                          2All About CD-ROMs0

     MultiMedia PCstraditionally have two hardware elements in common:
a  Sound  Card and a CD-ROM drive.  A Sound Card's purpose is obvious,
but CD-ROM is another matter.  What discs are available?  Are they the
same as audio CDs? What should you look out for in a drive anyway?
     Conventional audio CDs store sound information digitally.  It  is
picked up by a laser, converted to pulses of electricity and processed
by a digital to analog convertor (DAC) ready for listening.  A  CD-ROM
is essentially the same - a disk on which digital computer information
can be stored and retrieved in the same way as, say, a hard disk. TThe
ROM (read only memory) part of the term is important,  indicating that
the information may not be erased or modified in any way, just as with
an audio CD.
     CD-ROM drives work in the same way as audio CD players,  where  a
laser  picks up the information and converts it into digital pulses of
electricity.  At this point,  however,  the digital information may be
passed   directly   to  the  computer,  bypassing  the  DAC  found  in
conventional audio CD players.  DACs are available  very  cheaply  and
most CD-ROM manufactures include one anyway, providing the facility to
play audio CDs if requiered.
     The CD-ROM  drive  may  be  connected to your computer in several
ways, including daisychainable SCSI  or  a  manufacture's  proprietary
interface. Most  common  in  this  group  test  are those drives which
conform to   one   of   three   standarts   -   Sony,    Mitsumi    or
Matsushita (Panasonic),  enabling  direct  connection to certain sound
cards.
     Audio CD  players  are  single-speed  devices in that a 60-minute
disc takes 60 minutes to play - you wouldn't want it  any  other  way.
However, CD-ROM  data  is  jjust  plain  computer data which you would
ideally like transferred as  fast  as  possible;  single-speed  drives
transfer a pitifully low 150 K/sec, around the same as a floppy disc.
     But it's possible to spin the disc twice as  fast,  doubling  the
transfer rate if the laser pickup can keep track.  Double-speed drives
are the current standart and should sustain a 300 K/sec transfer rate.
Recently, quad-speed  drives have become available,  delivering around
600 K/sec,  but these tend to cost at least twice as much  as  typical
double-speed. All  CD-ROM  drives  switch automatically down to single
speed when playing audio CDs.
     The discs are physically the same as audio CDs, measuring 12cm in
diameter, and  may  be  pressed  in  the  same  labs,   resulting   in
potentially cheap mass duplication.  Uncomplressed CD quality (16 bit,
44.1 kHz stereo) audio requires approximately 10Mb per  minute  and  a
CD's capacity  is  around  74  minutes.  Consequently,  a  CD-ROM  may
contain, if necessary, between 650 and 700Mb of information. A typical
disc, though, may contain much less.
     CD-R discs are recorded rather than pressed, which introduces the
possibility of  additional  information  being  added at a later date.
CD-R is a WORM  format  (Write  Once  Read  Many):  you  may  add  new
information, but cannot erase or modify existing information. Each new
recording is known as a section.  Pressed,  silver CDs are made in one
go, and hence are single-section. Discs with more than one session are
called multi-session.  A single-session drive  will  only  access  the
first session  -  you'll  need  a  multi-session  drive  to access any
additional sessions. Kodak's Photo CD records photographic images onto
a CD   in   potentially   numerous   sessions,   thereby  requiring  a
multi-session drive.   Luckily,   almost   all   CD-ROM   drives   are
multi-session.
     CD-ROM software exists  in  many  guises.  Most  common  are  the
encyclopedias or  complete  works on a single disc.  Thanks to the low
price of disc  duplication  along  with  their  large  capacity,  many
developers supply  application or operating system code on a CD-ROM as
well as conventional floppies. CorelDraw 5.0, for example, ships on 12
HD floppies.  Alternatively  you  may  install the entire suite with a
single CD-ROM, which also contains many extras.
     Duplicating and  distributing  single CD-ROMs instead of multiple
floppies is much cheaper for the manufacturer and far easier  for  the
user to install. Consequently, Corel's pioneering lead to putting apps
on a CD-ROM has been taken up by many others. Many sophisticated games
are available on CD-ROM, usially featuring video and audio clips.
     The numerous standards for different types of CDs  are  known  by
the colour of the cover of the manual that describes them. CD audio is
known as Red Block and  CD-ROM  as  Yellow  Book;  unfortunately  this
rather merry naming procedure now becomes a little complicated. CD-ROM
Yellow Book comes in two track varieties: Mode 1 for computer data and
Mode 2 for compressed video/audio information.  Mode 2 sectors contain
no error correction,  resulting in a larger data capacity.  To  ensure
compatibility, the data on both Modes is laid out in a format known as
ISO 9660.
     Next, there's  a  Green  Book  CD-i  (Compact  Disc  Interactive)
containing almost anything Philips sees fit and playable only on  CD-i
machines. Then  there are so-called CD-i ready discs which feature Red
Book audio,  playable  on  normal  CD  audio  machines,  but  offering
additional text and graphics when used with CD-i players.
     Recently, CD-i has been touted as a movie-playing medium with the
help of a Digital Video, DV cartridge, although a similar standard for
storing fullmotion video and audio using the  same  highly  compressed
MPEG-1 system has been defined as the White Book.
     The whole CD-i movie/White  Book  story  has  been  fraught  with
difficulties from the start, with only certain discs made at a certain
time working on certain machines, and then probably only those made on
a Wednesday.  As  far as the official word is conxerned,  the definite
standard from now on for consumer MPEG-1 video/audio on a CD is  White
Book, and  yyou'll  need  a  White  Book  compatible player and MPEG-1
decoder to read it.
     The probblem  is finding a CD-ROM drive which is definitely White
Book-compatible, or harder still,  a  manufacturer  or  supplier  that
understands what you're on about.  Right now,  we know the Sony 33A is
definitely equipped for the job, but more are expected shortly.
     So saying,  White  Bokk  incompatibility  is  not  the end of the
world, since I suspect few people  will  seriously  boot  a  computer,
install hardware  and  drivers  just  to  watch  a  film on a small PC
monitor with a fan whirring in the background.  Other uses  for  White
Book would  be undeniably nice but not for everyone,  especially since
you'd also need MPEG-1 decording hardware.
     CD-ROM is established as a format and is here to stay. Very soon,
every PC will ship with a CD-ROM drive  fitted  as  a  standard.  They
probably won't be White Book-compliant, though.


                                                Gordon Laing
                                                PCW, January 1995
