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From: steven@pacific.csl.uiuc.edu (Steven Parkes)
Newsgroups: comp.misc
Subject: Re: Exabyte (8mm) versus DAT (4mm)
Message-ID: <1990Jul7.165608.18474@ux1.cso.uiuc.edu>
Date: 7 Jul 90 16:56:08 GMT
References: <9007061713.AA01816@stc06.CTD.ORNL.GOV> <1881@proa.SV.DG.COM>
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Organization: Coordinated Science Laboratory, University of Illinois
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In article <1881@proa.SV.DG.COM>, gary@dgcad.SV.DG.COM (Gary Bridgewater)
writes:
|> In article <9007061713.AA01816@stc06.CTD.ORNL.GOV> de5@ornl.gov (Dave Sill)
|>     writes:
|> >DEC seems to be pushing DAT's, and argues that they're inherently
|> >better since they were designed to record digital data, whereas 8mm is
|> >an analog video format.
|> 
|> Why wasn't this sent to rec.humor.funny?  This is the funniest thing I've
|> seen on the net this year.  Did someone actually say that?
|> 
|> Digital Audio Tape was designed to record SOUND.

Hmmm ... we're looking at getting a 4mm tape drive too, so I have some of the
same questions ...

There is a fairly big difference between analog and digital technology, and the
implications have a potentially big effect on the control of the underlying
drive-motor hardware.

Performance in analog hardware is usually measured in things like RMS error and
other `flutter'-like error measures.  On the other hand, digital is different
in that it is usually more of an `all-or-nothing' technology and therefore
often has error-detection-and-correction added at a very low level.

Also, analog video is a very highly-structured format ... all the analog video
tape recorders that I know of use the sync structure of video to phase-lock the
tape drive motors implying that the data and control interact in a possibly
significant degree.

Clearly, the underlying hardware is analog in either case and that digital
controllers added to video-designed drives are not theoretically incapable
of providing the same results as digital-from-the start products ... however,
its also a pretty challenging task.

My particular questions relate exactly to this matching of the digital to
analog in 8mm drives, and in particular are all the features normally found
in digital tapes avaialable from these drives.

Of most concern:

1) can video tapes be reused `many' times?  2? 5? 10? 100?  In VHS anyway, it
   seems like there is a noticiable degredation from many reuses.

2) are all the positioning commands used in digital tape available, i.e.
   forward and backspace by record and block as well as file?

3) how are blocks of constant/varying size handled?  What are the performance
   implications?