World's Crappiest Drives (was Re: A&J Microdrive)

From: Eric Smith <eric_at_brouhaha.com>
Date: Fri Dec 8 17:34:23 2000

Clint Wolff wrote:
> Scratches on the bottom of the disk cause the servo system to lose
> focus and/or tracking. This is why you will occasionally get a CD
> to repeat a short section of music, similiar to skipping on an LP.

Small scratches on the bottom of the disc do NOT impair the ability to
focus on the pits (near the top of the disc) by any substantial amount.
This is a deliberate factor in the designed thickness of the disc, the
geometry of the pickup, and the characteristics of the objective lens.
Furthermore, when they do interfere, this does NOT interfere with
tracking because the loop filter for the servo deliberately does not
react that quickly.

The CD will not repeat unless:

1) The scratch is along a long enough section of track to overrun
    the interleave, and prevent both error correction (ECC) and
    concealment (interpolation). It is fairly uncommon for a single
    scratch to do this, but multiple scratches may well do so.

2) The player has to have badly written firmware, that forgets the
    timecode where it left off before the error. This is unfortunately
    far more common than it should be.


References:

  standard IEC 60908 (1999-02): Audio Recording - Compact disc digital
      audio system - the definitive reference on the format, based on
      the original Red Book - available for purchase online at www.iec.ch

  _The Art of Digital Audio_ by John Watkinson, 2nd Edition (3rd Edition
      to be published this month)

  _Principles of Digital Audio_ by Ken C. Pohlman, 4th Edition

Note that the CD-ROM format (standard ECMA-130, available free online
from www.ecma.ch) is layered on top of the CD-Audio format, so an
understanding of the latter is necessary to fully understand the former.

The next layer up is usually the ISO 9660 file system.
Received on Fri Dec 08 2000 - 17:34:23 GMT