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From: pchow@Shasta.ARPA
Newsgroups: net.audio
Subject: Re: error correction in cd's
Message-ID: <6103@Shasta.ARPA>
Date: Mon, 10-Jun-85 20:34:10 EDT
Article-I.D.: Shasta.6103
Posted: Mon Jun 10 20:34:10 1985
Date-Received: Wed, 12-Jun-85 07:47:47 EDT
References: <679@charm.UUCP> <1214@hound.UUCP>
Organization: Stanford University
Lines: 40

> []
> I think there is some question, though, if the laser can't read at least
> some of the information even when the aluminum backing is missing. After
> all the "bits" are apparently deformations in the plastic <before> the
> aluminum is put on. They are not pits in the aluminum as I used to think.

Some more information further to the above...

In IEEE Spectrum March 1984 there is a diagram showing the following
layers.  (Not to any scale).

	______________
                        label
        ______________
                        protective film
        ____      ____
            |____|      aluminum reflection film
        ___        ___ 
           |______|     quarter wavelength pits
   

                        transparent layer
        ______________

             /\
            /||\ light direction
             ||

This shows that the pits are indeed in the plastic and the aluminum
layer is coated on top of them.  The quarter wavelength pits cause
destructive interference between the part of the beam hitting a pit
and the part that doesn't.  This is detected at the photodetector.

If the aluminum is missing, then the amount of light reflected back
should depend on the differences in the indices of refraction of
the transparent layer and the protective film.

	Paul Chow
	decwrl!shasta!pchow
	pchow@shasta