This page is designed to give detailed DVD information, answering your questions and giving full descriptions with all the 'techno-babble' we could find. 

What Is DVD
How Does DVD Work
DVD vs CD
DVD Basic Info

What Is DVD

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DVDs are of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc. A DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. 


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 (Figure 1). 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 no readable side. Double-sided discs are printed only on the no readable area near the hole in the middle.

What needs to be impressed upon you is how incredibly tiny the data track is just 740 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 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high (Figure 2).

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 (Figure 3). 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 7.5 miles long! That means that a double-sided, double-layer DVD would have 30 miles of data!  To read bumps this small you need an incredibly precise disc-reading mechanism.

The disc-reading mechanism is your DVD player.  When your DVD is loaded into the player, a precision tuned laser, in combination with a tracking motor begin the process of reading the data on your DVD.  Each digital MPEG-2 encoded track is converted to an analog signal (if necessary) of pictures and sound that can be understood by your television.

DVD's can store more data than CDs for a few reasons:

• Higher-density data storage
• Less overhead, more area
• Multi-layer storage

Higher Density Data Storage
Single-sided, single-layer DVDs can store about seven times more data than CDs. A large part of this increase comes from the pits and tracks being smaller on DVDs.

Let's try to get an idea of how much more data can be stored due to the physically tighter spacing of pits on a DVD. The track pitch on a DVD is 2.16 times smaller, and the minimum pit length for a single-layer DVD is 2.08 times smaller than on a CD. By multiplying these two numbers, we find that there is room for about 4.5 times as many pits on a DVD. So where does the rest of the increase come from?

Less Overhead, More Area
On a CD, there is a lot of extra information encoded on the disc to allow for error correction -- this information is really just a repetition of information that is already on the disc. The error correction scheme that a CD uses is quite old and inefficient compared to the method used on DVDs. The DVD format doesn't waste as much space on error correction, enabling it to store much more real information. Another way that DVDs achieve higher capacity is by encoding data onto a slightly larger area of the disc than is done on a CD.  The combination of greater efficiency, technology and design allow DVD to hold a minimum of 7x more information than a Compact Disk.

Multi-Layer Storage
To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. Here is a list of the capacities of different forms of DVDs:

 

You may be wondering why the capacity of a DVD doesn't double when you add a whole second layer to the disc. This is because when a disc is made with two layers, the pits have to be a little longer, on both layers, than when a single layer is used. This helps to avoid interference between the layers, which would cause errors when the disc is played.