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From: mjs@eagle.UUCP (M.J.Shannon)
Newsgroups: net.micro
Subject: Re: CD-ROM speeds...
Message-ID: <1291@eagle.UUCP>
Date: Mon, 22-Jul-85 09:53:05 EDT
Article-I.D.: eagle.1291
Posted: Mon Jul 22 09:53:05 1985
Date-Received: Wed, 24-Jul-85 21:32:38 EDT
References: <9@brl-tgr.ARPA>
Organization: AT&T Bell Laboratories, Summit, NJ
Lines: 29

I, too have been looking at CD-ROMs and CD-RAMs (coming in 1-2 years; some are
available now), but I see them not as an active disk, rather as an archival
store (like we currently use magtape).  For archive storage and retrieval,
these media would seem ideal.  The relatively long access times are of little
importance in this application, but the capacity and media lifetimes are ideal.
To give you an idea of the capacity, I present some `back of the envelope'
calculations:

	   75 minutes playing time (upper limit on current audio discs)
	   60 seconds/minute
	44100 samples/sec
	    2 bytes/sample
	    2 channels (stereo)
	----- (multiply)
    793800000 bytes/disk

or approximately 760 Megabytes per disk (that's 76 10-Meg winnies, folks)!  The
upper limit of 75 minutes is due to the higher level of surface defects on the
outer edge of current discs, but I'm betting on technology to clean up that
surface to make the outer inch fully usable (for the record (disc?), the audio
discs I've listened to that approach the upper limit have no errors).  Also,
the calculation above doesn't count the 6 bits of ECC code per sample (i.e., a
16 bit sample is represented as 22 bits) on the grounds that a data application
would need at least as much ECC.  My memory is a little hazy on the numbers
cited, but I believe them to be accurate.
-- 
	Marty Shannon
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