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Posting-Version: version B 2.10.2 9/18/84; site watnot.UUCP
Path: utzoo!decvax!bellcore!ulysses!burl!clyde!watmath!watnot!atoy
From: atoy@watnot.UUCP (Andy Toy)
Newsgroups: net.audio
Subject: Re: advice on CD players
Message-ID: <11575@watnot.UUCP>
Date: Wed, 5-Mar-86 02:17:05 EST
Article-I.D.: watnot.11575
Posted: Wed Mar  5 02:17:05 1986
Date-Received: Thu, 6-Mar-86 21:40:20 EST
References: <817@alberta.UUCP> <253@catnip.UUCP>
Reply-To: atoy@watdcsu.UUCP (Andy Toy)
Distribution: na
Organization: Mapping Analysis and Design Group, U of Waterloo, Ontario
Lines: 39
Summary: 

In article <253@catnip.UUCP> Ben Broder writes:
>This is just not true.  This salesman seems to suffer from the common
>misconception that 3 beams means 3 lasers.  This just isn't the case.
>The three beam units use half silvered mirrors to split the beam of a
>single laser into three individual beams.

The 3 beam players do indeed have only one laser which is split, but
I think it is done using a diffraction grating and not half silvered
mirrors.

>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.

Just to elaborate a bit on this, the beams on either side of the central
beam are half on and half off of the adjacent pit tracks.  If, for example
the beams start drifting to the left, the left beam will get darker since
it will be in the pits more and the right beam will be brighter since it
will be out of the pits more.  The 3 beams will not be right beside each
other, but one will be ahead of the centre beam and the other will be
behind.  This also aids in tracking since only one of the beams will be
in a radial scratch at any one time.
-- 
  Andy Toy, Mapping Analysis and Design Group (MAD), 
  Faculty of Environmental Studies, University of Waterloo,
  Waterloo, Ontario, CANADA N2L 3G1   (519) 885-1211 x6592
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