How does dvds work

Digital versatile discs DVDs can store more information than compact discs CDs because they have smaller pits, placed closer together. The tools of technology can seem like a confusing alphabet-soup.

Terms such as ZIP, CD or DVD are commonly used to describe the means available for storing and sharing information, ranging from text to audio to full-length films.

Of them all, DVD, which stands for digital versatile disc, is poised to become the most popular and reliable means for storing data, especially high-quality digital video. DVDs are now rapidly making their way into American homes as the state-of-the-art entertainment option for watching Hollywood releases. Before going into the mechanics of how a DVD works, it might be best to explain the why.

In other words, why is the industry replacing previous storage-unit standards, including the CD short for compact disc? Quite simply, the DVD's storage capacity is much greater. A high-quality digital video could simply never fit onto a CD. It can't even fit comfortably on a computer hard drive. Indeed, a high-quality digital video requires up to megabytes MB of data space each minute, depending on the amount of compression used.

MPEG2, a format commonly used for playing video via computer, compresses one minute of visual data into about 30 MB. So a two-minute video requires 60 MB, and a two-hour movie requires 3, MB. Compare that to the maximum storage space of a CD which is about MB.

The encoding of data onto a DVD may seem like a black art, but it's really not that much different from encoding data onto a videotape. A videotape stores and plays back information based on the physical placement and arrangement of iron oxide particles on the material of the tape.

A DVD similarly stores and plays information based on a particular "dot pattern" on its surface. A very precise laser burns these dots--actually, incredibly small pits--on to the master DVD.

It is the smallness of the dots that allows for the roomy storage capacity of the DVD. Practically speaking, let's say I had a virtual reality project of 75 MB on a disc. If I want to release my project to the public or exhibit it in museums across the country, I'm going to need a reliable means for viewing. Most of the optical industry service providers are, not surprisingly, located on the West Coast. But there are regional houses as well. They will feed the information from my disc via software algorithms to what is called a burner, which guides the laser that brands a glass-topped DVD with the data pattern of dots that vary in terms of spacing and in terms of brightness and darkness.

The spacing and variation of brightness and darkness of the dots are what makes the data readable to a computer or to a television set DVD player in the same way that the variety of shapes and spacing of these letters and words enables you to read this explanation.

Then, a photograph is taken of this master DVD and an etching is made from that photograph. In turn, the etching is used to create a metal stamper. The replication house simply uses the metal stamper to imprint the pattern into the plastic coating of all those DVDs. Smaller pits make it possible to store more information on DVDs. What most people find at the local movie rental or video store is called a DVD 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.

A DVD player is powered by electricity. DVD players can either be powered by electricity or batteries. DVDs are of the same shape and thickness as CDs, and they are made using some of the same materials and manufacturing methods.

A DVD is made up of several layers of plastic that is 1. Each layer is made by injection moulding plastic. This forms a disc that has tiny bumps often called pits arranged as a single very long spiral track of data. These bumps are where all the data is stored. The bumps and pits on a DVD are coded information.

Each writeable 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 4.

DVDs are read by a laser.