Digital Media Discs By Greg Galloway Above is the image of a dirty disc. Never touch the shiny surface unless you want to invoke the wrath of the owner. Summary: Digital media discs are used every day, but not everyone knows how information is stored on them. DVDs and CDs store immense amounts of music, video, and data, yet as close as you look, you cannot see any pictures. This paper aims at expounding upon the details intrinsic in these storage devices that are used daily. Brief Physical Description: Both types of disc have the same general measurements. Both are 120mm diameter discs of primarily polycarbonate. The center hole is 15mm in diameter. The clear area does not hold data, and ranges from a diameter of 15mm to 46mm. The data region begins at 46mm and ends at 117mm. The area from 46-50mm is the start, or lead in, region. The 116-117mm range is the end, or lead out, region. What Do They Do? Optical discs store information as a continuous line of bumps or dents (depending on perspective, explained later). The devices that read this information direct a laser that spirals outward counter-clockwise from the middle of the disc and determines what has been stored on the surface. For a CD, the tracks of the spiral are 0.5 microns wide, and separated by a distance of 1.6 microns1. For a DVD, the tracks are separated by 0.74 microns, with the tracks being 0.32 microns wide. Disc information is stored spiraling out from the center. A comparison of the surfaces of CDs and DVDs. 1 A micron is a millionth of a meter. For comparison, a human hair is 50 to 100 microns in diameter. (nanopedia.case.edu) Pits Or Bumps? From the perspective of the laser, the surface is covered in bumps. On the surface of a CD, the bumps are 125 nanometers2 high, for a DVD the bumps are 120 nanometers high. The minimum length of the bumps is 830 nanometers for a CD, and 400 nanometers for a DVD. They are sometimes referred to as pits due to the way audio CDs and video DVDs are mass produced. (See references) The surface of a compact disc, or CD. CDs are used primarily for music and data. The spacing between tracks on a CD is 1600 nanometers. On a DVD there are 740 nanometers between tracks. A DVD uses bumps that are only 400 nanometers in length, however a CD has bumps that are 830 nanometers in length (Over twice as long). Data Marking: The information is stored as bits3. Each bump stores information, where a change 4 in state signifies a one. The values to be stored are decided through a process called eight-to-fourteen modulation. This modulation gives the data more resilience to damage and mishandling, such as fingerprints and scratches. (See references for more information.) 2 A nanometer is one billionth of a meter, or 1/1000 th of a micron (Abbreviated nm). 3 A bit is a Binary digit. 4 A state is a bump or a land (flat spot). A change in state would be going from one to the other. Note: Since the label and acrylic are so close to the reflective layer, it is easier to damage the CD with scratches from the top. This is also why care should be taken when writing on CDs. CD cross section, not to scale. Layer Materials: A compact disc is composed of layers of various materials, listed from top down: Label: This is the visual section for human readable information. For professional productions this is silk-screened onto the top. The thickness of this layer is 20 microns. Acrylic: This is a protective layer to help prevent scratches on the label side from corrupting the data patterns on the other side. Aluminum: This is the reflective layer that allows the laser to register the height changes. The reflective layer varies from 50nm to 125nm in thickness. Gold and silver are sometimes used instead of aluminum. Polycarbonate plastic: This layer forms the mold that the aluminum is impressed upon. This Polycarbonate layer is 10 to 30 microns thick. It also provides a protective barrier limiting the effects of scratches and lending strength to the CD as a whole. Dual Layer DVD Technology: Dual layer DVD players use laser focusing to read the subsurface layer. The laser can travel through the polycarbonate layers without significant loss. The first reflecting layer is semi-transparent, allowing the laser to be refocused to ‘see through’ to the second layer. This is analogous to a one-way mirror. The light on one side, if bright and focused, reflects almost entirely. But if diffused, it will be reflected back from the focus point farther in. Layer Translations: Label Polycarbonate Base Metal Reflecting Layer Second Recording Layer Polycarbonate Spacer Semi-transparent Reflector First Recording Layer Polycarbonate Base Storage Comparison: DVD Capacity Movie Duration 5 Single-sided/single-layer 4.38GB 2 hours Single-sided/double-layer 7.95GB 4 hours Double-sided/single-layer 8.75GB 4.5 hours Double-sided/double-layer 15.9GB 8-9 hours CD Capacity Music Duration Single-sided/single-layer 650-754MB 64-75 minutes Further Information6 and References: General DVD information: http://electronics.howstuffworks.com/dvd.htm General CD information: http://electronics.howstuffworks.com/cd.htm Modulation information: http://en.wikipedia.org/wiki/Eight-to-Fourteen_Modulation More on pits and bumps: http://www.kitchenmastering.com/database/1140477795.html In-Depth CD information: http://www.chipchapin.com/CDMedia/index.php3 Images in this document were drawn from the above sources. 5 One GB (gigabyte) is 1,000 Megabytes, or 1,000,000,000 bytes. 6 These web pages are valid as of 02/25/2007.