An optical drive is a computer that uses a laser to read optical disks (1) like CDs, DVDs, and Blu-ray discs (2). It was envisioned by David Gregg in 1958, who named his invention the Videodisc. David Gregg’s company, Gauss Electrophysics, was bought by MCA around the 1960s. Then in 1978 MCA released the first consumer optical disk in Atlanta and Georgia (3). To the right is an image of David Paul Gregg.
The first optical discs specifically used for computer was the CD-ROM (1), which was released in 1985 (4). The CD-ROM reads only data files or software with a maximum storage of 700 MB. Around the early release of the CDs, people used their CD-ROM disc to listen to music or other media. Then its use shifted to distribute software and data files (1).
DVDs and Blu-rays
DVDs and Blu-rays, released after the 20th century (4) introduced the use of optical drives to store movies for computer system. DVDs and Blu-rays have a double layer format which allows them to store much more data. The double layer format will be discussed later on. DVDs can have a capacity of 4.7 GB. While a Blu-ray has a capacity of 25 GB. Because of popularity and better storage DVDs and Blu-rays have been replacing (6) CD-ROMs, CD-I, CD-ROM/XA, CD-WO, CD-MO, CD-R, Photo-CD, MMCD, and SD over the years (5).
Timeline from source 4
Date release: What type of Optical disc - name of Optical disc (Shortened name)
1982: Compact Disc - Digital Audio (CD-DA)
1985: Compact Disc - Read Only Memory (CD-ROM)
1988: Compact Disc - Interactive (CD-I)
1989: Compact Disc - Read only Memory/Extended Architecture (CD-ROM/XA)
1990: Compact Disc - Write Once (CD-WO) and Compact Disc - Magneto-Optical (CD-MO)
1991: Compact Disc - Recordable (CD-R)
1992: Photo Compact Disc (Photo-CD)
1995: Multimedia Compact Disc (MMCD)
1996: Super Density (SD)
1997: Digital (Video) Disk developed : DVD-ROM and DVD-Video
2000 and onward: DVD-Audio, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-RAM
Optical Drive Parts Edit
The optical drive has many components, but the basics are the laser, the mechanism to read, the drive motor, and a tracking mechanism (1).
The laser is used to read and write the information stored on the optical disc. This electromagnetic wave has a specific wavelength that is usually within or near the visible light spectrum. Depending on the disc, the wavelength will be different. In a compact disc or CD the wavelength is 780 nanometers (nm), which is in the infrared range. The digital disc or the DVDs are read with a wavelength of 650 nanometers (nm), which is a red color light. Blu-ray disc use the shortest wavelengths of 405 nanometers (nm), which is a violet light (1).
If an optical drive needs to read different types of disc they have to use a different lens. CDs and DVDs use the same lens, while Blu-ray discs use a different lens (1).
Reading the Optical Disc
As laser continuously fires on the disc, a mechanism uses photodiodes to read the reflections and translates then to binary code. If there is a reflection the computer read the data as binary ‘0’. If there is a lack of reflection from the disc there is a binary ‘1’ (4). To the right is an image showing how the computer read the reflections and why it is important to keep your optical disc in good condition.
The drive Motor spins the disc at a calculated speed to read or write the data. The rotations spin between 200-500 rotations per minute (rpm) (1). The optical drive spins at a constant linear velocity (CLV), which means it has to vary speeds depending on where the laser is reading. For example the while the laser reads the outer rim of the disc the rpm is 200, while the rpm in the inner rim is 500. The rotation is important for data like music or movies, which need to be read at a certain speed. For discs containing reading, writing, or downloading data the rotation is not as important (2).
A tracking mechanism moves the laser to follow the spiral of data on the disc. The mechanism moves the laser outward and gradually becomes faster. The spindle motor slows the CD down. If you were to untangle the spiral data coiled on the disc it would make a straight line that is 3.5 miles (5 km) long (1).
Optical Discs Edit
Optical discs like CDs or DVDs can hold much more data than their predecessors the classic floppy disc. Most optical discs you encounter are stamped CDs. These are created by injecting a molded polycarbonate plastic onto dye with bumps on the surface. These bumps are called ‘lands’ (5) are 0.5 microns wide, a minimum of 0.85 microns long and have a height of 125 nanometer (6). The laser in the optical drive shines through the transparent polycarbonate to a thin reflective metal layer. The bumps cause the laser’s waves to reflect at an angle so the optical drive cannot detect any reflection. The image to the right shows how the optical drive reads lands and pits.
Next the CD is coated with a thin layer of metal (like silver or aluminum) creating a reflective surface. Then a thin layer of plastic tops the metal and a label for the name or company (5).
CDs to store data
CD that store data cannot be created in the same way stamped CDs are. The disc are made almost exactly the same except a thin layer of dye is added between the polycarbonate and the reflective metal layer. When the layer shines on the dye it causes it to darken, thus creating a non-reflective surface (5).
CD-RW or Rewritable CD
A CD-RW or a Rewritable CD reacts to various levels of heat from the laser to ‘erase or write’. The default is a crystalline or transparent dye, but when it is heated it becomes non-reflective (5).
DVDs and Blu-Ray discs
Commercial DVD and Blu-ray DVDs use multiple layers of polycarbonate mold to hold more data. The disc looks semitransparent with gold that is reflective and a fully aluminum coating for a second laser to reflect from the gold layer for more data. This is called double layers allowing for twice as much data capacity (5). The double layering format uses two layers of polycarbonate to store more data. Although both DVDs and Blu-rays have the same thickness of 1.2 millimeter, however Blu-rays have five times more capability to store data. To the right is an image of a Blu-ray disc and a DVD.
Information is stored between two polycarbonate layers each 06 millimeters thick. With two polycarbonate layers on top of each other there can cause a problem in which the laser's light becomes split. This is caled birefringence. When the beam splits to much the data cannot be read.
Blu-Ray Disc Edit
The name 'Blu-ray' is used for this disc because, unlike DVDs and CDs, a blue laser to read the data. However companies cannot trade make an everyday word like 'Blueray', so the name Blu-ray was given. With a smaller laser to read the data the disc can have pits twice as small as the pits on DVDs.
Blu-ray discs avoid birefringence by having a 1.1 millimeter thick polycarbonate layer so that the recorded layer is closer to the lens.
Works Cited Edit