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From: jayj@hpisla.UUCP (Jay Johannes)
Newsgroups: net.audio
Subject: Re: Why is only one side of a compact disk used?
Message-ID: <59800002@hpislb.UUCP>
Date: Thu, 25-Apr-85 06:20:00 EDT
Article-I.D.: hpislb.59800002
Posted: Thu Apr 25 06:20:00 1985
Date-Received: Fri, 3-May-85 05:13:44 EDT
References: <106@daisy.UUCP>
Organization: HP Instrument Systems Lab - Loveland, CO
Lines: 18


The reason only one side of the compact disk is used is because
the laser reads from the bottom of the disk and the image of the
'pit or valley' is reflected off the disk.  If both sides were
'reflectable', the laser couldn't read either side.
/**********/

Close, but no cigar.
  True, the laser pickup is on the bottom, but so is the laser. The
  system is reflective, so it is irrelevent what is on the top of the 
  disc, when reading the bottom. What laser videodiscs do is in fact to
  bond two single sided discs back to back to make a double sided disc.
  No reason why they couldnt do the same thing for CDs. I suspect the
  real reason is 1. They have to put the label someplace. 2. Most releases
  only require a single side, 3. There is increased tooling costs to
  make double sided discs. For a long time yield was very low on the
  video discs. One of the big problems was that concentricity would be out
  of spec on one side. I guess its hard to bond accurately.