Optical discs
- Creighton Barrett
Overview
Optical discs are digital storage formats that encode binary data. Most formats are 12 cm in diameter:
Image source
There are three primary types of optical disc:
| Type of optical disc | Description | Examples |
|---|---|---|
| Prerecorded CDs and DVDs | Disc is prerecorded by injection molding and metallization. Requires production of an encoded master and injection molding stampers. Data is stored in "pits" impressed into the base layer and protected with a very thin metal layer. | CD-Digital Audio, CD-ROM, DVD-ROM |
| Recordable CDs and DVDs | Disc is recorded with laser. | CD-R, DVD-R, DVD+R |
| Rewriteable CDs and DVDs | Disc is recorded with laser at higher temperature. | CD-RW, DVD-RW, DVD+RW |
Timeline
| Date | Event | Reference |
|---|---|---|
| 1978 | Phillips, MCA, and Pioneer release LaserDisc format. LaserDisc was the first time optical discs were used for storage. They had many benefits over VHS, but they did not gain in popularity due to high cost, limited storage (approximately 1 hour of video), and lack of ability to record video. | |
| 1990 | CD-Digital Audio discs allow for multiple sessions (i.e., linear series of tracks bounded by a "table of contents" with location and descriptive metadata) | Alex Duryee, "An introduction to optical media preservation," Code4Lib 24 (April 16, 2014). |
| 1997 | DVD format is introduced. DVD quickly became the leading format for distributing pre-recorded video and could also store computer information and data. One DVD can hold up to 4.7 GB of data (2-3 hours of standard video). | |
| 2006 | Sony introduces Blu-ray format | |
| 2006 | Toshiba introduces HD DVD format |
Physical composition
Basic layers of prerecorded CDs and DVDs
The following table summarizes the basic layers of prerecorded CDs and DVDs:
| CD-DA / CD-ROM (single-sided) | DVD-ROM (single-sided, one recorded layer) | DVD-ROM (single-sided, two recorded layers) | DVD-ROM (double-sided, one recorded layer per side) | DVD-ROM (double-sided, two recorded layers per side) |
|---|---|---|---|---|
| Label, optional | Label, optional | Label, optional | Label, optional (hub area only) | Label, optional (hub area only) |
| Lacquer | Polycarbonate | Polycarbonate | Polycarbonate | Polycarbonate |
| Metal | Centre adhesive | Metal (fully-reflective) | Metal | Metal (semi-reflective) |
| Polycarbonate | Metal | Centre adhesive | Centre adhesive | Adhesive |
| Polycarbonate | Metal (semi-reflective) | Metal | Metal (fully-reflective) | |
| Polycarbonate | Polycarbonate | Centre adhesive | ||
| Label, optional (hub area only) | Metal (fully-reflective) | |||
| Adhesive | ||||
| Metal (semi-reflective) | ||||
| Polycarbonate | ||||
| Label, optional (hub area only) |
Table adapted from
Byers, Fred R. Care and handling of CDs and DVDs: a guide for librarians and archivists. Council on Library and Information Resources and National Institute of Standards and Technology (2003).
https://www.clir.org/pubs/reports/reports/pub121/pub121.pdf.
Basic layers of recordable and rewritable CDs and DVDs
The following table summarizes the basic layers of recordable and rewriteable CDs and DVDs:
| CD-R, CD-RW | DVD-R, DVD-RW, DVD+R, DVD+RW, DVD-RAM (single-sided) | DVD-R, DVD-RW, DVD+R, DVD+RW, DVD-RAM (double-sided) |
|---|---|---|
| Label, optional | Label, optional | Label, optional (hub area only) |
| Lacquer | Polycarbonate | Polycarbonate |
| Metal | Centre adhesive | Recording/writing layer (dye) |
| Recording/writing layer (dye) | Metal | Metal |
| Polycarbonate | Recording/writing layer (dye) | Centre adhesive |
| Polycarbonate | Metal | |
| Recording/writing layer (dye) | ||
| Polycarbonate | ||
| Label, optional (hub area only) |
Source
https://www.clir.org/pubs/reports/reports/pub121/pub121.pdf.
Base layer
The base layer is polycarbonate.
Dye layer
| Dye | Colour | Description |
|---|---|---|
| Cyanine | Blue-green | Synthetic dye with many uses in industry. Green and light blue dyes used in CD-R and DVD-R media are chemically unstable and must be stabilized. Least desirable dye. |
| Phthalocyanine | Gold | Was available for many years, but was not widely used until the early 2000s. |
| Azo | Silver-blue |
Types of optical discs
Risk assessment
- Optical media formats were conceived with data preservation in mind, but digital material stored on optical disc is now considered to be at high-risk of loss or degradation.
- Scratches from handling or faulty machines can result in data loss.
- Many discs are manufatured with low cost dyes and metals.
