Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: miker@polari.UUCP (Mike Ranta)
Newsgroups: rec.audio.high-end
Subject: Re: Photo-detectors in CD players
Message-ID: <12885@uwm.edu>
Date: 7 Jun 91 12:59:34 GMT
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Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu


In article <12783@uwm.edu> acgd@ihlpf.att.com (Andrew Charles) writes:

>What factors affect the output voltage of a photocell such as those used
>in CD players? What factors affect the time it takes a photocell to reach
>its "expected" output?

The photodiodes used to "receive" the reflected laser beam are very fast
devices.  I know the devices used in the fiberoptics communication area
must respond at several Megahertz which is at least an order of magnitude
above CD frequencies.

>Is it possible that circumstances could arise in a CD player which would
>affect the photocell sufficiently in these ways to either cause a misreading
>of a bit (wrong voltage value reached) or cause "jitter" (photocell reaction
>time is too long or too short)? 

I doubt the photodiode could cause any problems.  Again, other factors, such
as the optics in the transport are far more likely to be marginal.  Speed
would not be the problem, but signal level from say dirty lenses might be.

>Another way to approach this is to ask: how can a photocell fail? and if it
>does (or is degrading over time) what symptoms would one expect to see at
>the system level?

As with most semiconductors, I would expect the photodiode to either work,
or not work.  The sensitivity of some optoelectronic devices can be effected
if they are overloaded.  In the case of the photodiode, this would mean
being hit with a much higher intensity of light than is probably possible.

>Generally, I'm wondering how much the choice of photocell matters when
>a CD player is being designed.

I would say it's one of the easier things to specify.  The widespread use
and technology of communication laser/fiberoptic equipment has made this
sort of thing fairly "cookbook".  The more critical aspects are the servo
design for tracking and focus, optics design, and the laser diode itself.

miker@polari.uucp (Mike Ranta)