Radboud Verberne wrote:
>
> Hi Guys, (Are there any women around here?)
>
> Quite a time ago I saw in an advertisement a Add-On card for your PC to
> Store data on a Common VHS-VCR. I was quite cheap, around 80 dollars. I have
> quite a lot of information about the signal on the VCR and the video-signal.
> The only problem with data stroing on a VCR is the errorcorrection I
> think... Where can I found good information about "error" free datastorage?
> An easy way to do this is ofcourse the parity bit.
>
> What are good methods to ensure that the data read is correct? CRC? What
> Other methods are normally used?
>
> Has anyone aready done this with a uP?
>
> (I Was thinking of a project to play digital audio on a VCR...)
>
> Greetz,
>         Radboud Verberne
>
> ICQ     : 918640 (I Seek You)
> HAM  : PE1RUH

  Having Parity just serves to tell you that you have a 1-bit error,
you'd need some sort of redundancy to let you recover the missing data
(Parity doesn't catch the case of 2 bits being toggled, and it doesn't
tell you WHICH bit was changed, which is rather important!)

  There are codes specifically built for this purpose (IIRC "Hamming
codes" is the name of one of these.) that can, given a halfways good
read of your data, reconstruct the munged bits & give you the original
data - which is what you want, not just "Uh-Oh, I'm doomed, last June's
business records are totally unreadable and unrecoverable", but, "How
annoying, this videotape's going old, I recovered all my data safely,
but now I need to schedule a full recover/re-backup session on the tape
mongering machine, with new videotape."

  With parallel port and/or SCSI Travan type tape drives available, Zip
Drives and Jazz drives, and even better drives coming, though, why use
Videotape? <G>  (Just MY bias.)  I'll always just use several machines
(some only powered up at backup time) on the home LAN here, to back up
critical data.  (If a 10 Mb file is all you need to back up, why not
just safely stow it on a few spare home LAN machines, so it takes a
major disaster to wipe all copies?)  And nowadays, just burn a CD-R on
occasion to clean out the really critical backup archives...

  (Anyone want a circa 1984 Irwin tape drive "aka time-waster" with a
whole 10 Mb capacity, they can talk me out of it really easily, having
sat through a few tape formatting sessions & then finding the rig
wouldn't always recover tapes it had just written - having it take
forever to access the tape, no matter what - and discovering that the
software that came with was disk IMAGING software that couldn't deal
with things like bad spots on the HDD, etc., sorta spoiled tape drives
for me I guess.)

  Media life is important, too;  Gold CD-R's last about 50 years, video
tapes last nowhere near that much.  I guess this is turning into a "no
VHS tapes for me" rant though so I'll stop <G>

  Mark, mwillis@nwlink.com