ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/TEXT/INFO/CDROMSUM.ZIP/CD.2

Text File | 1991-03-12 | 5.2 KB | 113 lines

CD Summary Part 2

CD  Data  Hierarchy

Storing data on a CD may be thought of as occurring through a data
encoding hierarchy with each level built upon the previous one.  At the
lowest level, data is physically stored as pits on the disc.  It is
actually encoded by several low-level mechanisms to provide high storage
density and reliable data recovery.  At the next level, it organized into
tracks which may be digital audio or CD-ROM.  The High Sierra
specification then defines a file system built on CD-ROM tracks.  Finally,
applications like HyperCard specify a content format for files.

The Physical Medium

The Compact Disc itself is a thin plastic disk some 12 cm. in diameter.
Information is encoded in a plastic-encased spiral track contained on the
top of the disk.  The spiral track is read optically by a noncontact head
which scans approximately radially as the disk spins just above it.  The
spiral is scanned at a constant linear velocity thus assuring a constant
data rate.  This requires the disc to rotate at a decreasing rate as the
spiral is scanned from its beginning near the center of the disc to its
end near the disc circumference.

The spiral track contains shallow depressions, called pits, in a
reflective layer.  Binary information is encoded by the lengths of these
pits and the lengths of the areas between them, called land.  During
reading, a low power laser beam from the optical head is focused on the
spiral layer and is reflected back into the head.  Due to the optical
characteristics of the plastic disc and  the wavelength of light used, the
quantity of reflected light varies depending on whether the beam is on
land or on a pit.  The modulated, reflected light is converted to a radio
frequency, raw data signal by a photodetector in the optical head.

Low-level Data Encoding

To ensure accurate recovery, the disc data must be encoded to optimize the
analog-to-digital conversion process that  the radio frequency signal must
undergo.  Goals of the low level data encoding include:

1.  High information density.  This requires encoding that makes the best
possible use of the high, but limited, resolution of the laser beam and
read head optics.

2.  Minimum intersymbol interference.  This requires making the minimum
run length, i.e. the minimum number of consecutive zero bits or one bits,
as large as possible.

3.  Self-clocking.  To avoid a separate timing track, the data should be
encoded so as to allow the clock signal to be regenerated from the data
signal.  This requires limiting the maximum run length of the data so that
data transitions will regenerate the clock.

4.  Low digital sum value (the number of one bits minus the number of zero
bits).  This minimizes the low frequency and DC content of the data signal
which permits optimal servo system operation.

A straightforward encoding would be to simply to encode zero bits as land
and one bits as pits.  However, this does not meet goal (1) as well as the
encoding scheme actually used.  The current CD scheme encodes one bits as
transitions from pit to land or land to pit and zero bits as constant pit
or constant land.

To meet goals (2) to (4), it is not possible to encode arbitrary binary
data.  For example, the integer 0 expressed as thirty-two bits of zero
would have too long a run length to satisfy goal (3).  To accommodate
these goals, each eight-bit byte of actual data is encoded as fourteen
bits of channel data.  There are many more combinations of fourteen bits
(16,384) than there are of eight bits (256).  To encode the eight-bit
combinations, 256 combinations of fourteen bits are chosen that meet the
goals.  This encoding is referred to as Eight-to-Fourteen Modulation (EFM)
coding.

If fourteen channel bits were concatenated with another set of fourteen
channel bits, once again the above goals may not be met.  To avoid this
possibility, three merging bits are included between each set of fourteen
channel bits.  These merging bits carry no information but are chosen to
limit run length, keep data signal DC content low, etc.  Thus, an eight
bit byte of actual data is encoded into a total of seventeen channel bits:
 fourteen EFM bits and three merging bits.

To achieve a reliable self-clocking system, periodic synchronization is
necessary.  Thus, data is broken up into individual frames each beginning
with a synchronization pattern.  Each frame also contains twenty-four data
bytes, eight error correction bytes, a control and display byte (carrying
the subcoding channels), and merging bits separating them all.  Each frame
is arranged as follows:

Sync Pattern24 + 3channel bits
Control and Display byte14 + 3
Data bytes12 * (14 + 3)
Error Correction bytes 4 * (14 + 3)
Data bytes12 * (14 + 3)
Error Correction bytes 4 * (14 + 3)

TOTAL588channel bits

Thus, 192 actual data bits (24 bytes) are encoded as 588 channel bits.

Editorial:  A CD physically has a single spiral track about 3 miles long.
CDs spin at about 500 RPM when reading near the center down to about 250
RPM when reading near the circumference.

Disc with a 'c' or disk with a 'k'?  A usage has emerged for these terms:
disk is used for eraseable disks (e.g. magnetic disks) while disc is used
for read-only (e.g. CD-ROM discs).  One would presumably call a frisbee a
disc.

--andy