Xref: utzoo sci.electronics:18434 comp.dsp:1396
Path: utzoo!news-server.csri.toronto.edu!utgpu!cunews!mitel!melair!dataco!mcphail
From: mcphail@dataco.UUCP (Alex McPhail)
Newsgroups: sci.electronics,comp.dsp
Subject: Re: A question about the Nyquist theorm
Message-ID: <504@dcsun21.dataco.UUCP>
Date: 13 Mar 91 21:27:48 GMT
References: <20408@shlump.nac.dec.com> <625@ctycal.UUCP> <11515@pasteur.Berkeley.EDU> <wilf.667759065@rigel.sce.carleton.ca> <1180@aviary.Stars.Reston.Unisys.COM>
Reply-To: mcphail@dcsun18.UUCP (Alex McPhail,DC )
Organization: Canadian Marconi Company (Datacomm), Ottawa, Ontario
Lines: 49

In article <1180@aviary.Stars.Reston.Unisys.COM> gaby@Stars.Reston.Unisys.COM (Jim Gaby - UNISYS) writes:
>>jbuck@galileo.berkeley.edu (Joe Buck) writes:
>>
>>
>
>I think the oversampling is not time interpolation (which by Nyquist
>does not add any more information to the originial signal), but more 
>error correction oversampling.  I.e. the same bit is sampled multiple
>times to determine its value.  I assume that this is done by sampling
>over the duration (space on the CD) of the bit.  Since the same bit
>value is sampled multiple times (eight in the case of 8 times over
>sampling) I assume some voting procedure is used to determine the
>"true" (or best estimate) of the bit value.  I assume this results in
>less tracking and sample errors.  For an ideal system, it also implies
>that if the CD had a higher density (say 8 times) the laser can read 
>it at this resolution (i.e. you could put 8 time the music on one
>CD).

Actually, this is not true.  You can not increase the density of information
on a compact disk without changing the technology.  Right now, each bit
of information occupies an area 1.6 microns square (ie adjacent bits must
be seperated by at least 1.6 microns).  If you attempt to compress the
data using closer seperation, the optical interference patterns will produce
intolerable noise on adjacent bits, even with oversampling.  You must use
a much higher frequence laser (producing a higher energy output, thus 
requiring more robust material in the compact disk, thus requiring even
higher energy writing lasers, etc., etc.) to achieve a closer seperation
of information in the compact disks.

The bottom line is the optical disk media has already reached physical
bandwidth saturation, and will not support an increase in binary density
without degradation to the desired signal.
============================================================================
________
|\     /  Alex McPhail
| \   /    
|  \ /    mail to mcphail@dataco              
|---X     (uunet!mitel!melair!dataco!mcphail)
|  / \     
| /   \   The opinions are mine alone.
|/_____\  The rest is yours.
  Alex     

***--------------------------------------------------------------***
* DISCLAIMER:                                                      *
* ==========:                                                      *
*    The opinions expressed are solely of the author and do not    *
*    necessarily reflect the opinions of Canadian Marconi Company. *
***--------------------------------------------------------------***