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Re: [AV Media Matters] migrating media

>Hey DV gurus- I'm absolutely convinced DV has "dropouts" like any other
>dang magnetic tape. What I wonder is whether we are seeing (when observing
>"pristine" playback), recovery success, or error concealment.  We do not
>really know that an image "looks the same" unless we are comparing them
>frame by frame, side by side.  Comments?

DV does have dropouts, absolutely. Due to the low C/N needed, the
low tape tensions used to begin with (less abrasion -> less cause
for dropout), and the high recovery rate of usable bits, dropouts
are not as large a problem as they are in most analog formats.
Visible dropouts are typically limited to around one occurrence per
hour in SP mode (with, of course, a large variation on a per-tape

Most DV transports and hardware codecs include dropout-detection and
error concealment, but none of the first-generation software codecs
did. When doing a 1394 transfer, you're doing a data dump of raw
bits; people using 1394-based NLEs noticed audio beeps on disk
playback that didn't occur listening to the analog audio out of the
deck. Sure enough, these were dropouts. They typically last for one
frame. I will typically see one or two such errors in a one-hour
dump to disk, so my take on it is that recovery is pretty darned
good and that concealment isn't called upon that often. Other
peoples experiences vary.

The 1394 data dump, bypassing error concealment, means that (a) your
dub is a clone of the original (verified in multigeneration dubbing
by quite a few parties who didn't trust the claims of digital
transparency), and also that (b) any momentary dropouts on playback
of the source tape are faithfully recorded as bad data on the dub.
Fortunately for most tapes (so far; we only have 4 years on the
format!) the dropouts are minimal.

For best results/longevity/interchange, I would suggest dubbing to
DVCAM. DVCAM appears to be much more resistant to dropouts (that
extra 50% track width really makes a difference) and the wider
tracks of DVCAM make for better chances of recovery when you allow
for tape distortion over time. DVCAM will record DV's unlocked
audio, though with some machines you need to trick it into doing so
(see http://www.adamwilt.com/tidbits.html). The tape formulation is
the same, and DVCAM will play back in DVCAM, DVCPRO, and most Sony
DV VTRs and camcorders, at least as long as you use mini cassettes.

Adam Wilt

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