Posted by Richard Crowley on 01/12/11 11:52

"PTravel" wrote ...
> I'm just looking at the resulting copy. I take a black box approach to
> these things, i.e. if the output is the same as the input, it doesn't
> matter to me what happens inside the black box to make it so.

And that is my attitude as well. DV is more than adequate for all the
different kinds of video production I do as long as I stay within the
parameters.

> In this discussion, if I wind up with a bit-for-bit accurate copy, it
> doesn't matter to me whether the ECC guessed, used psychic prediction, or
> had employed a time machine -- a bit-for-bit accurate
> copy means no generational loss.

Perhaps you don't understand the difference between real data
*correction* (via ECC) and migitagion/extrapolation. The former is
bit-perfect, and the latter takes advantage of the nature of the data
to make educated guesses. Most of us (me included) won't know
or care whether the data is bit-perfect as long as it is good enough,
and that is the nature of DV (and audio CD, and DVD, etc.)

> We're not disagreeing about that. My question is this: how often, in
> duping from a DV tape, are there data failures that cannot be corrected
> exactly as they are for computer files? I've never heard anything other
> than, "so rarely that it's not a concern."

If everything is optimal (equipment within interchange adjustment,
clean, not worn-out, etc.) then it very likely is "so rarely that it's not
a concern". But that is not the same as "bit perfect". If comptuer
data had the same uncorrectable error rate as audio and video
digital data, we would have abandoned using computers for anything
but casual gaming.

> All I know about audio CDs is that my computer employs error correction
> when it reads them.

Every CD player down to the $15 one from Wal-Mart employs
error correction when it reads them. They would not be readable
without sophisticated ECC.

> However, that's still audio CDs -- I'm asking about digital video tape.

Same thing happens to all digital media, whether magnetic or
optical. Dunno whether you had seen this before?
http://www.adamwilt.com/pix-defects.html

> I never said it was zero, as people in this ng have reported the
> occassional data-losing dropout. Heck, cosmic rays could probably cause
> it, but how often do cosmic rays effect video playback (answer: not
> often).

When you used the terms (previously in the message I am replying to)
"if the output is the same as the input", and "bit-for-bit accurate copy",
I took that to mean "zero". If we agree that there is a non-zero rate
of uncorrected errors, then we also agree that there is some non-zero
"generation loss" (or whatever you want to call it).

If you want to believe that DV tape technology is bit-perfect, I don't
see any reason to try to persuade you otherwise. I'm sure that the
engineers who designed the system dearly wish it was as simple as
your concept of it.

In any case I will continue to use DV tape technology without
regard to its <100% bit-perfect accuracy. Since I never intend
to abuse the format by doing anything past 2nd or 3rd generation
tape-to-tape dubbing, my only concern is something physiclly
happening to the equipment (like when my DSR-300 lost one of
its tape guides). Regardless of how much we know about the
internals, we both (and millions of other users)

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