Computer Storage Medium: CDs
The Difference Between Various CDs
CD-R stands for Compact Disk Recordable. This type of Compact Disk works best when the recording data will not change. CD-RW, however, stands for Compact Disk Rewritable. These optical compact disks work like a floppy disk and are great for storing small amounts of data overt time, like daily backups of a couple critical files. In the early days of CD-ROM, the recordable drives were CD-R only but most drives today support both CD-R and CD-RW (Wempen).
Quality of CDs as Volatile or Non-Volatile and Random Access
Non-volatile storage can be defined as memory that isn’t lost when the power is turned off. CDs fall under the non-volatile storage category. Like RAM CDs can be accessed directly or “randomly” (“What is Non-Volatile Memory?”). RAM, however, is volatile and the random access can’t be applied to CDs.
The average compact disc usually has a reflective layer of aluminum. When aluminum is exposed to air, it oxidizes, leading to disc rot. Disc rot also happens as a result of damage to disc surfaces, a galvanic reaction between layers, chemical reactions with contaminants, UV light damage or the general break down of disc materials. Most cases of disc rot are a result of inappropriate use or storage. The estimate of a CD’s life span ranges from just a few years to decades depending on material quality and user handling (Sieber).
CDs and the Number of Read/Write Cycles
CD-Rs are mostly a read only and can only be written to once, except with multisession or packet writing. The success of the recording stage depends on if the CD-R is fed an uninterrupted stream of data at the right pace. If the buffer empties and no data is coming from the PC, the writing laser becomes idle, causing a buffer error, and ultimately ruining the disc (Wempen).
A CD-RW, on the other hand, can be both written and read many times. These discs need to be erased before reuse. There are a few different methods for blanking such as full (where the entire surface is cleared) and fast blanking (where only the metadata areas are cleared). As one would imagine, fast blanking is much quicker and, in most cases, sufficient enough to allow rewriting to the disc. Full blanking is beneficial for confidentiality as it removes traces of the former data, although it may still be possible to recover data from this method. Such a recovery is very expensive and is generally only used by government agencies. CD-RWs also have about 1,000 rewriting cycles in their life span, considerably less than other types of storage media. Noting that these discs are usually written and erased in totality makes this somewhat less of a drawback as wear leveling is normally not a problem (“CD-RW”).
Data Bit Encoding Scheme Used for CDs
A CD-ROM (Read Only Media) stores data in binary form and the CD’s surface reflects light. Data stored on the CD with a pattern of pits and unpitted areas. The drive reads the data by shining a laser on the disc in order to measure the amount of light that is bounced back. When the laser notices a transition from pitted to unpitted areas it sends information to the computer as binary data. There is also an error correction in the read process to account for imperfections in the method (Wempen).
Example Usage
CDs are used to store and for a variety of reasons. CD Rs are blank when purchased and are designed to be recorded on once, such as storing family pictures or music playlists. CD-RWs are also blank when purchased and are beneficial for recording multiple times, such as in the case of editing. They were made to replace floppy disks used to store files and programs and are used ideally in the creation of test discs, short or mid-term backup and where an intermediate solution between online and offline storage is needed.
Advantages vs Disadvantages
CDs are small and portable, relatively inexpensive to produce and purchase, can be read by most CD and DVD drives, and are fairly speedy in terms of accessing data. However, CDs are fairly fragile as they can be easily snapped or scratched. They also contain a smaller storage capacity than a hard drive or DVD for example and they have a slower access rate than the hard disk (“Compact Disks”). Some CDs are copy-protected and most won’t play on CD-ROM drives or CD players that use CD-ROM mechanisms which can cause some difficulty (“Compact Disc”).
CDs in History and in the Future
CDs were an evolution of LaserDisc technology and prototypes were developed by Philips and Sony independently in the late 1970s but eventually co-developed by Philips and Sony and then released in 1982. They were later adapted to store data and in October of that same year Japan released the first commercially available audio CD player. At the time of the CD’s beginning, a CD could store more than a computer hard drive, which typically held 10 MB. By 2010, hard drives commonly stored the storage space as a thousand CDs.
As early as the 2000s CDs were being replaced by other types of storage and distribution and by 2010 audio CDs being sold in the U.S. dropped 50%, despite 200 billion CD sales worldwide as of 2007. However, by this time the CD player had largely replaced the cassette player as standard equipment in new cars, with 2010 being the final year for cassette equipped cars (“Compact Disc”).
General Manufacturing and Formatting Processes of CDs
CD-Rs are injection-molded with a blank data spiral. After which, a photosensitive dye is applied and then the discs are metalized and lacquer-coated. The CD-RW uses a metallic alloy instead of a dye. The write laser in this form is used to heat and alter properties of the alloy, changing the reflectivity (“Compact Disc”).