Two related questions:
(1) CD players don't use a stylus and grooves like old vinyl LPs, so
why do they skip?  You'd think that the player's laser would just scan
in a continuous motion and, if it encountered a scratch, it would just
play a "click" but then resume playing music. Why on earth would it
jump to another location on the CD or (in extreme cases) seem to go
berserk?
(2) Is it my CD player or my CDs?  Some of my CDs never cause
skipping, while other always skip at the same location.  That makes me
think that it's my CDs that are at fault, and not my CD player.  I try
to handle my CDs carefully (but I am a heavy smoker).  Is there a
proper (or improper) way to clean CDs?  And if I bought a new CD to
replace a CD that always skipped, why on earth would the new CD skip
at the exact same location? (This happened with Sarah Brightman's
"Time To Say Goodbye" CD.)

rambler-ga:

When I first noticed your question, guzzi-ga had already posted his
comment. In his comment, he covered most of the information required
to answer your question, so I thought that it would be better not to
proceed with providing a proper Answer in case you were already
satisfied with guzzi's free comment. However, your follow-up comment
implies that you are still looking for help, so please allow me to
provide you with an Answer that will hopefully give you the knowledge
you seek.

The parts of your question are:

--------------------------------
(1) CD players don't use a stylus and grooves like old vinyl LPs, so
why do they skip?  You'd think that the player's laser would just scan
in a continuous motion and, if it encountered a scratch, it would just
play a "click" but then resume playing music. Why on earth would it
jump to another location on the CD or (in extreme cases) seem to go
berserk?

(2) Is it my CD player or my CDs?  Some of my CDs never cause
skipping, while other always skip at the same location.  That makes me
think that it's my CDs that are at fault, and not my CD player.  I try
to handle my CDs carefully (but I am a heavy smoker).  Is there a
proper (or improper) way to clean CDs?  And if I bought a new CD to
replace a CD that always skipped, why on earth would the new CD skip
at the exact same location? (This happened with Sarah Brightman's
"Time To Say Goodbye" CD.)

(3) Are some CD players better than others at handling damaged CDs or
badly-pressed CDs?  If so, how can you tell which ones are better? Is
there a feature or specification on the CD player that indicates that
it's better?

I'll address them in order.

--------------------------------

(1) How CD players work, and what happens when they skip

If a picture is work a thousand words, then an animated graphic should
be worth an entire thesis paper. So, let's start with an animated
graphic and a lot of useful info to boot. Please check out:

  Stuffo "How CDs Work" @ Howstuffworks.com
  http://stuffo.howstuffworks.com/cd.htm

and in particular, the graphic of the spiral pattern on:

  http://stuffo.howstuffworks.com/cd2.htm

and the graphic of what the 'data' looks like on the CD on:

  http://stuffo.howstuffworks.com/cd3.htm

In a nutshell, the audio data is stored on the CD as a series of bumps
and flat spots arranged in a tight spiral pattern that starts at the
inside of a CD, and works its way outwards, just like an old vinyl
record/LP. By using a laser to 'read' the bumps rather than a stylus,
a lot more data can be stored on a CD than on a record. This is
illustrated here:

  http://stuffo.howstuffworks.com/cd5.htm

However, now you're probably wondering, "if the bumps are inside the
plastic of the CD, how does the laser manage to follow the spiral when
the CD is spinning at several hundred RPM?". Sure, the optical sensor
is moved precisely by the tracking motor, as shown here:

  http://stuffo.howstuffworks.com/cd6.htm

But how does the CD player know where to position the sensor? After
all, a CD is round, so how does the CD player know where the beginning
of the spiral is, and how to find the exact starting position of the
next track when you hit the 'next track' button? The answer is
illustrated in this Electrical Engineering lecture note, here:

  http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm

Now I'll warn you, this link takes you to the lecture notes for a
fourth year engineering course. However, the important part is to be
found in the illustrations. At the very bottom of the page, the
professor mentions that the optical sensor actually has three sensors
in it. There is one sensor that detects whether the laser is scanning
a bump or flat spot (ie. a digital 1 or 0). That sensor's readings go
to the digital decoding circuitry to be converted into an analog
signal from which the sound is reproduced. There are also two
additional sensors, located to the 'inside' and 'outside' of the main
sensor. These two sensors are used to make sure the optical sensor is
centred over the spiral line of bumps and flat spots. By sensing the
difference between these two side sensors, the CD player's control
program can detect if the optical sensor assembly needs to be moved
inwards or outwards in order to follow the spiral. It's like how the
'sides' of the groove on an LP help to keep the stylus centred in the
groove.

