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