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Posted by Richard Crowley on 10/05/44 11:52
"PTravel" wrote ...
> But that's the whole point -- if it's corrected, it's not a drop out, and
> there's no data loss.
That is a philosophical difference and you appear to be using
the existentialist definition. You can be sure that the engineers
who design the guts of these things take a far more "rationalist"
view where they are indeed dropouts. :-)
> There's no generational loss, because, by definition, the data was
> corrected.
If you are talking about computer files, you would be exactly
correct. But I believe the context here is video (and audio)
data on DV tape, where the rules are different. Some data
failures can be corrected exactly (as they are for computer
files). But then if the data cannot be corrected, the failure is
mitigated with other kinds of recovery.
For example, even audio CDs, which contain uncompressed
16-bit samples of the audio very very rarely play back perfectly.
Data errors are routinely mitigated by extrapolating the missing
values and you very rarely hear the difference. But the result is
NOT bit-for bit. Same with digital video.
> 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, 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.
DV tape equipment depends heavily on BOTH levels of error
detection, correction, and mitigation. DV wouldn't work otherwise
(or it would hold only 30 minutes of 100% reilable video where
we now get 60 minutes of "very good" reproduction).
> 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.
It actually is inevitable with digital also. Digital relies on the same
analog technology for recording as analog video did. Extensive
ECC in hard drives keeps this inevitable loss to a vanishingly low
rate. And digital video tape uses mitigation to mask the higher
rate of failure in DV recordings. But the mitigation is usually so
good that we think we are geting 100% reliable data storage and
recovery.
> Okay, this gets into statistics. If you agree that, for a given digital
> tape, drop out is not inevitable,
We will have to agree to disagree at that point. No playback
of digital tape is 100% accurate. Dropouts ARE inevitable, but
the built-in ECC of DV makes us think we are geting 100%
back.
> 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,
You can't do that "without an error". You can do it well enough
that most of us won't see a visible error, but guaranteed it is not
100%. But if it is close enough to 100%, you can go many
generations before you start seeing the effects of all those
corrected errors.
> 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.
But not low enough to be "zero" (or even invisible) after 20
generations of digital dubs.
>> The expectation
>> for data is that you will get one uncorrectable error in several
>> trillion bits whereas most of us likely have seen and/or heard
>> uncorrectable errors in digital audio/video media in recent memory.
>
> As I said, they happen. Just not often enough so that they should be
> considered a source of generational loss.
Then we have come full circle back around to semantics.
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