Posted by Giraud on 01/05/07 18:12

I found this old thread while googling for MiniDV copy errors because I
have just done an exhaustive investigation into this very thing after
some recent video capturing I did. I hope I can contribute at least
one data point about DV reliability (i.e. "losslessness"). The results
are quite surprizing!

It started when we had to get a new inexpensive camera. Our old TRV900
starting to show audio (and some video) dropouts consistently (i.e.
every few seconds). Cleaning didn't help, and I didn't want to spend
more money and time getting that camera looked at, etc. We needed a
camera *now* to be able to produce videos reliably (and also capture our
old tapes). We have about 20 old MiniDV tapes, and I figured I'd
capture these digitally to hard disk for archival purposes (not trusting
tapes much at this point).

Anyway, I knew that many of the dropouts on recently recorded tapes were
there from the recording phase using that bad camera (but it was hard to
tell if it was recording or playback that was the problem), so I
expected them on capture with the new camera, but one particular tape
had new recordings made on this brand-new camcorder as well. I did a
couple of captures over firewire of this tape, and for kicks, I decided
to do a bit-for-bit compare of the two captures. Mind you, this is a
brand new camcorder (although inexpensive: Sony DCR-HC26) with a very
new tape.

I started by comparing the AVI files using "cmp" in Linux (the AVIs are
"Type 1" captured using WinDV). Noting that the sizes of the AVIs were
exactly the same (number of bytes), I figured I had a shot at exact data
without frames dropped (WinDV always seems to report exactly 1 dropped
frame). But I also knew that the AVI format might have slight
differences in header data or whatever, making comparing AVIs an invalid
process. As expected, the files had some byte differences.

So I went further. I used a free Ulead tool to convert both captures to
Type 2, and I used "mplayer" (Linux) to convert videos into individual
frame images and audio wav files. I then logged all differences. There
were some clips made with the old camera and 9 clips made with the new
camera. One new clip had audio differences, but the other 8 did not.

As expected, the old recordings had bit differences in audio and frames.
More unexpectedly, the new clips also had differences, but fewer. The
new recordings matched better, with only 4 out of the 9 clips showing
any bit differences. One new clip had audio differences, but the other
8 did not. We are talking only a few frames out of thousands, of course.

So I delved into the differences between the frames, and many were
completely invisible to the eye without *really* looking. I used a
Linux tool called "compare" to create difference images to point out the
areas in the image where there were numeric pixel mismatches. This
clearly showed the small DCT (8x8) squares that tended to be in groups
on the image. One case showed differences in horizontal bands as well.
But quite often there was *no* obvious glitch to the naked eye. I had
to stare at the right place in the two images to see the difference,
looking back and forth between them. In many cases, I could detect no
difference at all. I think this is a case of the camera concealing
errors rather than correcting them and doing a pretty good job in many
cases. Of course, some images had visible differences (like shifted
sections), sometimes happening at the start of the clip.

Bottom line:

1) Uncorrectable errors were frequent, even within one fresh tape with
new equipment. I know there are uncorrectable errors because two
captures produced different bits. There may have been other
uncorrectable errors that were concealed the same way on the two
captures, but my test would not have detected that.

2) Many (most?) uncorrectable errors are invisible. So the claim made
earlier in this thread that "I did not see glitches, so the copy is
bit-perfect" is not a good claim. In other words, "concealed" errors
could very well go unnoticed but still cause bit differences/loss over
generations.

Now maybe I have a bad camera (actually two: my old one that got bad at
some point, and a new one that is bad out of the box), but I am assuming
for now that this is the nature of MiniDV. On the new recordings, I
never *noticed* any dropouts at all, but the bit comparrison showed a
few mismatches here and there. Most of the tape was probably copied
bit-perfectly, but obviously not all of it. In fact, there were enough
differences to make me call this "generational loss". It might go many
generations before the loss becomes obvious, but it's still there.

If anyone wants to look at the images I saved, let me know, and I could
put them up on a site.

-Giraud


Binba wrote:
> Thanks for all the responses... apparently this did stir a heated
> debate :-)
>
> I think there's truth in what everyone says: what can cause "generation
> loss" in a firewire dub will be accumulative tape errors and cable
> errors. The result won't be a general degredation of the picture,
> rather more dropouts.
> It will be actually interesting to do a binary comparison of the tape
> data after 1,5,10 etc. copies. I'm sure someone has done it...
>
> PTravel, I have to disagree with you though. At least in the earlier
> days (circa 1998), we were working with the VX-1000, and I saw a LOT of
> dropouts. It's the same reason that made me write this post: I'm now
> cutting a short film shot with the XL-1, and 2 out of the 5 DV original
> tapes are in bad condition - they play with dropouts throughout on any
> deck which isn't in mint condition.
> There's a reason why when it comes to masters people are strict about
> stock choice, not recycling tapes, DVCAM instead of DV etc.
>
> I moved the first question into a new post.
>

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