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'[OT] Data on VCR?'
1998\11\09@052829 by Radboud Verberne

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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

1998\11\09@054118 by Russell McMahon

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I have here an 8 bit ISA bus  "Backer" card which alleges it can
store 1500MB (1.5GB) to a 3 hour VHS tape.
It belongs to a friend but I think it has never been tried -
presumably they work.
Transfer rate is said to be up to 9MB/minute (which approximately
matches the stated capacity).
Maybe this would suit your purpose - I don't know how they handle
error correction but they must do so.

This is dated 1996 so they may still be around and may have expanded
to audio-digital.
Single IC is a Danmere DAN0428A (that figures) in a QFP68 pkg (counts
pins with magnifying glass ...).


Maker is 'Danmere" in UK
Dunno web address (if any).
Snail mail is Danmere Ltd, Whitehall, 75 School lane, Hartford,
Northwich, Cheshire, England CW8 1AZ.

Has anyone tried these?

regards

               Russell McMahon


{Original Message removed}

1998\11\09@060951 by Nigel Orr

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At 23:31 09/11/98 +1300, you wrote:
>Transfer rate is said to be up to 9MB/minute (which approximately
>matches the stated capacity).

CD audio is 2 channels at 44.1ksps, with 2 bytes per sample- a total of
176.4kbytes per second (slightly faster than a floppy disc!) or 10584
kbytes per minute.

http://www.danmere.com is the website for the Backer

Nigel
--
Nigel Orr                  Research Associate   O   ______
       Underwater Acoustics Group,              o / o    \_/(
Dept of Electrical and Electronic Engineering     (_   <   _ (
    University of Newcastle Upon Tyne             \______/ \(

1998\11\09@082339 by cousens

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Russell McMahon wrote:
SNIP
> Maker is 'Danmere" in UK
> Dunno web address (if any).
> Snail mail is Danmere Ltd, Whitehall, 75 School lane, Hartford,
> Northwich, Cheshire, England CW8 1AZ.
>
> Has anyone tried these?

I have one, I have tried it but the data rate is affected by long leads

I have given up on it for these reasons:

I haven't got room for a video (and TV) next to my computers.

I can't be bothered to plug in and out all the plugs every time.

It's more practical and almost as cheap to fit a second hard disk
as a backup.

Peter Cousens
email: spam_OUTcousensTakeThisOuTspamher.forthnet.gr  phone: + 3081 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

We cannot blame God for creating a World with so many problems,
He had just upgraded from win3.11 to win95.
No wonder Eve recommended Apple.

1998\11\09@113137 by Radboud Verberne

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>>Transfer rate is said to be up to 9MB/minute (which approximately
>>matches the stated capacity).

Have you ever tried to put a backup tape into your VCR? What did you see?

>CD audio is 2 channels at 44.1ksps, with 2 bytes per sample- a total of
>176.4kbytes per second (slightly faster than a floppy disc!) or 10584
>kbytes per minute.

In the PDIF signal you have for each sample 16bits of audiodata and 16 bits
of extra data (preamble, copy protectionbit, userbit etc.). So you come up
to a transferrate of 44100*2*32 --> 2.8244 Mbit/s /8 --> 353k/s.

But I was thinking to first encode it with MPEG1 layer 3. (See another mail
in the list) So i get an enormous datareduction. Since the bandwith of a VCR
is about 3Mhz it should be suitable for the job.  The most important thing
for me is how to put the data in a NORMAL video signal, so there are no
changes needed inside the VCR. I was thinking to encode a 1 as a white dot,
wich is 1Vpp and a zero as an Black signal, 0.3Vpp. For datareduction you
could also use a Gray color for a 01 or 10. (When the signal is 010101010
you have maximimum bandwidth with this method). Then you have still the
problem of the SYNC pulses, when the sync is generated there no data
possible. So with  a continues datastream you should use an buffer.

The mostimportant thing is how to be sure that the decoded sigal is correct?
I do need quite a good errorcorrecton I think... Any suggestions on this?

