taylorj@yvax.byu.edu (11/30/90)
>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