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From: taylorj@yvax.byu.edu
Newsgroups: comp.ivideodisc
Subject: Re: CD-ROM vs. Videodisc (was Interactive videodisc project--American Sign Language
Message-ID: <1852taylorj@yvax.byu.edu>
Date: 30 Nov 90 07:13:22 GMT
Lines: 39

>About CD-ROM: it's my understanding that a videodisc holds about 3.5
gigabytes >of information, and a CD-ROM about 0.6 gigs. Can anyone confirm
or dispute >these figures? My grey matter is notoriously unreliable...

It's almost impossible to compare CD-ROM to videodisc because CD-ROM is
digital and videodisc is analog.  (Contrary to what people may tell you,
videodiscs are NOT digital storage; they store an analog video signal. If
you attempted to store video on a videodisc at full resolution without
compression, you'd only be able to fit a few seconds of video on a disc.
Digital audio takes up much less space, which is why audio CDs are practical
and why newer discs can have a digital audio track.)

Saying a videodisc holds about 3.5 gigabytes of information is kind of like
saying a room holds about 20 animals.  You don't know if you're referring to
elephants or cats.  With a videodisc you don't know if you're referring to
24-bit, full screen, full motion, uncompressed video (which would take
around 796 gigabytes!), or 8-bit, small screen, 15 frames/sec, compressed
video.

But I'll still try to answer the question.  A CD-ROM holds about 650
megabytes (0.6 gigabytes) of data.  If you digitize video to a standard
640x480 8-bit image, you can fit about 2200 images on a CD-ROM, which at 30
frames/sec gives you 73 seconds, or just over 1 minute of video.  Obviously,
compression is sorely needed.  The latest DVI chips can achieve video
compression of around 100:1 to 120:1.  Accounting for the space taken up by
the digitized audio track, this gives you about an hour to an hour and a
half of video on a CD-ROM.  Unfortunately, a 640x480 8-bit image is really
not all that good (it's what you get on an IBM Super-VGA system or a
standard color Mac II).  So you can't fit nearly as much high quality video
on a CD-ROM.

So the answer is that you can get about the same amount of
video on a CD-ROM as you can get on a videodisc (counting both sides), but
the picture quality will suffer noticeably.


Jim Taylor
Microcomputer Support for Curriculum  |
Brigham Young University              |  Bitnet: taylorj@byuvax.bitnet
101 HRCB, Provo, UT  84602            |  Internet: taylorj@yvax.byu.edu