From: tel1568@tamuts.tamu.edu (Thomas Edward Landrum)
Newsgroups: rec.audio.high-end
Subject: Re: Two great mysteries of audio
Date: 23 Jan 1993 02:28:15 GMT
Organization: Texas A&M University, College Station
  (5.65b/IDA-1.4.3 for rec-audio-high-end@uunet.uu.net); Fri, 22 Jan 93 20:28:16 -0600
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Originator: tjk@csd4.csd.uwm.edu

In article <1jov32INN53@uwm.edu> ken@isgtec.com (Ken Newman) writes:
>
>
>Two things I've been wondering about or been annoyed by:
>
>1) Why are there no pre-recorded metal cassettes, or no pre-recorded
>   Dolby C cassettes? Every decent cassette deck for many years
>   (even car decks, for crying out loud) has had metal and Dolby C
>   capability. There must be some demand? Sure metal tape is expensive
>   but so are CD's.
>
>2) Why do they not "record" or "press" or "download" :) or whatever
>   the appropriate term is, on both sides of a CD? Again, this would
>   be more expensive, sure, but probably much less than two CD's?
>   I certainly wouldn't mind flipping a CD over if I could save 
>   several bucks and obtain the several other obvious advantages
>   of this.
>
>--
This debate raged on alt.cd-rom...The upshot was that because of two
factors, this would not be feasible.  The first and probably most
damning in this situation is that the laser in a drive is focused
to the actual depth in the substrate that the vapor deposited aluminum
layer is found.  The tolerances are very close.  The thicknes of a CD 
is critical to proper operation.  One benefit to this is that scratches
on the plastic side are less likely to cause problems.  However, it is
important to note that scratches on the label side that penetrate to 
the aluminum layer (just under the painted label and the layer of 
laquer) are usually terminal.  

The second factor related to a scenario where one would use both
sides at the same time, much like the multiple heads and platters in
a hard drive.  The killer here is the fact that the CD standard
calls for a constant velocity no matter where the head is.  As a
result, the r.p.m. of the disk is completely dependent on the position of
the head.  To see this, find one of those boom-boxes that have a CD 
drive in them that has a window through which the cd is visible.  Watch
how the much slower the disk spins as you quickly step from the first 
to the last track.  The only way I can see a way to use both sides 
of a CD is to rewrite the entire standard thus obsoleting every CD-
audio, CD-ROM, CD-I, CD-WORM, etc.... drive out there.  If you
are interested in the exact specifications used for several of the
different types, try looking in MIX magazine a couple of years
back.  MIX is a high end type publication for the recording industry.
I know they had an article once that described the details right down
to the use of each bit in a packet.  BTW, did you know that for each
stereo sample used on a CD (and we're talking 44,100 of'em per second!)
a total of 59 bits are allocated?  2x16 for each channel audio, the rest
are used or reserved for future expansion by such things as cut #, time
remaining, time elapsed, stuff like that.  There is even a subsection
that is solely dedicated to carrying video information.  I am not
making this up!  GRP came out with a jazz sampler that had the lyrics
and bio notes on the artists.  Only catch is that you have to have
a CD player that can decode this info and send it to your TV set
via an NTSC output.  It was called CD+G I think.  
Hope this provides some insite for you

Rock on
T



>
>ken@isgtec.com   [ ...!uunet.ca!isgtec!ken ]  Ken Newman
>