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From: sjc@mips.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: advice on CD players
Message-ID: <382@mips.UUCP>
Date: Thu, 6-Mar-86 22:26:04 EST
Article-I.D.: mips.382
Posted: Thu Mar  6 22:26:04 1986
Date-Received: Sun, 9-Mar-86 20:37:38 EST
References: <817@alberta.UUCP> <253@catnip.UUCP>
Distribution: na
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 29

> The three beam units use half silvered mirrors to split the beam of a
> single laser into three individual beams.  The center beam is used for
> reading audio information -- it is equivilent to the single beam used
> in other machines.  The other two beams are aimed just to the left and
> right of the track.  The outputs from the photodetectors from the side
> beams are compared.  If the unit is tracking properly, the difference
> should be zero.  But if the pickup is off slightly, one of the auxiliary
> beams will be dimmer (because it will be catching the edge of the pits).
> The sign and magnitude of the difference between the two auxiliary beams
> tells the player how much of a correction it needs to make and in what
> direction.  This mechanism gives a three beam player a big advantage in
> tracking eccentric discs.

The argument above would seem to apply equally to one-beam or three-beam
units: it's the number of detectors, not the number of beams, that matters.
(For reference, the papers on the CD system in the Philips Technical Review
volume 40 number 6 describe a mechanism wherein one wide beam impinges on
four photodiode detectors; it too compares the levels from the diodes to
the left and right of the track to decide whether to servo the optical
mechanism to the right or left.)

I doubt that any CD player could do away with such servoing, since
there's nothing else to keep the beam aligned with the track. A
servoless system accurate enough to advance the optics at just the
right speed would cost a lot more than a servo system! Does a
three-beam unit with three detectors really offer advantages over a
one-beam unit with four detectors? If so, why?
-- 
...decwrl!mips!sjc						Steve Correll