At the same time, the 'index' information written onto the CD contains
the starting positions (relative to the beginning of the spiral track)
of each song. It's like how you can visually see the gap between songs
on an LP, and you can position the stylus approximately near the start
so that the stylus can start playing and find the next song on its
own. With the CD player, it knows that if it goes to a specific
distance from the zero position, and starts looking for the spiral
there and tracks it, it can pick up the next track relatively quickly.
In fact, the way that the audio data is encoded on a CD includes more
than just audio information. It also includes timing and error
correction information so that even if the CD player makes a mistake
in reading a little bit of data, you might not even notice it in the
audio output due to error correction.

So, if there is all this error correction, why do CDs skip? If you
visualize the spiral track on a CD is being many many times more fine
than the groove on an LP, and if you see the laser as being many many
times smaller than a stylus, then you can imagine just how huge even a
small scratch on the surface of the CD would appear to the laser and
optical sensor assembly in the CD player. Grand Canyon-like. Depending
on the size of the scratch, it may be enough to make the CD player
accidentally think that it needs to move the sensor further than it
should in order to stay 'on track'; this extra movement actually
causes it to lose the spiral once the scratch has gone by, and all of
a sudden the CD player needs to go search for the spiral again.
Sometimes, it finds it again so quickly that you don't even notice.
Other times, it 'thinks' it finds it, but actually ends up 'skipping'
either forward or backwards in the actual song. This, you will indeed
notice. And, if it REALLY loses its way, some CD players will even
return to the 'zero' position and use the index information to move
the head back to the approximate location where it lost the track, in
the hopes of reacquiring it. That's when it seems like it is going
berserk with the head assembly going back and forth, back and forth.

--------------------------------

(2) So is it the CD player or the CDs

In my description above, I used a scratch in the CD as an example of
something that can cause the CD player to skip. This is not the only
cause, though. In that Electrical Engineering link, there is also some
discussion about the ability for the CD player to move the optical
sensor up or down to let it 'focus' on the laser light reflected from
the CD. This is because the laser light is focused through the lens of
the sensor to precisely hit the CD in as small a spot as possible.
Since CDs are mass produced, manufacturing tolerances mean that the
distance between the sensor and the reflective surface inside the CD
cannot be precisely predicted. Instead, the CD player has to move the
sensor up and down to ensure that the light reflecting back is an
accurate reflection of the bumps and flat spots in the CD. This is
accomplished by looking at the shape of the reflected light (you can
see an illustration of this in the Electrical Engineering link). Now,
since the ability of the CD player to 'see' the data depends on the
ability for the laser to pass through the lens, hit the CD, and be
reflected back through the lens to the optical sensor, anything that
gets in the way of the laser will affect the ability of the CD player
to play uninterrupted. This means that any dust or dirt in the
environment that ends up on the optical sensor's lens, will make it as
difficult for the CD player to 'drive' down the spiral track as it is
to drive a car down a highway with a fogged up windshield. Eventually,
it gets too difficult to see, and you end up driving off the road.

So, the answer to this part is that it can be either the CD player, or
the CDs, or both. Even if the CD has no scratches, if the CD player
has a dirty lens (or even dirt on the plastic gears used by the
stepper motor that controls the position of the optical sensor), it
can have difficulty staying on top of the spiral.

You also asked why the problem seems to be repeatable using two
different purchased copies of the same audio CD. While it is not
possible to give an absolutely definitive answer to this without
seeing the CDs in question, the most likely reason is because the
'master' molds used to mass-produce the CDs, shared a common problem;
the bumps in that part of the spiral track are a little too low, and
your CD player thinks that it's 'driven off the road'. This is a good
indication that you need to clean the lens and/or blow the dust out of
the tracking mechanism.

