CHAPTER 13

CD/DVD Drives and Specifications Chapter 13
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the data is located on the disc. When the read mechanism is positioned to a portion of the disc nearer
to the narrower center, the access rate is faster than when it is positioned at the wider outer perimeter.
Access rates quoted by many manufacturers are an average taken by calculating a series of random
reads from a disc.
Obviously, a faster (that is, a lower) average access rate is desirable, especially when you rely on the
drive to locate and pull up data quickly. Access times for CD and DVD drives have been steadily
improving, and the advancements are discussed later in this chapter. Note that these average times are
significantly slower than PC hard drives, ranging from 200ms to below 100ms, compared to the 8ms
access time of a typical hard disk drive. Most of the speed difference lies in the construction of the
drive itself. Hard drives have multiple-read heads that range over a smaller surface area of the
medium; CD/DVD drives have only one laser pickup, and it must be capable of accessing the entire
range of the disc. In addition, the data on a CD is organized in a single long spiral. When the drive
positions its head to read a track, it must estimate the distance into the disc and skip forward or backward
to the appropriate point in the spiral. Reading off the outer edge requires a longer access time
than the inner segments, unless you have a CAV drive, which spins at a constant rate so the access
time to the outer tracks is equal to that of the inner tracks.
Access times have fallen a great deal since the original single-speed drives came out. However, recently
a plateau seems to have been reached with most CD/DVD drives hovering right around the 100ms
area, with some as low as 80ms. With each increase in data transfer speed, you usually see an
improvement in access time as well. But as you can see in Table 13.23, these improvements are much
less significant because of the physical limitation of the drive’s single-read mechanism design.
Table 13.23 Typical CD-ROM Drive Access Times
Drive Speed Access Time (ms)
1x 400
2x 300
3x 200
4x 150
6x 150
8x–12x 100
16x–24x 90
32x–52x or greater 85 or less
The times listed here are typical examples for good drives; within each speed category some drives are
faster and some are slower. Because of the additional positioning accuracy required and the overall
longer track, DVD drives usually report two access speeds—one when reading DVDs and the other
when reading CDs. The DVD access times run usually 10ms–20ms slower than when readingCDs.
Buffer/Cache
Most CD/DVD drivesinclude internal buffers or caches of memory installed onboard. These buffers are
actual memory chips installed on the drive’s circuit board that enable it to stage or store data in larger
segments before sending it to the PC. A typical buffer for a CD/DVD drive is 128KB, although drives
are available that have either more or less (more is usually better). Recordable CD or DVD drives typically
have much larger buffers of 2MB–4MB or more to prevent buffer underrun problems and to
smooth writing operations. Generally, faster drives come with more buffer memory to handle the
higher transfer rates.