive.
How long will they last: Writable CD and DVD discs kept in clean, dry conditions, like a safety deposit box at a bank, should last 30 to 50 years. But in real life conditions it is common for them to fail in 3 to 5 years. Commercial CD, DVD, and Blu-ray discs for movies and software use a different material that will last longer. Milleniata, the creator of M discs, claims that they will last 1000 years! They should definitely last longer than other types of discs because M discs do not use an organic layer.
Keep in mind: Manufacturers say that the discs that use an organic layer have a shelf-life of 5 - 10 years before they are used for recording.
Rewrite:
An option for backup storage of changing data is rewritable or erasable discs, CD-RW, DVD-RW, DVD+RW, DVD+RAM, BD-RE. M discs do not come in a rewritable type. [May 2014]
The data layer for these discs uses a phase-changing metal alloy film. This film can be melted by the laser's heat to level out the marks made by the laser and then lasered again to record new data.
In theory you can erase and write on these discs as many as 1000 times, for CD-RW, and even 100,000 times for the DVD-RW types.
Advantages of Optical Disks
- Physical: An optical disc is much sturdier than tape or a floppy disk. It is physically harder to break or melt or warp. It's somewhat harder to lose than a USB flash drive.
- Transport and Storage: Discs are easy to take with you and to store away. They have a long shelf-life.
- Delicacy: It is not sensitive to being touched, though it can get too dirty or scratched to be read. It can be cleaned!
- Not Magnetic: It is entirely unaffected by magnetic fields.
- Capacity: Optical discs hold a lot of data, especial the double-sided DVDs.
Plus, the non-data side of the disc can have a pretty label! But that's only for commercially produced discs. Paper labels corrode the protective layer quickly!
For software providers, an optical disc is a great way to store the software and data that they want to distribute or sell.
Disadvantages of Optical Disks
- Cost: Drive [May 2014] You have to have a drive that will write. DVD burning drives cost under $30 and are a standard part of most new computer systems. Blu-ray drives range from $50 to $100. Some inexpensive optical drives have M-disc support.
- Cost: Discs [May 2014] The total number of discs needed depends on how much you need to write to the discs and the capacity of discs you choose. The cost per disc ranges from just a few cents each for CD and DVD to $3 each for M discs.
- Copying Speed: It is not quite as easy or as fast to write files to an optical disc as it is to copy files to a USB flash drive or an external hard drive or SSD drive.
- Software: You need the software as well as the hardware for writing discs! (This is an advantage as far as commercial software providers are concerned!) Recent versions of Windows include disc writing software but third party programs offer features you might want.
Care of Optical Disks
Your CDs and DVDs are not going to last forever. M-discs seem likely to last a long, long time. They certainly store data longer than other storage media! but mishandling any type of optical disc can quickly make your data unreadable. Even fingerprints can do damage over time.
Data loss comes from:
-
Physical damage - breaking, melting, scratching...
-
Blocking of laser light by dirt, paint, ink, glue...
-
Corrosion of the reflecting layer
Here are some do's and don'ts for keeping your optical discs healthy.
-
Cleaning:
- Keep it clean!
- Handle by the edges or center hole.
- Put it back in its case as soon as you are finished with it. No laying around on the desktop!!
- Remove dirt and smudges with a clean cotton cloth by wiping from the center to the outer edge, NOT by wiping around the disc. Wiping in a circle can create a curved scratch, which can confuse the laser.
- For stubborn dirt, use isopropyl alcohol or methanol or CD/DVD cleaning detergent.
-
Labeling:
- Don't use an adhesive label. The adhesive can corrupt your data in just a few months!
- Don't write on or scratch the data side of the disc - ever!
- Don't scratch the label side.
- Don't write on the label side with a pencil or pen (scratches!)
- Don't write on the label side with a fine-point marker or with any solvent-based marker. Use markers for CDs or a fast-drying permanent marker. Sharpies seem to do well. Solvent in a regular permanent marker may dissolve the protective layer.
-
Storage:
- Store optical discs upright on edge, like a book, in a plastic case designed specifically for them. Not flat for long periods!
- Store in a cool, dark environment where the air is clean and dry. NO SMOKE! Low humidity.
-
How you treat it:
- Keep away from high heat and high humidity which accelerate corrosion.
- Keep out of sunlight or other sources of ultraviolet light.
- Keep away from smoke or other air pollution.
- Don't bend it!
- Don't use a disc as a coaster or a frisbee or a bookmarker!
Recording
- Inspect the disc for flaws and dirt BEFORE recording on it.
- Only open a recordable disc just before you plan to record on it.
- After recording, make sure the disc works as you expect: Read data; run programs.
Complete recommendations from NIST 
and Quick
Reference Guide as part
of the "Digital Preservation Program" sponsored by the US National
Institute of Standards and Technology, designed to give libraries and
museums guidance on how to preserve digital data.
(in pdf format. Requires Adobe
Acrobat Reader)
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Last updated: March 24, 2017