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From: sjc@angband.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: cd players?
Message-ID: <74@angband.UUCP>
Date: Wed, 14-Aug-85 21:46:27 EDT
Article-I.D.: angband.74
Posted: Wed Aug 14 21:46:27 1985
Date-Received: Mon, 19-Aug-85 04:45:48 EDT
References: <226@geowhiz.UUCP>
Distribution: net
Organization: S-1 Project, LLNL
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> 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?

In lieu of a "groove", the disk has a spiral track of pits which
alternately do and don't reflect laser light.  To track this "groove",
the player shines three (sometimes more) beams onto the disk and
adjusts their position to maximize the modulation of the center beam
and minimize the modulation of the surrounding beams.  Some models use
multiple lasers whereas others use beam splitters. I have seen no
evidence that one approach is superior to the other.

> 2.  What is "oversampling"?  It seems that every sample on the disk
>     should be read by all machines...

Every machine reads every sample. "Oversampling" is signal-processing
jargon.  In this case, it means that samples from the disk go into a
digital transversal filter at 44.1k samples/sec, and filtered data
comes out of the filter at 88.2k samples/sec (for 2x oversampling) or
176.4k samples/sec (for 4x oversampling). Roughly speaking, the filter
generates additional points by interpolating between the original ones.

> 3.  "Error Correcting Code"...  How do different manufacturers ECC differ?

Digital circuitry within the player can correct many errors by taking
advantage of redundant information included on the disk, roughly
as a computer disk-drive controller or memory controller does. The
algorithm for doing this is straightforward, so I doubt that players
differ in this respect. However, if the redundant information proves
insufficient, a player may mute, or repeat a section of the program, or
interpolate; how cleverly it does so may make a great deal of
difference. Similarly, if the player loses its lock on the "groove"
entirely, the algorithm it uses to recover makes a big difference. On
one defective disk, my early-model Technics player performed better
than Sony and Yamaha players because it apparently punted the defective
section and skipped to a later, intact one, whereas the other players
got in serious trouble attempting to track it. In another case, the
Technics compressed several minutes of music into one because it kept
skipping over defects, whereas the other players merely produced
ticking sounds during the music. Sometimes the Technics produces ticks,
too; the behavior of players in the presence of defects is fascinating.

> 4.  Output filtering, digital versus analog.

Digital filter systems (in practice, they're hybrid digital-and-analog
systems) usually exhibit less group delay (loosely speaking, "phase shift")
at high frequencies than purely analog systems, and are less likely to
deteriorate with time as components age.

> 5.  Manufacturers:  Apparantly there are two designs used, Phillips and Sony,
>     which are manufactured by a limited number of companys...

There seem to be many designs. Old Sony models use analog filtering; new
Sony models use digital filtering; all Philips models use 4x oversampling
digital filters, but some other manufacturers use 2x oversampling digital
filters; some manufacturers switch one D/A converter between channels
whereas others use two; and so on.

> 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 at a varying rate so that the bits need not be crammed
closer together near the center, where the diameter of the "groove" is
smaller, than at the edge. The player compares the bit stream from the
disk against a crystal-derived clock and adjusts the motor speed to
maintain a constant bit rate. This feedback mechanism (similar to that
of some "quartz-locked" turntables) largely overcomes motor
inaccuracies.

A good reference on the CD system is:

    M G Carasso, J B H Peck, J P Sinjou, "The Compact Disc Digital
    Audio System," Philips Technical Review, Vol. 40, No. 6, 1982.
-- 
                                                           --Steve Correll
sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc