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<p align="center"><font size="6" color="#0000ff">cd-rom technical summary</font></p>
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<p> cd-rom technical summary<br>
from plastic pits to "fantasia"<br>
<br>
andy poggio<br>
march 1988<br>
<br>
<br>
abstract<br>
<br>
this summary describes how information is encoded on compact disc (cd)<br>
beginning with the physical pits and going up through higher levels of<br>
data encoding to the structured multimedia information that is<br>
possible with programs like hypercard. this discussion is much<br>
broader than any single standards document, e.g. the cd-audio red<br>
book, while omitting much of the detail needed only by drive<br>
manufacturers.<br>
<br>
<br>
salient characteristics<br>
<br>
1. high information density -- with the density achievable using<br>
optical encoding, the cd can contain some 540 megabytes of data on a<br>
disc less than five inches in diameter.<br>
<br>
2. low unit cost -- because cds are manufactured by a well-developed<br>
process similar to that used to stamp out lp records, unit cost in<br>
large quantities is less than two dollars.<br>
<br>
3. read only medium -- cd-rom is read only; it cannot be written on<br>
or erased. it is an electronic publishing, distribution, and access<br>
medium; it cannot replace magnetic disks.<br>
<br>
4. modest random access performance -- due to optical read head mass<br>
and data encoding methods, random access ("seek time") performance of<br>
cd is better than floppies but not as good as magnetic hard disks.<br>
<br>
5. robust, removable medium -- the cd itself is comprised mostly of,<br>
and completely coated by, durable plastic. this fact and the data<br>
encoding method allow the cd to be resistant to scratches and other<br>
handling damage. media lifetime is expected to be long, well beyond<br>
that of magnetic media such as tape. in addition, the optical servo<br>
scanning mechanism allows cds to be removed from their drives.<br>
<br>
6. multimedia storage -- because all cd data is stored digitally, it<br>
is inherently multimedia in that it can store text, images, graphics,<br>
sound, and any other information expressed in digital form. its only<br>
limit in this area is the rate at which data can be read from the<br>
disc, currently about 150 kbytes/second. this is sufficient for all<br>
but uncompressed, full motion color video.<br>
<br>
<br>
cd data hierarchy<br>
<br>
storing data on a cd may be thought of as occurring through a data<br>
encoding hierarchy with each level built upon the previous one. at<br>
the lowest level, data is physically stored as pits on the disc. it<br>
is actually encoded by several low-level mechanisms to provide high<br>
storage density and reliable data recovery. at the next level, it<br>
organized into tracks which may be digital audio or cd-rom. the high<br>
sierra specification then defines a file system built on cd-rom<br>
tracks. finally, applications like hypercard specify a content format<br>
for files.<br>
<br>
<br>
the physical medium<br>
<br>
the compact disc itself is a thin plastic disk some 12 cm. in<br>
diameter. information is encoded in a plastic-encased spiral track<br>
contained on the top of the disk. the spiral track is read optically<br>
by a noncontact head which scans approximately radially as the disk<br>
spins just above it. the spiral is scanned at a constant linear<br>
velocity thus assuring a constant data rate. this requires the disc<br>
to rotate at a decreasing rate as the spiral is scanned from its<br>
beginning near the center of the disc to its end near the disc<br>
circumference.<br>
<br>
the spiral track contains shallow depressions, called pits, in a<br>
reflective layer. binary information is encoded by the lengths of<br>
these pits and the lengths of the areas between them, called land.<br>
during reading, a low power laser beam from the optical head is<br>
focused on the spiral layer and is reflected back into the head. due<br>
to the optical characteristics of the plastic disc and the wavelength<br>
of light used, the quantity of reflected light varies depending on<br>
whether the beam is on land or on a pit. the modulated, reflected<br>
light is converted to a radio frequency, raw data signal by a<br>
photodetector in the optical head.<br>
<br>
<br>
low-level data encoding<br>
<br>
to ensure accurate recovery, the disc data must be encoded to optimize<br>
the analog-to-digital conversion process that the radio frequency<br>
signal must undergo. goals of the low level data encoding include:<br>
<br>
1. high information density. this requires encoding that makes the<br>
best possible use of the high, but limited, resolution of the laser<br>
beam and read head optics.<br>
<br>
2. minimum intersymbol interference. this requires making the<br>
minimum run length, i.e. the minimum number of consecutive zero bits<br>
or one bits, as large as possible.<br>
<br>
3. self-clocking. to avoid a separate timing track, the data should<br>
be encoded so as to allow the clock signal to be regenerated from the<br>
data signal. this requires limiting the maximum run length of the<br>
data so that data transitions will regenerate the clock.<br>
<br>
4. low digital sum value (the number of one bits minus the number of<br>
zero bits). this minimizes the low frequency and dc content of the<br>
data signal which permits optimal servo system operation.<br>
<br>
a straightforward encoding would be to simply to encode zero bits as<br>
land and one bits as pits. however, this does not meet goal (1) as<br>
well as the encoding scheme actually used. the current cd scheme<br>
encodes one bits as transitions from pit to land or land to pit and<br>
zero bits as constant pit or constant land.<br>
<br>
to meet goals (2) to (4), it is not possible to encode arbitrary<br>
binary data. for example, the integer 0 expressed as thirty-two bits<br>
of zero would have too long a run length to satisfy goal (3). to<br>
