From: Benjamin Koh (benkoh_at_stanford.edu)
Date: Sat Nov 03 2001 - 14:08:05 EST
Michael Geary wrote:
>
> > > What causes the noise? I believe the CD is seated as tightly
> > > on the spindle as possible. It is a clean, factory CD (no
> > > paper label).
>
> The 24X isn't a CLV drive, is it? If so, then it spins REALLY fast. A 1X
> drive spins at 200 RPM to 530 RPM, so a 24X drive would be 4800 RPM to
> 12,720 RPM.
>
> If the 24X is a CAV (Constant Angular Velocity) drive--that is, if 24X is
> its *maximum* transfer speed, not a *constant* transfer speed--then it would
> be spinning at 4800 RPM.
>
> At these speeds, a CD doesn't have to be very far off balance to vibrate
> loudly.
Almost all laptop drives faster than 4x are CAV drives because higher
speeds in a CLV design require more powerful motors and brakes to
accelerate and decelerate the CD as needed. More powerful motors and
brakes require more battery power. Ergo, CAV. Sometimes the drive can be
adjusted to run faster on AC power. My portable drive is 8-20x on AC,
but only 4-6x on laptop battery power.
Much as we like to think of factory CDs as perfect vs burned CDs, they
are still items of mass production and it is not rare for CDs to be out
of balance (the center of gravity is not in the exact center). Thus the
faster the disc spins the more wobbly/noisy it gets. It has been
reported that caddy loading drives seem resistant to this
vibration/noise problem. Quality control is out of our hands since the
same CD-stamping equipment has been making audio CDs for ages, and at 1X
the vibrations are not an issue.
CDs were never designed to be spun faster than 1X - their use today as a
data medium is more coincidental than deliberate. Above 50x on a CAV
drive the disc itself begins to disintegrate. Desktop drives that today
report speeds of more than 50x use various tricks such as multiple
pickup lasers to read from more than one location on the disc at once.
Our equipment today is a kludge designed to work around the limitations
of the CD medium (access speed, recordability and so on).
Benjamin
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