- Pens or markers with hard tips or solvents can affect the protective data layer.
- Optical discs can be affected by ambient heat and direct sunlight.
- Risk associated with the lack of expertise in identifying optical media formats.
- Scratches from handling or faulty machines can result in data loss.
Stability of optical discs depends on the type of disc and the combination of dyes and metals used to manufacture the disc:
Image source
MAM-A, Inc. CD-R Dyes: How to Tell What's What. Accessed November 3, 2017.- Digital material stored on optical disc should be appraised or re-appraised to determine the nature and significance of the material and whether it has sufficient value to justify reformatting and long-term preservation as digital files.
Types of damage and deterioration
| Problem | Description | Cause | Remedy |
|---|---|---|---|
| Breaks or cracks | Breaks or cracks usually cause optical discs to become imbalanced, which makes it impossible for the disc to spin in the drive. | ||
| Delamination | CDs are more vulnerable to delaminiation than DVDs because the thin layers susceptible to delamination are at the surface. | Adhesive label removal Rapid and extreme fluctuations in temperature and/or relative humidity (RH) Physcal stress (e.g., bending of the disc) | No remedy. Error correction systems in playback equipment may compensate for minimal delamination. |
| Scratches | |||
| Dirt/debris | |||
| Warping | |||
| Damage to moulded pits in read-only discs | Pre-recorded discs store digital information in "pits" that are moulded into the top of the base layer. | Pressure from sharp objects (e.g., pencil or pen) Physical stress (e.g., bending) | No remedy. Optical disc systems rely on the error correction system to compensate for errors introduced by damage to moulded pits. |
| Fading of dyes in recordable discs | |||
| Damage to or degredation of the base layer | |||
| Damage to or degredation of the lacquer layer | |||
| Physical damage to or oxidization of the metal reflective layer | |||
| Manufacturing defects | Manufacturing defects include impure reflective metal layers, thin layers, flaws in the pit moulding process, inconsistent dye layers. | Defects are introduced at the time of manufacture. | No remedy. Optical disc systems rely on the error correction system to compensate for errors introduced by manufacturing defects. |
| Damage to the table of contents | The table of contents contains file system information and other data that is essential for successful playback. | Chemical or physical damage to inner portion of discs | No remedy. If the table of contents is damaged in a manner that cannot be compensated for by the error correction system or repaired by one of the previously stated remedies, the disc will not be be playable. |
Source
Adapted from Joe Iraci, Remedies for deteriorated or damaged modern information media carriers - Canadian Conservation Institute (CCI) Technical Bulletin Number 27, Ottawa: Minister of Public Works and Government Services Canada (2005).
Technical standards
General standards
- ISO standard 9660:1988. Information processing -- Volume and file structure of CD-ROM for information interchange.
CD-DA (Digital Audio)
- Red Book - IEC standard 60908:1999. Audio recording - Compact disc digital audio system.
CD-ROM
- Yellow Book - ISO/IEC standard 10149:1995. Information technology -- Data interchange on read-only 120 mm optical data disks (CD-ROM)
Software tools for working with optical discs
| Tool | Open-source / Proprietary | Description | Uses |
|---|---|---|---|
| CueRipper | |||
| DBPowerAmp | Proprietary | DBPowerAmp's "CD Ripper" tool offers multiple audio encoders and other useful features. | Recover audio tracks as single WAV file with cue sheet |
| Exact Audio Copy (EAC) | Ripping audio tracks as WAV files | ||
| ISOBuster | Proprietary | Data recovery Review properties of disc (useful for identifying CD-DA formats) | |
| Opti Drive Control | Proprietary | Opti Drive Control is a program for testing optical drives and optical media | Testing optical drives and optical media |
| Sound eXchange (SoX) | De-emphasis |
References
Bradley, Kevin. "Risks associated with the use of recordable CDs and DVDs as reliable storage media in archival collections - strategies and alternatives." Paris: UNESCO 2006). http://unesdoc.unesco.org/images/0014/001477/147782E.pdf.
Duryee, Alexander. "An introduction to optical media preservation." Code4Lib 24 (April 2014). http://journal.code4lib.org/articles/9581.
Eargle, J.M. The Compact Disc (CD). In Handbook of Recording Engineering. Boston: Springer (1996): 461-466.
Iraci, Joe. Remedies for deteriorated or damaged modern information media carriers - Canadian Conservation Institute (CCI) Technical Bulletin Number 27. Ottawa: Minister of Public Works and Government Services Canada (2005).
Iraci, Joe. Longevity of Recordable CDs and DVDs - Canadian Conservation Institute (CCI) Notes 19/1. Ottawa: Minister of Public Works and Government Services Canada (2010).
Pohlmann, K. The Compact Disc Handbook. Madison, WI: A-R Editions (1992).