CDs, DVDs and Blu-ray discs are called optical disks because they are read and written using a laser. A device which reads or writes an optical disk is called an optical drive.
The CD (Compact Disc) was originally introduced to distribute digital music, and is still used for that purpose. The same technology was later adapted to store or distribute computer files - software (in particular games), and documents.
CDs have a capacity of about 650 MBytes.
CDs have been mainly replaced by DVDs, which have a higher capacity for the same cost. They are still used for music.
DVD stands for Digital Versatile Disc (it was originally Digital Video Disk, but they are now used for much more than just video, so the V now stands for versatile). They are used for movies and storing or distributing computer files.
A standard DVD can hold about 4.7 GBytes. Double density discs are available, which can hold twice as much data, but they are less commonly used.
There are 3 main types of DVD.
A DVD-ROM (DVD - Read Only Memory) is a read-only data DVD, used for distribution of software and games. The data is pressed onto the DVD as it is manufactured. DVDs produced in this way are very cheap to mass produce, but each DVD must contain exactly the same data. The data cannot be altered after the DVD has been created.
To produce DVD-ROMs, it is necessary to create a glass master disc which is used to press the DVDs. This makes it quite expensive to produce a small number of DVD-ROMs (it is cheaper to use writeable DVDs), but when the cost of creating the master is spread over thousands of copies it is not significant.
A DVD-R (DVD - Recordable) is a writeable DVD. You can write data to it once, but it cannot be erased or rewritten afterwards. It is a useful alternative to DVD-ROMs if not many are required because it avoids the cost of creating a master disk. It is also useful for creating backups and archives, because individual blank discs are relatively inexpensive.
A DVD-RW (DVD - Rewritable) is a DVD which can be written many times, similar to a hard drive. It can be used for creating backups or for transporting data around, much like a memory stick.
Blu-ray Discs are a successor to DVDs, with a higher capacity, which allows them to store high resolution videos. When used for data storage, they can hold more data than a DVD.
A standard BD (Blu-ray Disc) can store 25GByte, a double layer BD can store 50GBytes.
In addition to the standard BD, there are 2 other types. BD-R (BD - Recordable) is a disc which can be written to once only (like a DVD-R). BD-RE (BD - Recordable Erasable) is a disc which can be written, and then erased and rewritten many times.
Computers usually have optical read/write drives:
Read and write speeds are usually expressed as a multiplier on the standard speed of the drive when it is playing music or a movie.
For example when a CD drive is playing a music CD, it holds up to 74 minutes of music, and it reads data from the disc at 0.15 MBytes per second.
Of course, when you are playing music, you don't need the CD to transfer data any faster than its normal rate. You don't usually want to listen to an entire album in 2 minutes, you just want the music to play at its normal speed.
But when you are reading and writing computer files you want the CD, you want to transfer the data as quickly as possible. The drive will have a speed, for example it might say it is a 32x drive. This means it can transfer data at 32 times the normal rate (0.15 MBytes/sec) which is 4.8 MBytes/sec.
The normal data rate for a CD is defined to be 0.15 MBytes per second. This is the data rate required to read 74 minutes of music from the CD in 74 minutes of real time. You could have the fastest CD drive in the world, it would still take it 74 minutes to play 74 minutes worth of music.
The multiplier tells you how fast the CD player is for transferring files. An 8x drive is quite slow, a 32x drive is much better.
A DVD has a normal rate of about 1.3 MBytes/sec when playing back a movie at normal speed. This means that a 16x DVD can transfer data at a little over 20 MBytes/sec.
The 1x rate for a DVD drive is about 9 times faster than the 1x rate for CD drive. Blu-ray is even faster. You can only use the multiplier to compare the same type of drive.
A Blu-ray disk has a normal rate of about 4.3 MBytes/sec.
Higher transfer rates require the disc to spin faster. There is a physical limit to how fast the disc can spin. At high speed it will vibrate and become impossible to read.
Optical discs face competition from other technologies:
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