CD VS DVD

Have you ever wondered how does a combo drive play both cd and dvds .?

Why do combo drives often stop reading DVDs but are still able to read CDs?

First lets figure out the essential differnces between CD/ DVDs

DVDs can store more data than CDs for a few reasons:

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

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.

Multiple-Layered 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:

COMBO DRIVES

Every CD/DVD ROM has a lens which is used for reading Data.In case of combo Drive

The same read head lens is used to read both CD's and DVD's

There are two essential physical differences between CD and DVD disc.

First, the smallest DVD pits are only 0.44 micron in diameter; the equivalent CD pits are nearly twice as large, or 0.83 micron wide.

And DVD data tracks are only 0.74 micron apart, wherea 1.6 microns separate CD data tracks. So although a DVD is the same size as a CD,its data spiral is upward of 11 kilometers long-more than twice the length of a CD's data spiral.

To read the smaller pits, a DVD player's readout beam must achieve finer focus than a CD player's does. In order to do this, it uses a read semiconductor laser that has a wave length of 635 to 650 nanometer. In contrast, CD players use infrared laser with a longer wavelength of 780 nanometer. Also, DVD players employ a more powerful focusing lens-one having a higher numerical aperture than the lens in a CD player

In that case after a particular time if the lens weakens only CD's are read and not DVD's.