Unlike HDDs that use the electromagnetic field to read and write data from/to disk platters, optical discs utilize laser beams with a specific wavelength to read and write data from/to compact discs (CDs). Always try to differentiate between the optical disk drives (ODDs) and optical discs, such as a CDs or DVDs. The first is the device where the CD or DVD is inserted. CDs contain tracks in the form of a spiral. The starting point for storing data on CDs is the inner track. As is the case with HDDs, optical discs too are measured by capacity in bytes (B). Usually, the capacity of common CDs is between 650 MB and 700 MB, while common DVDs range from 4.7 GB to 8.5 GB. In contrast to HDDs, the speed of optical discs is measured in KB/s and is determined by an x symbol that is equal to 150 KB/s. That said, if your optical drive has a speed of 24x, then its speed is 24 x 150 KB/s = 3600 KB/s = 3.6 MB/s. There are three recording types of optical disks:
- CD-ROM and DVD-RAM are read-only optical discs
- CD-R and DVD-R/DVD+R are write-once optical discs
- CD-RW and DVD-RW/DVD+RW are rewritable optical discs
The most common forms of optical media at present are DVDs and Blu-ray discs. The latter has been designed to supersede DVD technologies, thus achieving tremendous capacities where a single layer holds 25 GB, a dual-layer holds 50 GB, 100 GB for triple-layer discs, and 128 GB for quadruple-layers.