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Path: utzoo!decvax!bellcore!ulysses!mhuxr!mhuxt!houxm!ihnp4!mmm!schley
From: schley@mmm.UUCP (Steve Schley)
Newsgroups: net.audio
Subject: Re: advice on CD players
Message-ID: <601@mmm.UUCP>
Date: Thu, 13-Mar-86 13:37:42 EST
Article-I.D.: mmm.601
Posted: Thu Mar 13 13:37:42 1986
Date-Received: Sun, 16-Mar-86 16:26:46 EST
References: <817@alberta.UUCP> <253@catnip.UUCP>
Reply-To: schley@mmm.UUCP (Steve Schley)
Distribution: na
Organization: 3M Company, St. Paul, Minn.
Lines: 51
Summary: 

In article <253@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes:
>In article <817@alberta.UUCP>, luca@alberta.UUCP (Luca Vanzella) writes:
>> One thing in particular - a local dealer (carries JVC, Sanyo, and Sony) 
>> suggested to me that 3-beam laser units would not last as long as 1-beam
>> laser units due to "the power configuration".  He suggested 6-8 years
>> life expectancy vs 8-10 years.
>
>This is just not true.
  
This is a pretty strong statement, without any hard facts to back it up.
It _is_ possible that three-beam pickups might die young.  See below.

>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 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 is, to the best of my knowledge, an accurate description of how
three-beam pickups work.  Now, let's try a thought experiment,
comparing the two types of pickups.  To get the same raw data S/N from
the disc, I suggest that the intensity of the three-beam pickup's
center beam must equal the total intensity of the single-beam pickup's
laser.  Since the three-beam pickup is also generating two side beams,
its total output must be greater than that of the single beam pickup,
which means higher power dissipation in the diode.  This generally
reduces life expectancy of an electronic component.

I do not know of any studies that discuss actual failure rates of CD
player laser diodes.

>This mechanism gives a three beam player a big advantage in
>tracking eccentric discs.

This statement, while possibly true in specific comparisons, is not
true in the general case.  A while back, reviewers were finding that
certain Philips single-beam players would out-track certain Japanese
three-beam players on defective discs.

-- 
	Steve Schley

	ihnp4!mmm!schley