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Posted by Martin Heffels on 10/05/36 11:52
On Mon, 10 Jul 2006 17:27:07 GMT, "PTravel" <ptravel@travelersvideo.com>
wrote:
>But that's the whole point -- if it's corrected, it's not a drop out, and
>there's no data loss.
No, it is not. Because if the drop-out is severe enough, data will be
copied from another part (interpolated from adjacent frames or areas), and
that means you have dataloss when compared to the original.
From Keith Jack's "Video Demystified":
<quote>
Error Concealment
The ability to hide transmission errors that corrupt the content beyond the
ability of the receiver to properly display it. Techniques for video
include replacing the corrupt region with either earlier video data,
interpolated video data from previous and next frames, or interpolated data
from neighboring areas within the current frame. Decoded MPEG video may
also be processed using deblocking filters to reduce blocking artifacts.
Techniques for audio include replacing the corrupt region with interpolated
audio data.
</quote>
>If a particular section of tape exhibits flaking, but
>not severe enough so that the ECC can handle it, there's no drop out, and a
>copy made from that tape will be a bit-for-bit copy of what's supposed to be
>on the tape. There's no generational loss, because, by definition, the data
>was corrected.
See above.
>We're talking apples and oranges here. On the one hand, there's everyone's
>DV deck that can invent data if it's missing,
There is no deck which "invents" missing data. They interpolate, based on
what is there. Inventing means coming-up with something completely new
which didn't exist before.
> and on the other hand there's
>ECC which can _repair_ data, i.e. restore it to its original state, if
>there's an error. The former will result in generational loss (to the
>extent that it happens). The latter will not.
There is no guarantee that data can be restored to it's original form,
because the protocols try to correct and if this fails, conceal the error.
>That's what I said. The OP made a later post in which he was talking about
>the relative loss between, IIRC, SuperBeta and digital. SuperBeta, as good
>as it is, will result in generational loss because it's analog -- the loss
>isn't the result of any failure in the video chain, but of the inherent
>imprecision in trying to match analog levels.
That is a possible reason. But also the analogue signal will be amplified,
from a few micro-volts from the head, to 1 volt, and every time the signal
is amplified, noise is added. And this happens more than a mismatch of
analogue levels (by which I assume you mean impedance mismatch of the
cables).
>The big difference, though, whereas generational loss is inevitable with
>analog, loss due to drop outs or other uncorrectable errors is not
>inevitable with digital.
Indeed, but that again depends on the severity of the drop-out.
>Okay, this gets into statistics. If you agree that, for a given digital
>tape, drop out is not inevitable, then it is solely a question of the
>uncorrectable error rate for a particular digital tape medium which, I would
>think, is incredibly low. If you can copy a 60 minute tape to a computer
>and then back to another tape without an error, it is no more likely that,
>if you repeat the cycle, an error will be introduced, i.e. the statisical
>likelihood that you'll experience data loss, whether it's one dupe or 100,
>remains the same (just like the roulette wheel that has hit red 20 times in
>a row -- it is not more likely to hit black than red on the 21st spin).
Yes, the likelihood of an error is low, I agree with you. But the thing is
that it is not measureable for us. We have no idea how bad a tape can be,
because of all the error-correcting and concealing.
[...]
>And if it is completely and accurately reconstituted by the ECC, then there
>has been no data loss, and the copy made from the tape is bit-for-bit
>accurate.
Indeed :-) But only if the error can be repaired by using the inner and
outer-bits, which simply tell whether a row contains a certain amount of
1's or 0's.
[...]
>Again, we need to define terms. My camera does ECC (as does any D-25
>device). It does not synthesize video data from prior frames to make a drop
>out look unnoticeable, as due the digital decks that other posters have
>described.
Yes, it does. Every deck/camcorder does that.
>I don't think it's pristine. I do think that the ECC for D-25 is
>sufficiently robust so that, by using quality tapes only once, and
>maintaining my camcorder, I can produce digital transfers to and from the
>computer without any dataloss.
Yes and no.
>This was a discussion about generational data loss, not error correcting
>algorithms.
One determines the other.
>It also has to be more robust because (1) hard drives are constantly
>re-written, (2) data is packed more closely on hard drives, and (3) data is
>read more quickly on hard drives. The ECC on miniDV is sufficiently robust
>that the likelihood of losing any data on a transfer is very, very low.
It is a very loose ECC on tape. If it was really very tight, it would stop
the capture/copy on an error, rewind, and play again, untill it was sure
the data was 100% correct. A hard-disk does this.
cheers
-martin-
--
"If he can he'll smile 'cos he's a Royal Crocodile."
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