Overview of CD-ROM Capacity
CD-ROM is one of the most cost effective and reliable
methods of storing and distributing data and SW. The
ability to store hundreds of megabytes of data on a disc
seemed to be almost unlimited 10-12 years ago when CD-ROM
was being developed. Most early CD-ROM applications did
not begin to approach a disc's maximum capacity. However,
as data and applications have become more complex, the
requirements to store larger amounts of information onto
CD-ROMs has become even greater. When your data or
application goes beyond today's maximum capacity, you must
go to multiple discs or use some sort of data compression.
In the not to distant future however the current density
or maximum amount of data stored on a CD-ROM will be
increased by new mastering, laser and CD-ROM drive
technology.
Today's Maximum
The fundamental unit of data on a CD-ROM is the sector.
Every CD-ROM is composed of a given amount of sectors. The
amount of user data contained in each sector depends on
the mode type of the sector. Mode 1 sectors (the most
common) contain 2048 bytes of user data. Most DOS, Mac,
and UNIX CD-ROMs are mode 1. Mode 2 sectors contain 2336
bytes of user data. Examples of mode 2 CD-ROMs would be
XA, Photo-CD, and CD-I. The maximum amount of sectors that
can be currently mastered onto a DMI CD-ROM is 345,000.
Therefore the maximum amount of user-data for a DMI CD-ROM
(mode 1 or mode 2) is the following:
Mode 1 Mode 2
(345K x 2048) (345K x 2336)
Bytes 706,560,000 805,920,000
Mega-Bytes (MB) 673 768
It should be noted that a MB (Mega-Byte) is made up of
1,048,576 bytes. Millions of bytes should not be confused
with MB. MB is the more common unit used to describe
CD-ROM capacity. Since there are 75 sectors per second the
maximum length of a DMI CD-ROM in time would be:
345,000 sectors/75(sectors/second) = 4600 seconds = 76
min. 40 sec.
The maximum length of a CD-ROM is usually given in
data. However, data and time are interchangeable.
Compressed CD-ROMs
If the size of your data or application on your CD-ROM
is going to go over the above maximum then you will need
to make a decision on how you are going to work around
this issue. First of all you could decrease the size of
your data or application. This works but is probably not
practical! Second of all you could split your data or
application across two (or several) CD-ROMs. This approach
depending on the application might or might not be
practical. Having more than one disc will impact your cost
and packaging considerations. If the above two approaches
are not satisfactory then data compression might be your
next best approach. Depending on the type of data you have
on your disc, data compression might be the way to go. If
you have data on your disc that is already in a compressed
form like MPEG video then data compression will probably
not work to well. However, if you have data that is not
already compressed then data compression will probably
bring you benefits. Some CD-ROM data compression companies
claim you can increase the amount of data stored on a
CD-ROM by up to a factor of 4 or more. This technology
could increase your maximum amount of data stored on a
CD-ROM to over 2 GB. Remember the type of data you are
compressing will affect how good of compression you will
achieve.
Most CD-ROM compression technologies have to process on
the data you want to compress for several hours. Some can
compress in real-time. When the compression is done you
usually have one big file with all of the compressed data
in it. You then can distribute this file along with SW
that can perform the decompression. Once the decompression
SW is installed you can view or access all of the
compressed data on the CD-ROM. The decompression SW only
needs to be installed once on the systems you want to use
compressed CD-ROMs on. Accessing the data is usually as
fast or faster than on non-compressed CD-ROMs.
Decompressing the data takes time but this is usually
overcome by having more data in a smaller amount of space
on the CD-ROM.
The requirement of having special SW or drivers to
access a compressed CD-ROM seems to be the biggest
drawback to this technology. Some compression companies
also charge a royalty for each disc that uses the
compression technology.
High Density CD-ROMs (coming soon)
In order for CD-ROM to continue to be the low cost
reliable media for data distribution the maximum capacity
will eventually have to be increased. Many applications
are pushing the current maximum of CD-ROM technology.
Probably the most talked about application pushing the
CD-ROM data storage maximum is Video-CD. The goal of the
CD-ROM industry is to be able to put a full length (over 2
hrs.) MPEG compressed motion picture onto a single CD-ROM.
The current density or capacity of today's CD-ROMs does
not allow for this. Currently you can fit approximately 75
minutes of MPEG I compressed full-motion video onto one
CD.
Philips and Sony have recently announced a new
specification for High-Density CD-ROM. Their new single
sided (1.2mm thick) dual layer (3.7 GB per layer) disc is
called MMCD (Multimedia CD). Philips/Sony's proposed
specification will increase the maximum amount of
non-compressed data storage on a CD-ROM to 7.4 GB.
Increased data capacity along with increased data transfer
rates will allow for full length motion pictures (over 2
hrs., MPEG 2) to be stored on one CD-ROM. Philips/Sony's
specification will increase the amount of data that can be
stored on a CD-ROM by decreasing the pit size and track
pitch on the CD-ROM. This new disc will also support a
variable bit rate which will increase the quality and
length of digital video playback.
Philips/Sony's new disc will require new CD-ROM drives
based on new laser technology (red lasers) that will be
able to read the smaller pits. Current CD-ROM drives are
based on infrared laser technology. These new CD-ROM
drives will be backwards compatible with current CD-ROM
technology. In other words you will be able to read low
and high density CD-ROMs on the new drives. Philips/Sony's
new discs and players will probably be available in mid to
late 1996. Of course this timing can change.
Toshiba and Time Warner (TTW) are also working on a
high density CD-ROM which they are calling SD (Super
Density). There solution is created by gluing (hot melt)
two .6mm discs together. Each single layer side can hold
up to 5GB. Therefore you can have a 5GB (one side blank)
or 10GB solution. They are also working on a 9 GB dual
layer (single sided) solution and a 18 GB dual layer
(double sided) solution.
All SD solutions implement MPEG 2 for their video
compression. The SD disc also supports variable bit
playback. SD video discs will have many features including
multiple languages, multiple subtitles, copy protection,
parental lockout, etc.
As with the Philips/Sony MMCD disc a new player will
have to be developed to play SD discs. SD discs and
players will probably be available by mid 1996. Of course
this timing could change.
*The "CD-ROM Capacity" paper is a proprietary
document authored by Disc Manufacturing - A Cinram
Company.
Updates: Most CD-R discs are now 80 minute discs
instead of 74 minutes. These discs are
actually cheaper to make because the licensing fees are
much lower for 80 min. media. There are also 90
minute cds on the market however there are still playback
issues with many players with these large discs.
List of Compression Companies
The following is a list of some of the companies that
offer the CD-ROM compression solutions similar to those
discussed above:
EWB & Associates PC Guardian
2111 Palomar Airport Rd., Suite 220 San Rafael, CA 94901
Carlsbad, CA 92009 Phone: 800-288-8126
Phone: 1-619-930-0440 FAX: 415-459-1162
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