Since the introduction of the Multimedia PC, CD-ROM and DVD drives have become a standard peripheral on nearly all computers. Even most laptops now come with built in 24x CD-ROMs. Drives in the market differ in speed, capacity and read/write abilities. This section explains some basic differences and concepts. The CD-ROM scene is changing very rapidly, and as such, few hardware sites contain any information about them - a benchmark would be outdated before it is even published because new drives would simply have advanced to the next level of speed by then!

The definition of a CD-ROM

CD-ROM describes a drive that is able to read digital data stored on a compact disk. Like an audio compact disk player, the CD-ROM uses a laser pickup mechanism to read the stored data. A laser beam shines on the compact disk, and if it is deflected by the disk to a sensor, the computer registers this as a "1". If no light is deflected, it is assumed as a "0". Note that these drives are only able to read data, but not able write data. Up to 680MB (ISO 9660) of digital data can be stored on a single compact disk. The basic CD-ROM spins the compact disk at the same rotational speed that a normal audio compact disk player spins at. This allows for a data transfer rate of 150Kb/s.

What is a 2x CD-ROM?

This is the same as the basic CD-ROM, but the compact disk has a rotational speed twice that of a normal audio compact disk player. This allows double the amount of information to be read from the disk per second - 300Kb/s.

What is a 3x CD-ROM?

This is the same as the basic CD-ROM, but the compact disk has a rotational speed three times that of a normal audio compact disk player. This allows three times the amount of information to be read from the disk per second. The same goes for 4x, 6x, 8x..... I don't think I need to explain this any further.

The 36x limit of single beam CD-ROMs

The 36x CD-ROM is the same as the basic CD-ROM, but it is able to read data up to thirty-six times as fast by using higher rotational speeds via a clever trick known as CAV. Read more about CAV later on in this section. The 36x CD-ROM is the fastest conventional single-beam CD-ROM avaliable in the market. Faster drives are not possible based on current technology because irregularities in the compact disk media make the compact disk wobble at higher speeds. When the compact disk wobbles too much, the laser pick up mechanism is unable to read data from the disk.

Does this mean that the 100x CD-ROM a fake?

There is a CD-ROM in the market called the smart 100x CD-ROM. This is actually a normal 12x CD-ROM with a data cache feature. When a compact disk is read, the data will be stored on the hardisk automatically. This means that the next time the data is needed, it will be read from the hardisk instead of the CD-ROM. Hardisks being many times faster than CD-ROMs, allows this feature to boost the data transfer rate, under certain conditions, to the equivalent data transfer rate of a theoretical 100x speed CD-ROM.

Breaking the 36x limit with TrueX

TrueX is a multi-beam technology introduced by Zen Research that allows the 36x speed limit to be overcome by diffracting a single laser beam into several seperate beams. Each of the beams then reads a seperate track on the compact disk and the resulting data collected is than reassembled together by means of signal processing. The first TrueX CD-ROM released was a CAV 40x unit.

What about CDR Drives?

Most modern CDR drives are able to do multisession packet writing and can be used many times until the compact disk fills up. Because of this, although data once written cannot be erased, it is still not exactly a WORM (write once, read many times) drive. It is both able to read and write on compact disks, but the compact disks it writes on has to be a special type of disk (CDR media). These CDR disks can store up to 650MB and recording a full 650MB disc can take between 15mins (4x) to 1hr (1x). When writing, a high powered laser is beamed into the recording layer which turns the dye darker, thus creating dark spots that simulate pits found in normal compact disks.

Definition of a DVD-ROM

A Digital Video Disk (DVD) is similar to a normal compact disk, but it is able to store more than 4Gb of digital data on a single disk. The drive has a different frequency laser from normal CD-ROMs and hence DVD media cannot be used on a normal CD-ROM. DVD drives however, are backward compatible, the exception being CD-R media, with normal compact disks.

CAV CD-ROMs spin the compact disk such as to maintain a constant angular rotational speed of the compact disk. Since compact disks store more data at its edges (because of a larger circumference) than at near the center, the speed at which the data is read increases as the laser moves to read data stored at the edges of the compact disk. Typically twice the amount of data is read in 1 revolution at the edges than at near the center. Speeds quoted for CAV drives are the maximum speed at which data can be read (at the edges!). So a 32x CD-ROM drive actually has a maximum speed of 32x and a minumum speed of 16x. When playing audio compact disks, these CD-ROM drives are able to switch to CLV mode.