Re: CD question

Many times the
complaint is "CD not working". Before doing any cleaning I test them
with a variety of my own CD's and often times I cannot confirm any
problems with many of these machines. I have asked that the teachers
include with the particular offending player any particular CD which
has failed to play for them but usually I never get any. My own test
CD's are just standard CD's and vary in that they are different sizes.
Some consist of two tracks, others are four, one is ten, etc. Is there
any relationship between how many tracks are recorded on the CD and
whether or not it may or may not play on a given questionable
machine ?

Sometimes yes, sometimes no. Here's my take on it.

I don't think that the issue is the number of tracks. Rather, it's
the total duration of the contents, and how the CD was made.

CDs play from the inside out. Those CDs which are almost "full" (up
to 72 minutes) have data which goes quite far out towards the edge of
the CD. There are a couple of reasons why the outer edge may be more
difficult for a CD player to play:

- Dust and grease and tobacco-smoke tar can build up on the "rails" on
which the laser mechanism slides back and forth. The inner
portions of the rails may be less gunked up, because the sled
"wipes" these portions of the rails during almost all playbacks.
The outer portions are wiped less often... and so if you try to
play a CD which uses the whole disc diameter, the sled can run into
a sticky portion of the outer rail, and fail to track evenly...
hence, skipping.

- CDs can wobble, if they aren't sitting flat-and-even on the center
spindle. This can happen if the spindle is damaged, has debris on
it, or if the CD's center hole is damaged or isn't evenly punched.
The effect of a vertical wobble will be more pronounced out at the
outer edge, and the laser will have more difficulty tracking the
disc and maintaining a proper focus on the groove.

The discs themselves can be a big part of the problem:

- CD-R and CD-RW discs are inherently harder to track than a
"manufactured" (molded and aluminized) CD. The dye layer (CD-R) or
phase-change layer (CD-RW) has a lower contrast than the
molded pits on a manufactured CD. This leads to a lower signal
level as seen by the playback electronics, making it harder for the
servo to maintain position and focus on the track, and raising the
bit-error rate. If a CD player's laser is getting old and its
output level is dropping, the lower reflectivity can become a
significant problem.

- Many (most) CD-R discs these days are of the "extended time"
variety - they hold up to 80 minutes of content, as opposed to
the 72 minutes of a standard CD or CD-R. These long-play discs
push right up against the edge of the Red Book standard in
several ways - the spiral tracks are packed together quite closely,
and the "pits" and "lands" burned into the dye layer are at
the lower size limit of the standard. This tight packing once
again makes it harder for the player to maintain tracking and focus
lock.

- "High speed" CD-R discs use a dye laser which is quite laser-
sensitive, so it can be "burned" with a short exposure...
it may have even a lower contrast than a standard CD-R's dye layer.
Vibration of the disc and drive during a fast burn can further
reduce the quality of the pits and lands, making tracking even
more difficult.

- Discs which are used in an institutional setting (classrooms,
public libraries) are often in poor physical condition...
fingerprints and scratches. Tracking problems abound, as do
low-level data errors which can cause popping and muting.

My wife frequently borrows books-on-CD from our city library.
They're all CD-R, mostly extended-length, probably burned at very high
speeds, and they're usually dirty and scratched.

She can't use a portable CD player with these... we've tried several
and they all suffer from skipping problems. Our home CD deck will
usually play them, but even it suffers from skipping occasionally
(which never happens on CDs from our own home collection).

On a few occasions I've "ripped" a troublesome disc, and then burned a
replacement CD-R. The replacements play fine on all of the CD players
we've tried. The ripping of these damaged discs often takes a very
long time... the drive and software end up retrying certain sections
of the disc dozens of times in order to reconstruct the data correctly.

I've found that a good, careful cleaning of the lower CD surface with
plastic cleaner often helps (I use the Novus brand). Usually a
clean-and-polish is enough to allow for a quick rip-and-burn. In a
few cases, I've had to use the Novus 2 scratch-remover to buff out big
scratches.

So... to sum it up, I suspect that the problem you're seeing may be an
interaction of several things: an aging laser whose output is
dropping, hard-to-read discs (low-reflectivity extended-play CD-R,
probably in poor physical condition), and maybe some crud buildup on
the lens or rails.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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