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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: net.audio
Subject: Re: cd players? (answers)
Message-ID: <4159@alice.UUCP>
Date: Fri, 16-Aug-85 11:40:36 EDT
Article-I.D.: alice.4159
Posted: Fri Aug 16 11:40:36 1985
Date-Received: Mon, 19-Aug-85 08:32:10 EDT
References: <226@geowhiz.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 79

1.  Are there really more than one laser in some of these things or is
    it true (as someone has said) that one laser is used and its beam is
    split (optically?).  Furthermore, what do the additional beams/lasers
    give you that you don't get with one?

Probably.  What you gain with multiple beams is resistance to gunk
and scratches on the surface.  The bits are recorded as tiny changes
in a reflective surface a short distance below a transparent layer.
If you have, say, three beams, each entering the covering layer at
a different angle, and you use a 2-out-of-3 vote to decide what the
bit should be, then perhaps a surface defect that blocks one of the
beams will still let the other two get through.

2.  What is "oversampling"?  It seems that every sample on the disk
    should be read by all machines, is oversampling the re-reading of
    a single sample multiple times, perhaps to confirm it's value?
    And if that is the case, what happens to that sample in the event
    of a descrepency?

A sample is never read more than once from the disk.  However, some
machines use the same value several times running.  This is one of
several ways of reconstructing the analog signal: the main other way
is to use each sample only once.  The latter is conceptually simpler,
but requires extremely high-precision analog filters to work.  These
are hard to make and may drift with time.  Some people say they can
hear differences between the two schemes.  I can't.

3.  "Error Correcting Code"  This is probably the most obtuse point to
    me, does "Code" imply that there is a CPU inside these things deciding
    what to do in case of a disk error?  How do different manufacturers
    ECC differ?

Every disk has redundant bits recorded on it in order to make it possible
to mask defects in the surface.  These bits are recorded in a standard
format and are based on the same theory as error-correcting codes used
in various other forms of computer memory.  Where players differ is in
how much of this redundant information they use.  There are several
things one can do:  use multiple beams, look at some or all of the ECC
information, interpolate when the signal is lost for a short time,
and so on.  The intent of the ECC codes is to reduce the rate of
genuine errors to less than one per month of continuous playing.

4.  Output filtering, digital versus analog.   I think I have a reasonable
    grasp on this section.  Personal preference seems to be the main
    discriminator since both types of filtering can do the job.

This is closely related to the questin of oversampling.  See #2.

5.  Manufacturers:  Apparantly there are two designs used, Phillips and Sony,
    which are manufactured by a limited number of companys.  These companys
    products are then marketed under many names.  Could someone come up
    with a semi-comprehensive cross-reference list of who makes what
    machine behind the trim and which design they use?

I'm quite sure that other manufacturers are making innards, too.
I don't know who buys from whom.  I'm sure the situation changes often.

6.  Moving parts:  Does the disk itself spin or does the laser beam
    move?  If the disk spins how fast does it go and how accurate need
    the motor be?  Is this a point between which one can discriminate
    a "good" cd player from a "bad" one?

The disk spins.  The stuff on the inside, near the hole, gets played
first, and the player then works toward the outside.  The reason for
this decision is that manufacturing defects are more likely to be near
the outside, and disks usually aren't completely full.  Thus playing
from the inside out reduces the defect rate.

The rate ranges between about 200 and 500 RPM depending on where on
the disk you are.  The information density is constant per unit
distance along the track, so the disk starts spinning quickly and
slows down as it goes.  The speed accuracy of the motor is unimportant,
because the bits from the disk go into a big buffer and they are clocked
out at the other end by a crystal oscillator.  The motor speed is controlled
by how full the buffer is.  The buffer is always necessary to allow
error correction to be done.  If the motor is so inaccurate as to allow
the buffer to empty, the result would be gross aberrations in the sound
(like the thing stop playing altogether, or at least cutting out for
short periods).