How do double sided dvds work

More on the bumps later. Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers.

After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the nonreadable side.

Double-sided discs are printed only on the nonreadable area near the hole in the middle. Cross sections of the various types of completed DVDs not to scale look like this:. Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside.

That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired. What the image to the left cannot impress upon you is how incredibly tiny the data track is -- just nanometers separate one track from the next a nanometer is a billionth of a meter. And the elongated bumps that make up the track are each nanometers wide, a minimum of nanometers long and nanometers high.

The following figure illustrates looking through the polycarbonate layer at the bumps. You will often read about "pits" on a DVD instead of bumps. They appear as pits on the aluminum side, but on the side that the laser reads from, they are bumps. The microscopic dimensions of the bumps make the spiral track on a DVD extremely long. If you could lift the data track off a single layer of a DVD, and stretch it out into a straight line, it would be almost 7.

These pits are roughly half the size of those found on a compact disc, requiring a special laser to read this data. Dual-layer discs or DVD-9 media are effectively two single-layer DVDs fused back-to-back, although are the same overall thickness. This increase in data is achieved by having the reading laser focus on a semi-transparent reflective layer.

The laser then shifts focus as needed, reading past the semi-transparent layer to the second. You can see this happen when watching a dual-layer DVD, with the movie pausing briefly as the laser switches layers.

Not all DVD burners are dual-layer compatible. When ripping data that would normally exceed 4. If you're archiving video, you can encode the data at a higher quality level with less compression using dual-layer media. For those computer drives that do support dual-layer burning, you can purchase slightly more expensive dual-layer media that also prevents potentially having to use multiple discs for a single application.

These drives are labeled accordingly. When looking at single and dual-layer DVD media, you will encounter different designations. The technical difference for the designation refers to the manner in which data is stored. Dual-layer DVDs use both, with the reverse spiral typically on the second layer.