5. Why are CD-ROM drives so slow?

   Compact discs were originally designed for music.  When you are

   listening to "Willie Nelson's Greatest Hits", you are accessing the

   data sequentially and at a very regular speed.  The only time you need

   to seek is when you decide to skip over a song, or back up and listen

   to "Always on my Mind" one more time.



   In order to fit as much music as possible onto the disc, the data

   is recorded at the same linear density near the outer edge of the disc

   as it is near the center, so there is more information in the outside

   tracks than in the inside tracks.  In order to deliver a steady rate

   of data, the linear velocity of the disc moving under the head is

   constant, so the angular velocity of the disc changes when the head

   moves from the center toward the outside tracks.  This is no big deal

   when you are playing music, but when you are trying to do random access

   to a CD-ROM, the need to accelerate and decelerate the disc is the biggest

   obstacle to making it faster.  Most magnetic discs spin at a constant

   angular velocity, so the data density decreases toward the outside of the

   disk, but seeks are faster.



   A few other reasons that CD-ROMs are slow:  Optical disc heads tend to be

   heavier than magnetic disk heads, so they have more inertia, and take

   longer to stablize onto a new track.  Many CD-ROMs contain too much data

   to make effective use of RAM caches.



   As time goes by, CD-ROM drives will get a little faster, but don't expect

   any miracles.

			Copyright ©1995 by ISOMEDIA, Inc. For Information, contact Ken Cheney