Hardware Reference
In-Depth Information
What the Yellow Book adds is the grouping of 98 frames into a
CD-ROM sec-
tor
, as shown in Fig. 2-26. Every CD-ROM sector begins with a 16-byte pream-
ble, the first 12 of which are 00FFFFFFFFFFFFFFFFFFFF00 (hexadecimal), to
allow the player to recognize the start of a CD-ROM sector. The next 3 bytes con-
tain the sector number, needed because seeking on a CD-ROM with its single data
spiral is much more difficult than on a magnetic disk with its uniform concentric
tracks. To seek, the software in the drive calculates approximately where to go,
moves the head there, and then starts hunting around for a preamble to see how
good its guess was. The last byte of the preamble is the mode.
…
Symbols of
14 bits each
42 Symbols make 1 frame
Frames of 588 bits,
each containing
24 data bytes
…
98 Frames make 1 sector
Preamble
Mode 1
sector
(2352 bytes)
Data
ECC
Bytes 16
2048
288
Figure 2-26.
Logical data layout on a CD-ROM.
The Yellow Book defines two modes. Mode 1 uses the layout of Fig. 2-26,
with a 16-byte preamble, 2048 data bytes, and a 288-byte error-correcting code (a
cross-interleaved Reed-Solomon code). Mode 2 combines the data and ECC fields
into a 2336-byte data field for those applications that do not need (or cannot afford
the time to perform) error correction, such as audio and video. Note that to pro-
vide excellent reliability, three separate error-correcting schemes are used: within a
symbol, within a frame, and within a CD-ROM sector. Single-bit errors are cor-
rected at the lowest level, short burst errors are corrected at the frame level, and
any residual errors are caught at the sector level. The price paid for this reliability
is that it takes 98 frames of 588 bits (7203 bytes) to carry a single 2048-byte pay-
load, an efficiency of only 28 percent.
Single-speed CD-ROM drives operate at 75 sectors/sec, which gives a data rate
of 153,600 bytes/sec in mode 1 and 175,200 bytes/sec in mode 2. Double-speed
drives are twice as fast, and so on up to the highest speed. A standard audio CD
has room for 74 minutes of music, which, if used for mode 1 data, gives a capacity
of 681,984,000 bytes. This figure is usually reported as 650 MB because 1 MB is
2
20
bytes (1,048,576 bytes), not 1,000,000 bytes.
As usual, whenever a new technology comes out, some people try to push the
envelope. When designing the CD-ROM, Philips and Sony were cautious and had
the writing process stop well before the outer edge of the disc was reached. It did
