Optical Drive Facts

When selecting an optical drive (CD or DVD), consider the following factors.

Factor / Description
Interface / For internal drives, IDE is the most common interface. You can also purchase SCSI CD drives. For external drives, USB or Firewire are common interfaces.
Drive Function / The most important consideration when choosing an optical drive is the types of discs that will be used with the drive as well as the function the drive will perform:
  • Will the drive be used for both CDs and DVDs?
  • Will the drive be used for read-only discs (ROM)?
  • Will the drive be used for recording (CD-R, DVD-R)?
  • Will the drive be used for rewriting (CD-RW, DVD-RW)?
Today, you can typically purchase a drive that supports both CDs and DVDs and can both read and record without much additional cost.
Speed / Drive speed is measured in multiples of the data transfer supported on the first drives.
  • 150 KBps for CD
  • 11 MBps for DVD
A single drive might have multiple speed ratings based on the following factors
  • Media type (either CD or DVD)
  • The operation being performed (record, rewrite, or read)
  • The DVD format supported (such as DVD-R or DVD-RW)
CD speeds often report the speed as a single set of numbers such as 24x10x70. When multiple numbers are listed together, the first number indicates the record speed, the second number is the rewrite speed, and the last number is the read speed. If the drive does not perform rewrite functions, the middle number will be omitted.
Tip: To remember what each number represents, just remember that the highest number is the read speed and the lowest number is the rewrite speed.
Format / When selecting the drive, identify the disc formats the drive supports.
  • On older CD drives, you had to identify whether the drive could read various CD book standards.
  • On DVD drives, you need to verify whether the drive supports the DVD+R/DVD+RW standards.

Loading Type / The loading type identifies how discs are inserted into the drive:
  • Tray loading drives are the most common. When you open the drive, a tray comes out of the drive.
  • Slot-load drives do not have parts that open and close. Instead, a motor inside the drive pulls the disc into the drive. This type of drive allows you to mount the drive vertically instead of horizontally.
  • Some older CD drives required a disc caddy. You would load the disc into the caddy and then insert the caddy into the drive.

Cache / Drives that are used to burn or record discs often include a buffer that is used during the recording process. The cache (or buffer) includes data that is waiting to be written to disc. Using a drive with a larger cache ensures that the drive has a constant stream of data during the recording process.
Note: If the drive runs out of data that is ready to be recorded onto disc, a buffer underrun condition will result. When this happens, the recording process will fail, resulting in a wasted disc (called a coaster).
Additional Features / Drives might also include additional features such as:
  • Bundled recording software
  • Slimline drives are half the size of standard drives

Be aware of the following when working with optical drives:

  • When you place a disc in the drive, it can take several seconds for the drive to recognize the new disc and spin up to speed. If you receive a message saying that the drive is not accessible after trying to access a recently inserted new disc, wait a few seconds and try again.
  • DOS and Windows 95/98 systems assign drive letters to hard drives first, then to CD-ROM/DVD drives. If you install a new hard drive, the drive letter for your optical drive will likely change. Software programs that rely on the old drive letter will likely not run properly until they have been told the correct drive letter for the drive.
  • Some optical drives adjust their rotation speed (RPM) when reading from the edge versus the center of the disc. This allows the drive to read data at a constant rate. These types of drives are called Constant Linear Velocity (CLV) drives. Most 12x and below CD-ROM drives are CLV drives. CLV drives typically operate at slower RPMs than CAV drives. Other drives operate more like hard drives. In other words, they always maintain the same rotation speed (RPM). This simplifies the way the drive rotates, but causes data to be read faster from the edge of the disc than from the center of the disc. These types of CD-ROM drives are called Constant Angular Velocity (CAV) drives. Most drives above 12x are CAV drives.
  • Access time is a general measure of drive performance. Like hard drives, average access time includes average seek time and average latency time. However, it also includes average spin up/down time. This is the time required for a CLV drive to spin up or down to the proper speed to read the data from that particular location of the disc.
  • CAV drives retrieve data more quickly from the edge of the disc than from the center of the disc. Also, latency time is greater toward the center of the disc. CAV try to make up for these factors by operating at a constant RPM that is higher than CLV drives.

Use the following precautions to protect discs:

  • Some discs use a foil placed on the top of the disc instead of imbedding the foil inside the plastic. Be very careful when working with these types of discs. A scratch or even some types of markers can damage this layer.
  • To help prevent scratching, keep the disc in its case when not being used.
  • To minimize the effect of scratches that might be generated while wiping a disc, wipe the disc from the center to the edge in straight lines (like the spokes of a wheel).
  • Keep the disc away from direct sunlight and other sources of heat.