Conference cookie::notes$archive:cd_v1

This is a totally non-technical answer.

For the following example you'll need a pair of glasses (if you don't already
wear them).

1) Remove glasses
2) Cover them with finger prints
3) Close your eyes
4) Wear the glasses
5) Open your eyes
6) This is what the laser is trying to track through.

Note:  Your eyes are trying to focus on a large area, the laser is attemping
       view a microscopic dot through this pea soup.

Someone else may be able to give a more technical answer.  However, I think
this will take care of most of your questions.

Steve

I understand why it would misread the data.  What I don't understand is why
it would skip ... the data moves at a constant velocity past the head, and
all the parameters of head and disk movement should be derivable from the
distance of the head from the center of the disk.  Of course, things like
detecting the end-of-disk,  or responding to a program would force the disk
to react to data it reads,  but for ordinary play,  I can't see why reading
bad data should cause the head to get "stuck" at a particular point.

-pd

The laser is constantly shifting to follow the spiral track.  If it gets
confused as to where it is, it may move forward or back one track.  (Note
that the tracks are VERY close together.  Moving forward gets you a skip,
moving back gets you "stuck".
					Steve

That doesn't explain why it would get confused as to where it is.  Yes, it
would read bad data, but it should still know where it is, shouldn't it?

I assume the answer is that it reads positional information as well as signal
information.  This is not obvious, since the position could be completely
determined by the location of the laser relative to the center of the disk.

Again, think of a magnetic disk ... it does not get stuck trying to read
a bad block.  It simply gets a data read error.  (Unless the servo information
on the disk is corrupt, but I don't think a CD works in that way.)

The CD engineers went through a lot of trouble to design a CD player so
that it would skip! I don't understand why you're questioning this. Don't
you want your CD player to sound as good as your turntable?

There's no pleasing some people! Sheesh!

--- jerry

   this one needs a picture:
   
   		----------------------------------------	CD
   				|_____|        \|/
                                   |            X
                                   |            |
                                   |            #
                                   |            #
                                              laser
   
   
    
             ----------------------------------
                    __       ___       __
                   |  |     |   |     |  |   pits
             ----------------------------------
                   / \        |        /
                  /   \       |       / \
                /      \      |      /  \
              /         \     |     /   \
            /            \    |    /     \
          /               \   |   /      \
        /                  \  |  /        \
                            \ | /          \
                             \|/           \
                              |             \
                              |             \
                              |              \
                              |              \   
                              |               \
              --xxxxx--------xxx--------xxxxx-----
                                              photo diodes (?) or something
   
   the laser is plit into three - the middle one reads the data, the other
   two are for tracking.  They arr reflect off the surface, and bounce back
   to a mumblefratz that determins if the laser is hitting it or not.  The
   outer laser beams will 'track' the middle row.  when the other two hit 
   the mumblefratz then they are off center and the 'tracking device' moves
   the laser over a bit.  If dirt gets in the way, it can get confused...
   
   	any better ?
   
   				 �dave�
   
   
   

Note that a magnetic disk is recorded after it is mounted on its spindle,
where as a CD is "recorded" before.  Thus it is possible to determine the
location of a track of a magnetic disk merely by distance from the spindle,
but effectively impossible to do it on a CD. Even if the hole were only a
smidgen off center it would fail.  Considering that a CD costs about $2.50
to make, and must sell for about $15.00, it is clearly neccessary to have
the player follow the tracks rather than expect them to be in a certain place.

		George Chaltas

Thanks to .6  ... that's the information I was looking for.

-pd

Hate to break the news to you guys, but most CDs are as off center as LPs.
As much as 2mm.  I can take the bottom off my player and watch the arm
move in and out as it plays.  Laser centerd before punching, european
disks are far better than hole molded in disks.

All this servo movement puts lots of load and noise on the same power
supply that is usually used for the analog audio circuitry.

Magnetic disks *do* mistrack sometimes, unless they have servo tracks (and
most early ones didn't).  The servo track allows the heads to follow the
eccentricities of the disk motion.  If there is a dropout or other error
in the servo track, a magnetic disk drive *will* mistrack.

CDs have a spiral track, while magnetic disks have concentric circular tracks.
The spiral nature of the CD track makes it orders of magnitude more difficult
to use a servo track to maintain proper head motion (laser motion in this
case).  Also, you'd need two lasers (one for reading the servo track, one
for data), which would drive costs up.  Even then, if the contamination hit
the servo track, you'd be out of luck.  If the dirt blotch is large enough
to cause a mistrack, chances are excellent that it would cause an audible
signal dropout, too, so there isn't much lost by having the mistracking
occur.

--PSW

    ANOTHER POSSIBILITY
    
    I had one disk that skipped and after repeated cleaning and inspection
    was about to return it, then I simply watched it turn thru the clear
    plastic (Technics portable unit) and noticed that the disk was turning
    slowly.  Apparently the disk was slippery and by simply clearning
    it and then putting some fingerprints on it it has gripped fine
    since (about 2 months).
    
    One other time I had a problem and found that I had forgotten to
    switch back to AC current from battery and that I had run out of
    power.  So occassionally this happens from the simple reason that
    the disk is not turning fast enough.
    
    Sam

    Personally I think the skipping CD problem is not long for this
    world. 
    
    It won't be long before manufacturers get smart and put caches in
    their players. This would allow the head to be reading data that
    isn't needed for a couple of seconds, therefore allowing re-seeks
    without loss of real-time performance.
    				- HBM