Data Storage: DVD vs. CD

DVDs 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.

Specification	CD	DVD
Track Pitch	1600 nanometer	740 nanometers
Minimum Pit Length (single-layer DVD)	830 nanometers	400 nanometers
Minimum Pit Length (double-layer DVD)	830 nanometers	440 nanometers

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?

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.

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:

Format	Capacity	Approximate Movie Length
Single Sided/Single Layer	4.38 GB	2 hours
Single Sided/Double Layer	7.95 GB	4 hours
Double Sided/Single Layer	8.75 GB	4.5 hours
Double Sided/Double Layer	15.9 GB	Over 8 hours

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.