The audio has a bandwidth of about 15Khz and is perfect to store the
Track/Album title using FSK...

>
>http://www.danmere.com is the website for the Backer


I did not look at it yet, in Holland we still have to pay for making
localareaphonecalls. (So i only connect to the internet after 20:00, because
it's two times cheaper then).

Tnx for the URL.


>Nigel
>--
>Nigel Orr                  Research Associate   O   ______
>        Underwater Acoustics Group,              o / o    \_/(
>Dept of Electrical and Electronic Engineering     (_   <   _ (
>     University of Newcastle Upon Tyne             \______/ \(

1998\11\09@115152 by Dave VanHorn

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> But I was thinking to first encode it with MPEG1 layer 3. (See another mail
> in the list) So i get an enormous datareduction.

That you do.. Just try restoring it.
MPEG is lossy compression, so you won't be able to use any files that
you compress.
With sound and pictures, all that matters is that the end product be a
lot like the original.  Programs are a bit more picky.

You'll have to compress, but lossless (ZIP type) compression is your
only choice. You'll also need forward error correction, and lots of it,
since dropouts are expected in the media. This will further cut the
available bandwidth.

1998\11\09@115821 by Peter L. Peres

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On Mon, 9 Nov 1998, Radboud Verberne wrote:

> 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

It's still around, only slightly cheaper. I think Jameco has it.

> 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.

Aiee. This is an extra large can of worms with no end to it. You are
looking for a framed protocol with some redundancy in it. This means ECC
usually. Then there is the issue of the protocol proper, block size, what
happens if an error is too big for ECC and much more. For a good start
(since you want music) take a good look at the CD Red Book (I think)
which solves many of the problems that will occur with your system.

BTW the real data rate useable with a VHS VCR is probably of the same
order of magnitude as on a music CD. The commercial version advertises
about 200kBytes/sec compressed speed, about as fast as a single speed
CDROM.

> What are good methods to ensure that the data read is correct? CRC? What
> Other methods are normally used?

As I have said, you need to build redundancy into the data, i.e. somehow
send each bit more than once, in case one of them does not make it. How
much of this you need to do depends on the error rates in the system. A
dumb method is to send each data packet with CRC twice, with a skew
between the copies (meanwhile you send more packets). Then, if the 1st
packet does not make it, you read the 2nd. This is because you can't/won't
rewind the tape to re-read anything in real time.

More elegant methods are Reed-Solomon and other error detecting/correcting
polynomial encodings. These do the same thing (build redundancy into the
data stream) but at a much lower cost in bandwidth reduction.

Then there is skewing or spreading of data packets such that a media error
will not cause enough damage to void any single self-correcting packet
(see the music CD recording format for a good explanation on this).

Peter

1998\11\09@120853 by Mark Willis

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Radboud Verberne wrote:
{Quote hidden}

 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, .....mwillisKILLspamspam@spam@nwlink.com

1998\11\09@140833 by Peter L. Peres

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On Mon, 9 Nov 1998, Radboud Verberne wrote:

> In the PDIF signal you have for each sample 16bits of audiodata and 16 bits
> of extra data (preamble, copy protectionbit, userbit etc.). So you come up

Nope protection etc is transmitted on the subchannels and in the headers,
the correct rate is 44.1k x 16 ~= 1.44 Mbps ~= 180 kBytes/sec

> But I was thinking to first encode it with MPEG1 layer 3. (See another mail
> in the list) So i get an enormous datareduction. Since the bandwith of a VCR
> is about 3Mhz it should be suitable for the job.  The most important thing

Think again. It's 1.5 MHz or so at a tolerable S/N.

> The audio has a bandwidth of about 15Khz and is perfect to store the
> Track/Album title using FSK...

imho, don't mix too many systems. You might spend a year or so evening out
the bugs... There is plenty of room for 32 characters in 10 minutes of
1.44 Mbps data stream.

Peter

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