CHAPTER 13

738 Chapter 13 Optical Storage
recommend a minimum of a 400MHz processor, and of course, even faster would be better. Although
the software decoders can work well, in most cases if you really plan on using your PC to watch DVD
movies, it is worth the extra cost for a hardware MPEG-2 decoder card.
Another performance issue is related to data transfer. Most ATAPI DVD drives support Ultra-DMA
transfers. Be sure you enable DMA transfers in your BIOS setup and in the DVD driver installed in
your operating system. Most installation programs automatically enable DMA support in the driver,
but it’s a good idea to check anyway. Enabling DMA dramatically reduces the load on your processor
and greatly enhances system performance when playing or reading DVDs.
CD/DVD Drives and Specifications
When purchasing a CD or DVD drive for your PC, you should consider three distinct sets of criteria,
as follows:
■ The drive’s performance specifications
■ The interface the drive requires for connection to your PC
■ The physical disc-handling system the drive uses
Performance Specifications
Typical performance figures published by manufacturers are the data transfer rate, the access time, the
internal cache or buffers (if any), and the interface the drive uses.
Data Transfer Rate
The data transfer rate tells you how quickly the drive can read from the disc and transfer to the host
computer. Normally, transfer rates indicate the drive’s capability for reading large, sequential streams
of data.
Transfer speed is measured two ways. The one most commonly quoted with CD/DVD drives is the “x”
speed, which is defined as a multiple of the particular standard base rate. For example, CD-ROM drives
transfer at 153.6KB/sec according to the original standard. Drives that transfer twice that are 2x,
40 times that are 40x, and so on. DVD drives transfer at 1,385KB/sec at the base rate, whereas drives
that are 20 times faster than that are listed as 20x. Note that because almost all faster drives feature
CAV, the “x” speed is usually indicated is a maximum that is seen only when reading data near the
outside (end) of a disc. The speed near the beginning of the disc might be as little as half that, and of
course, average speeds are somewhere in the middle.
With recordable CD drives, the speed is reported for various modes. CD-R drives have two speeds
listed (one for writing, the other for reading), and CD-RW drives have three. On a CD-RW drive, the
speeds are in the form A/B/C, where A is the speed when writing CD-Rs, B is the speed when writing
CD-RWs, and C is the speed when reading. The first CD-RW drive on the market was 2/2/6, with versions
up to 20/10/40 available today.
See the previous sections “CD Drive Speed” and “DVD Drive Speed,” earlier in this chapter, for more
information about speeds and transfer rates.
Access Time
The access time for a CD or DVD drive is measured the same way as for PC hard disk drives. In other
words, the access time is the delay between the drive receiving the command to read and its actual
first reading of a bit of data. The time is recorded in milliseconds; a typical manufacturer’s rating
would be listed as 95ms. This is an average access rate; the true access rate depends entirely on where