Xref: utzoo comp.periphs:1015 comp.sys.dec:690
Path: utzoo!attcan!uunet!husc6!cfa!wyatt
From: wyatt@cfa.harvard.EDU (Bill Wyatt)
Newsgroups: comp.periphs,comp.sys.dec
Subject: Re: Helical-Scan Query
Message-ID: <1044@cfa.cfa.harvard.EDU>
Date: 14 Jun 88 15:02:59 GMT
References: <2648@usceast.UUCP> <11959@mimsy.UUCP>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 30

>>     Several weeks ago, I posted a request to the net for information
>>concerning Helical-Scan backup devices. ...
> 
>>>I am very wary of such devices. Since there is NO way to
>>>verify (read after write) on a vcr [besides stopping the tape, [...]
> 
> Note that the Exabyte 8mm videotape backup (and another similar 8mm
> system---I forget whose, but it has a Pertec interface instead of SCSI;
> it is the same drive with a different controller) are not VCR backup
> systems.  Apparently most VCR systems do indeed have no motor control.
> The Exabyte does, hence it could do verification.  (Whether it in fact
> does, I have no idea.)

I missed the original request, or I would have supplied some info
here.  The Exabyte 8mm system uses a special head that DOES allow
read-after-write.  Thus, blocks in error are caught and rewritten
right away, some number of physical blocks down the tape.  Combined
with about a 40% ECC overhead (i.e. about 400 extra bytes are written
for each 1024 data bytes), the stated error rate of 1 in 10^13 is 
quite reasonable.

If you want a few hundred word summary of our experiences with these
(we've had a few for several months), e-mail me and I'll respond.
-- 

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