--------------------------------

(3) Are some CD players better at this than others?

If you do decide that it is time to buy a new CD player, then a
feature to keep an eye out for is a player that can play burned CD-R
and CD-RW discs. Why should this matter if you are only playing
purchased CDs? The reason is shown, to a certain extent, here:

  http://stuffo.howstuffworks.com/cd-burner.htm

The first few pages are a repeat of the links I discussed above in
section (1). The important part is on page 4:

  http://stuffo.howstuffworks.com/cd-burner4.htm

Here, the graphics (while not to scale) are a good indication of the
difference between a molded, store-bought audio CD, and a burnt audio
CD. A burnt CD emulates the bumps and flat spots of a molded CD using
a very thin layer of dye that is 'burnt' dark by the writing laser. In
order to accurately read a burnt CD, the design of the optical sensor
in the CD player is different to reduce the possibility of
mis-tracking even when the spiral track is barely visible. Sure, even
these players are succeptible to 'going blind' if the lens gets dirty
or the surface of the CD is dirty/damaged, but in normal use
environments, they will generally perform better than the older
players that were designed to 'see' physical bumps and flat spots,
instead of the light and dark spots of a burnt CD.

So, if you buy a CD player that is designed to work well with CD-Rs
and CD-RWs, then you will also have a CD player that can handle
store-bought CDs better, too.

While we are on the topic of CD player specifications to look out for,
you will also encounter a feature on better units called a "1-bit dual
D/A converter", and another feature called "over-sampling". Here is an
excellent explanation of the advantages of a 1-bit dual D/A converter:

  http://ibs.howstuffworks.com/ibs/bos/question620.htm

and, you can read a bit about oversampling here:

  http://www.tweeter.com/info/index.jsp?categoryId=1189803&infoPath=1139132

Incidentally, on this last link, you will read that while a DVD player
may also be capable of playing CDs, it is first and foremost designed
to work well with DVDs, and may compromise its abilities to play CDs.
So, if you want a good CD player, then shop for a CD player, not an
all-in-one CD/DVD/etc player.

Rambler-ga, I hope that this Answer helps shed some light on this
problem for you. Please let me know if any part of this Answer is
unclear.

Regards,

aht-ga
Google Answers Researcher

---

Request for Answer Clarification by rambler-ga on 01 Dec 2004 07:07 PST

Dear aht-ga,

It may be unfair of me to ask this, but if you were in my shoes
(desperate to find a CD player that is better than most at avoiding
skipping), precisely which CD player would YOU buy (make and model)?

I'm just looking for a starting point in my search for a new CD player.

---

Clarification of Answer by aht-ga on 01 Dec 2004 21:41 PST

rambler-ga:

Now, this is just my opinion; when it comes to audio equipment, there
are as many opinionated people out there as there are CD players on
the market. Based on your situation, here's a few options to consider.

First, if your budget will allow for it, consider getting a quality
'juke-box' style CD player like this one:

http://www.amazon.com/exec/obidos/tg/detail/-/B000069JWX/002-4058466-8687204

The reason I suggest this is because, once your CDs are all loaded
into the player, you do not need to handle them manually again. Since
you mentioned that you smoke, not having to handle the CDs means that
the CDs are less likely to get dirty, and more importantly, the smoke
residue is less likely to get onto the lens. The downside to this, of
course, is that these 'juke-box' style players tend to be expensive.
For this particular unit, a quick check on Froogle gives:

http://froogle.google.com/froogle?q=CDPCX455

a range from around $235 up to over $300. If you target a smaller
unit, say around 300-CD capacity, then it'll be around $200. In any
case, I give you these links only for visual reference; I strongly
suggest heading down to your nearest big-box electronics retailer
(Best Buy, Circuit City, etc) to see the systems in person so that you
can get a feel for how usable they would be in real life.