accommodate these goals, each eight-bit byte of actual data is encoded<br>
as fourteen bits of channel data. there are many more combinations of<br>
fourteen bits (16,384) than there are of eight bits (256). to encode<br>
the eight-bit combinations, 256 combinations of fourteen bits are<br>
chosen that meet the goals. this encoding is referred to as<br>
eight-to-fourteen modulation (efm) coding.<br>
<br>
if fourteen channel bits were concatenated with another set of<br>
fourteen channel bits, once again the above goals may not be met. to<br>
avoid this possibility, three merging bits are included between each<br>
set of fourteen channel bits. these merging bits carry no information<br>
but are chosen to limit run length, keep data signal dc content low,<br>
etc. thus, an eight bit byte of actual data is encoded into a total<br>
of seventeen channel bits: fourteen efm bits and three merging bits.<br>
<br>
to achieve a reliable self-clocking system, periodic synchronization<br>
is necessary. thus, data is broken up into individual frames each<br>
beginning with a synchronization pattern. each frame also contains<br>
twenty-four data bytes, eight error correction bytes, a control and<br>
display byte (carrying the subcoding channels), and merging bits<br>
separating them all. each frame is arranged as follows:<br>
<br>
sync pattern 24 + 3 channel bits<br>
control and display byte 14 + 3<br>
data bytes 12 * (14 + 3)<br>
error correction bytes 4 * (14 + 3)<br>
data bytes 12 * (14 + 3)<br>
error correction bytes 4 * (14 + 3)<br>
<br>
total 588 channel bits<br>
<br>
thus, 192 actual data bits (24 bytes) are encoded as 588 channel bits.<br>
<br>
editorial: a cd physically has a single spiral track about 3 miles<br>
long. cds spin at about 500 rpm when reading near the center down to<br>
about 250 rpm when reading near the circumference.<br>
<br>
disc with a 'c' or disk with a 'k'? a usage has emerged for these<br>
terms: disk is used for eraseable disks (e.g. magnetic disks) while<br>
disc is used for read-only (e.g. cd-rom discs). one would presumably<br>
call a frisbee a disc.<br>
<br>
<br>
first level error correction<br>
<br>
data errors can arise from production defects in the disk itself,<br>
defects arising from subsequent damage to the disk, or jarring during<br>
reading. a significant characteristic of these errors is that they<br>
often occur in long bursts. this could be due, for example, to a<br>
relatively wide mark on the disc that is opaque to the laser beam used<br>
to read the disc. a system with two logical components called the<br>
cross interleave reed-solomon coding (circ) is employed for error<br>
correction. the cross interleave component breaks up the long error<br>
bursts into many short errors; the reed-solomon component provides the<br>
error correction.<br>
<br>
as each frame is read from the disc, it is first decoded from fourteen<br>
channel bits (the three merging bits are ignored) into eight-bit data<br>
bytes. then, the bytes from each frame (twenty-four data bytes and<br>
eight error correction bytes) are passed to the first reed-solomon<br>
decoder which uses four of the error correction bytes and is able to<br>
correct one byte in error out of the 32. if there are no<br>
uncorrectable errors, the data is simply passed along. if there are<br>
errors, the data is marked as being in error at this stage of<br>
decoding.<br>
<br>
the twenty-four data bytes and four remaining error correction bytes<br>
are then passed through unequal delays before going through another<br>
reed-solomon decoder. these unequal delays result in an interleaving<br>
of the data that spreads long error bursts among many different passes<br>
through the second decoder. the delays are such that error bursts up<br>
to 450 bytes long can be completely corrected. the second<br>
reed-solomon decoder uses the last four error correction bytes to<br>
correct any remaining errors in the twenty-four data bytes. at this<br>
point, the data goes through a de-interleaving process to restore the<br>
correct byte order.<br>
<br>
<br>
subcoding channels and blocks<br>
<br>
the eight-bit control and display byte in each frame carries the<br>
subcoding channels. a subcoding block consists of 98 subcoding bytes,<br>
and thus 98 of the 588-bit frames. a block then can contain 2352<br>
bytes of data. seventy-five blocks are read each second. with this<br>
information, it is now straightforward to calculate that the cd data<br>
rate is in fact correct for cd digital audio (cd-da):<br>
<br>
required cd digital audio data rate: 44.1 k samples per second * 16<br>
bits per sample * 2 channels = 1,411,200 bits/sec.<br>
<br>
cd data rate: 8 bits per byte * 24 bytes per frame * 98 frames per<br>
subcoding block * 75 subcoding blocks per second = 1,411,200 bits/sec.<br>
<br>
the eight subcoding channels are labeled p through w and are encoded<br>
one bit for each channel in a control and display byte. channel p is<br>
used as a simple music track separator. channel q is used for control<br>
purposes and encodes information like track number, track type, and<br>
location (minute, second, and frame number). during the lead-in track<br>
of the disc, channel q encodes a table of contents for the disk giving<br>
track number and starting location. standards have been proposed that<br>
would use the remaining channels for line graphics and ascii character<br>
strings, but these are seldom used.<br>
<br>
<br>
track types<br>
<br>
tracks can have two types as specified in the control bit field of<br>
subchannel q. the first type is cd digital audio (cd-da) tracks. the<br>
two-channel audio is sampled at 44.1 khz with sixteen bit linear<br>
sampling encoded as twos complement numbers. the sixteen bit samples<br>
are separated into two eight-bit bytes; the bytes from each channel<br>
alternate on the disc. variations for audio tracks include<br>
pre-emphasis and four track recording.<br>
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