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What is a dual-layer DVD?
By Administrator | September 21, 2007
DVDs can be manufactured with several different physical configurations. A dual-layer DVD, also known as a DVD-9, is a DVD with two layers of storage. Even though a dual-layer DVD contains two layers, it is not two discs. A dual-layer DVD contains only one set of files. In other words, a dual-layer DVD is only one disc volume.
How is a dual-layer DVD made?
DVDs are manufactured by bonding together two polycarbonate plastic discs that are half the thickness of a finished DVD. Each of these “substrates” is 0.6 mm thick. When the two substrates are bonded together the finished disc is 1.2 mm thick.
Dual-layer DVDs have a track of digital content (represented by tiny “pits”) molded into each of the two disc substrates. When the DVD is bonded together these 2 layers are known as layer 0 and layer 1. Engineers who design systems that use digital storage and processing methods always start counting from 0 instead of counting from 1. This is because digital storage systems such as DVDs and computer memory store information using binary numbers. A binary digit (or bit) is either a one or a zero. So, it is easier to code the 2 layers of a dual-layer DVD as layer 0 and layer 1.
Layer 0 is the “first” layer. When you place a DVD in a drive or player, layer 0 is the layer that is closest to the optical pickup of the drive (it is on the underside of the disc, where the laser from the optical pickup will read the DVD). Layer 0 is molded into the substrate that is on the underside of the disc.
Layer 1 is the second layer, on the top side of the DVD. Since both of the substrates are bonded with the information track on the inside, the 2 layers are actually quite close together. A clear bonding lacquer is used to bond the discs together. This lacquer is applied as a liquid to the center of one of the disc substrates. The second substrate is placed on the first substrate, and the two discs are spun, causing the bonding lacquer to flow outward. The liquid layer spreads outward until it is covering the entire surface between the two discs and no air remains in between the discs. The two discs are pulled together as the liquid layer gets thinner. The bonding machine is adjusted to spin the discs at a speed that results in a separation between the discs of between 40 and 70 microns (a micron is one millionth of a meter, or one thousandth of a millimeter).
Dual layer DVDs can be created so that both layers have an information track that starts at the inside (where the track has a diameter of 48 mm, just under 2 inches) spiraling outward. This format is called Parallel Track Path (PTP). More commonly, DVDs are created so that the second layer (layer 1) starts at the same diameter that the first layer (layer 0) ends. In this case the second layer plays from the outside in. This format is called Opposite Track Path (OTP). The DVD disc always spins counterclockwise when viewed from the read-out side. So, for OTP discs the spiral track on the second layer has a spiral that goes the opposite direction as the first layer. Opposite Track Path discs allow the DVD player to quickly refocus the laser beam in the optical pickup onto layer 1 when the player reaches the end of layer 0. The disc continues to spin at the same rotation speed, since the beginning of layer 1 is at the same radius as the end of layer 0. The player can adjust the focus to layer 1, and then begin to read the track from the outside inward.
How do DVD players read dual-layer DVDs?
So how can DVD players read two layers of information on the same side of the disc? How can you shine a laser beam through the reflective surface on layer 0 to read the track on layer 1? The answer is that layer 0 is only partially reflective. While a single layer DVD has a reflective layer that will reflect 60 to 85% of the red laser light, a dual-layer DVD has a reflectivity of only 18 to 30% on layer 0. This means that layer 1 must be highly reflective, in order to allow the read laser to read through layer 0 while still allowing enough light to return to the optical pickup. While reading layer 1 the read laser will lose 18 to 30% of its energy while passing through layer 0, and then 18 to 30% of the light that is reflected from layer 1 will be lost as the returning beam passes through layer 0 again.
Note: Dual-layer DVDs are typically manufactured with a silver alloy instead of aluminum for the reflective metal layer. This is because aluminum is highly reactive to oxygen in air, and when aluminum oxides it turns into aluminum oxide which transmits the red laser light instead of reflecting it. When dual-layer DVDs were first developed manufacturers used pure gold for the reflective layer. Gold is extremely expensive, and so a more cost-effective material was sought. Silver has a reflectivity that is higher than gold or aluminum at the DVD player’s red laser wavelength of 650 nm. But silver is also reactive and it turns into silver oxide (it tarnishes).
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