Xref: utzoo comp.unix.wizards:6196 comp.arch:3198
Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!sgi!daisy!david
From: david@daisy.UUCP (David Schachter)
Newsgroups: comp.unix.wizards,comp.arch
Subject: Re: CD-ROM speed
Message-ID: <795@daisy.UUCP>
Date: 12 Jan 88 22:50:24 GMT
References: <1495@osiris.UUCP> <2126@haddock.ISC.COM> <1497@osiris.UUCP> <791@daisy.UUCP> <4603@ihlpg.ATT.COM>
Reply-To: david@daisy.UUCP (David Schachter)
Organization: Daisy Systems Corp., Mountain View, Ca.
Lines: 38

In article <4603@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) writes:
>In article <791@daisy.UUCP>, david@daisy.UUCP (David Schachter) writes:
>> According to "CD-ROM: The New Papyrus" by Microsoft Press, faster data
>> transfer from CD-ROMs is unlikely because the frequency of the data starts
>> to approach the frequency of the servo mechanisms used to keep the optics
>> on track and in focus.
>> 				-- David Schachter
>Yes.  But, that is a discussion of data transfer rate, which is not what makes
>CD roms slow.  What makes them slow is seek times.  Optical disks give you very
>dense packing of tracks, this means you need very fine advances, which means
>careful movement, which means slow (relatively, have you ever used a cassette
>with a C64?).
>
>Also, what is to say we can't remork CD roms with alternative servos?
>
>--j.a.tainter


The surface of a CD-ROM isn't too flat.  And the rotation rate of a CD-ROM
varies as the head moves in and out.  (Unlike magnetic disk drives which have
constant _angular_ velocity, the Compact Disk format has constant _linear_
velocity.)  And the CD-ROM isn't centered on the spindle too well, either.

These factors combine to make it difficult to seek on a CD-ROM.  To move to a
particular sector, you must be able to read the sector id.  To read the sector
id, you must be rotating at the correct rate.  To rotate at the correct rate
must know what sector you are on.  Combine this with a non-flat surface and an
off-center disk and you have a non-trivial engineering endeavor.  Fast seeking
is hard and not needed for consumer _audio_ applications.  Therefore, the price
break due to volume production won't be as great as you might want.

Note that changing tracks (and actually, CD-ROM doesn't have "tracks"-- it is
a single 3-mile long spiral groove, on a full disk) requires changing the
rotation rate of the disk, if you want to keep transferring at the full rate of
the CD-ROM.  There is a limit to how fast you can speed up or slow down a
CD-ROM, due to inertia.  (The full speed transfer rate is pitiful-- less than
seventy kilobytes a second, after taking out the error correcting code for CD-
ROM.)