CHAPTER 13

Figure 13.5 shows how this data would look when actually written to a CD.
Figure 13.5 EFM data encoding on a CD.
What Is a CD-ROM? Chapter 13 701
The edges of the pits are translated into the binary 1 bits. As you can see, each 14-bit grouping is used
to represent a byte of actual EFM encoded data on the disc, and each 14-bit EFM code is separated by
three merge bits (all 0s in this example). The three pits produced by this example are 4T (4 transitions),
8T, and 4T long. The string of 1s and 0s on the top of the figure represent how the actual data
would be read; note that a 1 is read wherever a pit-to-land transition occurs. It is interesting to note
that this drawing is actually to scale, meaning the pits (raised bumps) would be about that long and
wide relative to each other. If you could use a microscope to view the disc, this is what the word
“NO” would look like as actually recorded.
CD Drive Speed
When a drive seeks out a specific data sector or musical track on the disc, it looks up the address of
the data from a table of contents contained in the lead-in area and positions itself near the beginning
of this data across the spiral, waiting for the right string of bits to flow past the laser beam.
Because CDs originally were designed to record audio, the speed at which the drive reads the data had
to be constant. To maintain this constant flow, CD-ROM data is recorded using a technique called
constant linear velocity (CLV). This means that the track (and thus the data) is always moving past the
read laser at the same speed, which originally was defined as 1.3 meters per second. Because the track
is a spiral that is wound more tightly near the center of the disc, the disc must spin at various rates to
maintain the same track linear speed. In other words, to maintain a CLV, the disk must spin more
quickly when reading the inner track area and more slowly when reading the outer track area. The
speed of rotation in a 1x drive (1.3 meters per second is considered 1x speed) varies from 540rpm
when reading the start (inner part) of the track down to 212rpm when reading the end (outer part) of
the track.
In the quest for greater performance, drive manufacturers began increasing the speeds of their drives
by making them spin more quickly. A drive that spins twice as fast was called a 2x drive, one that
spins four times faster was called 4x, and so on. This was fine until about the 12x point, where drives
were spinning discs at rates from 2,568rpm to 5,959rpm to maintain a constant data rate. At higher
speeds than this, it became difficult to build motors that could change speeds (spin up or down) as
quickly as necessary when data was read from different parts of the disc. Because of this, most drives
rated faster than 12x spin the disc at a fixed rotational, rather than linear speed. This is termed
constant angular velocity (CAV) because the angular velocity (or rotational speed) is what remains a
constant.
CAV drives are also generally quieter than CLV drives because the motors don’t have to try to accelerate
or decelerate as quickly. A drive (such as most rewritables) that combines CLV and CAV technologies
is referred to as Partial-CAV or P-CAV. Most writable drives, for example, function in CLV mode
when burning the disc and in CAV mode when reading. Table 13.6 compares CLV and CAV.