If a large unit like this is not to your liking, and you would prefer
a stereo system with radio, tape, CD, and amplifier all in one,
consider getting one where you can easily access the lens to clean
it... and then remember to periodically clean it! This type of unit is
actually getting hard to find from the better-known brands, so its
best to just see what is available at your local retailer, listen to
it, and bring along some test CDs.

Regards,

aht-ga
Google Answers Researcher

Very thorough answer. Addressed each of my concerns specifically. Thank you!

Rambler,
Do you clean your CDs but not the player? For me, I believe it makes
good sense to have both cleaning material for both the CD and the
player. Sometimes the problem can be a clean CD, but not a clean lens.

Your CD probably skips because it is happy, but there again, I?d also
be likely to do something unpleasant to Sarah Brightman. No
accounting..... ;-)

I?d suspect it?s a bad pressing (musak content notwithstanding). Yes,
they are pressed, then protectively coated. Otherwise though, CD
players can get confused if there is a break in the data -- the
electronic equivalent of mechanical disks. There is actually a lot of
mechanical stuff going on it the laser head too though. The focus /
tracking lens is loosely spring mounted with electromagnets acting
against small rare earth magnets to align in vertical (focus) and
radial (tracking) directions. In addition to this, a little motor
(usually a stepper) moves the whole head assembly more slowly for bulk
movement. There?s a sensor array in the laser head which sends out
position correction signals. Take an old one to bits and push up and
down and back and forward on the lens and you?ll see what I mean.
Clever stuff.

CD failure is usually caused by laser degradation and perhaps sensor
misalignment caused by material creep. Bear in mind that tolerances
are down to a few microns. But even when things are working fine, it
doesn?t take much for the electronics to lose control. There is
variation in the quality of error correction in different machines,
mobile ones store a few seconds ahead so can pick up from where they
left off and you don?t get a ?break? in the play. There are also
predictive algorithms in the firmware to minimise clicks and things
but the functions can?t play completely corrupted data. The little
alignment mechanisms have to start almost from scratch (sorry for the
pun).

As for cleaning disks -- soapy water and a soft cloth works fine.
Don?t waste money on fancy cleaners. In severe cases, silver of brass
metal polish can restore unplayable disks. You?ll be left with small
surface scratches but not a problem. In extremely severe cased
(kids!), one can use very fine wet and dry abrasive paper (1000
grade), finishing off with metal polish. As long as you don?t break
through the surface coating and have thinned down the disk reasonably
evenly, it can work. Someone gave me a destroyed ?98 disk which worked
for years after I rubbed it down this way.

Cleaning the lens is often required, especially if you smoke (I too).
Proprietary cleaners are variable but the best way is to take it to
bits and examine the lens with an eye glass. Assuming there is nothing
lying on the lens (fibres etc), if it has a matt bloom on it?ll need
cleaning. Don?t use alcohol because it can strip the blue quarter
wavelength coating. Use the tip of a damp soapy tissue held in
tweezers, followed by cleaning off with clean water on a tissue tip.
Sometimes a little polishing will be necessary to remove water stains.
Very fragile mechanism so got to take great care (and don't scratch
the lens) but I have restored dozens this way. If a CD doesn?t work,
you?ve nothing to lose.

Best

Lasers work differently, if you use a Optical mouse you would know
this, on my Laser Mouse for my computer when it is on a surface that
reflects the laser, sometimes it SHOOTS the curser to a rondom point
on my screen, so when a laser gets Confused, like running into a
Scratch on a cd, it Throws it off and makes it jump, just as in my
laser mouse! I hope that answers your question.
Also check out
http://science.howstuffworks.com/laser.htm

Are some CD players better than others at handling damaged CDs or
badly-pressed CDs?  If so, how can you tell which ones are better?  Is
there a feature or specification on the CD player that indicates that
it's better?

Sorry for toe stepping aht-ga. I usually wait -- but maybe not long enough.

Best

guzzi-ga, no worries. Your comment was very informative and based on
your personal experience, something of great value for the client. So,
no toes were stepped on, in my opinion at least!

Regards,

aht-ga
Google Answers Researcher