Posted by Toby on 07/14/06 00:12

"PTravel" <ptravel@ruyitang.com> wrote in message
news:4hnbilFdriuU1@individual.net...
>
> "Toby" <kymarto123@ybb.ne.jpp> wrote in message
> news:44b5b15e$0$65941$bb4e3ad8@newscene.com...
>>
>> "PTravel" <ptravel@ruyitang.com> wrote in message
>> news:4hfsbtF1rcvsvU1@individual.net...
>>>
>>> "Richard Crowley" <richard.7.crowley@intel.com> wrote in message
>>> news:e8ucis$8o5$1@news01.intel.com...
>>>> "PTravel" wrote ...
>>>>> We're still talking about apples and oranges -- data reconstructed
>>>>> using ECC is accurate and bit-for-bit what was recorded (or supposed
>>>>> to have been recorded).
>>>>
>>>> No. That is the description of how ECC works for computer
>>>> data. There is another MAJOR error recovery layer used by
>>>> audio CDs, DVDs, DVD tape, etc. It is how they manage to
>>>> be cost effective.
>>>>
>>>> Audio/video data that cannot be accurately *corrected* gets
>>>> *mitigated* by extrapolation. This is the factor that is present
>>>> in audio and video digital recording but NOT in computer data
>>>> recording. It probably comes into play more often than we think,
>>>> but if it is good enough we don't notice. But multiple repetitions
>>>> of it amount to something approaching "generation loss".
>>>
>>> Can you site me to something that says how often this happens? I've
>>> never heard of DV video transfers being described as anything other than
>>> lossless.
>>>
>>>>
>>>>> Replacing pixels (or scan lines or averaging blocks) results in lost
>>>>> data.
>>>>
>>>> I think you have that backwards. Replacing pixels, etc. is one
>>>> of the major migitating actions taken as a result of lost data.
>>>
>>> I meant that the resulting data is not the same as the original, hence
>>> there is data lost. But how often does this happen?
>>
>> This reminds me of a story. How often do you think earthquakes happen in
>> Japan? Once a month? Once a week? We recently did a doco about
>> earthquakes here and visited one of their seismo centers. It turns out
>> that the rate of measurable earthquakes is about 100 an hour.
>>
>> Do you see what I am getting at?
>
> Yes, which is exactly what I've been getting at. Borrowing Martin's
> terminology, what is the ratio of mitigated errors to fully-corrected
> errors? If it's comparable to the ratio of major earthquakes in Japan
> versus small tremors that no one notices, then we're looking at Richard's
> guestimate that works out to 1 mitigated error per 666 hours of video.
> With that kind of rate, miniDV is, for all intents and purposes, lossless.
>
> However, we remain in the realm of speculation -- without hard data, this
> is all pointless, and I'll stand by my position, i.e. there is no
> generation loss for D-25 transfers.

OK, here are a couple of clues. First a quote from an ad for Sony tape:


"DVCAM tape has a 50% lower dropout rate vs DV resulting in a four- to
five-fold improvement in the error rate margin, which yields a better
picture."

So somebody is lying, is it Adobe or Sony?

Here's a quote that perhaps expands on that theme:

"Finally, banding or striping of the image occurs when one head of the two
on the scanner is clogged or otherwise unable to recover data. The image
will show 10 horizontal bands (12 in PAL countries), with every other band
showing a "live" picture and the alternate bands showing a freeze frame of a
previous image or of no image at all (or, at least in the case of the JVC
GR-DV1u, a black-and-white checkerboard, which the frame buffers appear to
be initialized with). Most often this is due to a head clog, and cleaning
the heads using a standard manufacturer's head cleaning tape is all that's
required. It can also be caused by tape damage, or by a defective tape. If
head cleaning and changing the tape used don't solve it, you may have a dead
head or head preamp; service will be required.

This sort of banding dropout occurs fairly often; about once per DV tape in
my experience. Usually it isn't even noticeable -- a single frame of banding
due to a momentarily clogged head won't be visible unless there's motion in
the scene to show off the frozen stripes. Have a look through your old tapes
frame by frame (on a slow day, of course!) and you might be surprised how
often you'll be able to find a single, subtly banded frame. For what it's
worth, I've only rarely found such a banded frame on any DVCAM footage I've
shot, which indicates to me that DV is right on the edge of reliability.
DVCAM, with its 15 micron track width, or DVCPRO with its 18 micron track,
are sufficiently on the safe side of the bleeding edge so that this sort of
droput is much less likely to occur.

Bear in mind that analog BetaSP typically has several dropouts per minute;
the last time I measured visible dropout rates on Hi8 and S-VHS I got
numbers in the range of a dropout every 3-5 seconds (Hi8) and every 7-20
seconds (S-VHS). One visible dropout per hour-long tape, on average, is not
something to get flustered about. But if it does bother you, shoot DVCAM or
DVCPRO instead."

"Digital Dropout: DV is highly resistant to dropout because the same data is
recorded multiple times within the track. The playback electronics compare
the multiple data streams, and reject those that are corrupted by an oxide
particle dropping off the tape. The dropouts are still there, but you don't
see them on the screen. But if the dropout is big enough, the correction
circuits don't have enough data to work with, causing digital dropout, which
results in a sudden blockiness in the picture. The worse the dropout, the
larger the blocks, until the picture is lost completely. They are much rarer
than analog dropouts, but they're much more noticeable when they do happen."

Finally, view this pdf file if you want to see dropout rates of DV vs. DVcam
graphically displayed with counts/min.

http://www.pmdmagnetics.com/displayfile.asp?id=48140.

It's pretty good, but it ain't perfect.

Toby

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