|
|
Posted by PTravel on 10/05/37 11:52
"Martin Heffels" <youwishyouwouldknow@nottellinya.com> wrote in message
news:o7b5b25qf7up8n6ffg8sj9oj76ehur1ovn@4ax.com...
> 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>
That's still apples and oranges. I didn't say that this kind of
"correction" isn't done. I said that it's extremely rare and the kind of
common "drop out" that you've been discussing is fixed by ECC in such a way
that no data is lost.
>
>>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.
You see above. You understand how ECC works, and you also understand that
ECC is capable of correcting small errors without data loss.
>
>>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.
Now who's arguing semantics? Interpolated data is data that is different
than what was actually recorded, hence it's invented.
>
>> 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.
Doesn't matter. The kind of gross data correction that results in
"interpolation" of data is extremely rare.
>
>>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).
No, I didn't. I meant that analogue waveforms get smoothed in an analogue
copy, that noise is introduced, etc. Impedence mismatch is one way that an
analogue signal can be degraded, but it's hardly the only way.
>
>>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.
That's right. How often do you think miniDV experiences drop out so severe
that it results in an uncorrectable error?
>
>>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.
If it's been error-corrected, it doesn't matter, because a bit-for-bit copy
results and there is no generational loss. If it has been concealed, it
does matter but I don't believe that happens often enough (with proper
technique -- I'm sure you can induce significant drop out if you put your
mind to it) to be a consideration when duping digital data.
>
> [...]
>>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, doesn't matter. All that matters is whether you wind up with a
bit-for-bit copy or not.
>
> [...]
>>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'll take your word for it.
>
>>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.
In my case, yes and yes, as well as in the case of others who have posted
about drop out to this newsgroup, e.g. drop out reported due to a bad tape
batch, etc.
>
>>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.
My capture software can stop on error -- never had it happen, though.
>
> cheers
>
> -martin-
> --
> "If he can he'll smile 'cos he's a Royal Crocodile."
Navigation:
[Reply to this message]
|