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Posted by Richard Crowley on 07/11/06 18:22
"Steve King" wrote ...
> Your comments about the DV standard allowing for bit-errors is over my
> head. Do all DV tapes have bit errors?
Average 100 per minute according to Sony. They rate
DVCAM at only 50 per minute. But almost all are corrected
by the ECC.
> If so, and if they accumulate with each transfer, then I guess that would
> be generational loss.
The ones that can't be bit-perfect corrected by the ECC will
accumulate. To my way of thinking it is the digital equivalent
of "generation loss". I never go past the 2nd generation "digital
clone" of a DV or DVCAM tape, so I anticipate that the error
rate is unlikely to ever affect my day-to-day activities.
> Oh, and is computer tape as used in backing up the most valuable data on
> the planet different from DV tape? Or, are we really interested any more?
> ;-)
The physical tape may be exactly the same. But, how the data is
handled on the tape (and how much raw capacity is devoted to
ECC) is different depending on the application (hard data vs.
audio/video data).
There are many different error detection and correction schemes
out there.
http://en.wikipedia.org/wiki/Category:Error_detection_and_correction
But as with anything, they come at the cost of taking up space that
you could have used for storing data. Media used for computer data
("Orange Book" CD-ROM, hard drives, computer data backup tape
formats, etc.) expend a significant portion of their capacity on the pure
overhead of storing checksums, redundant information, etc. so that you
are ensured of a "bit-perfect" data storage and retrieval experience. Of
course, nothing on this earth is "perfect", but computer data medium
are rated at a failure rate so tiny that you may see one only a few times
during your lifetime.
OTOH, some digital media which are intended for "soft data" like audio
or video can get away with a higher failure rate because it is both easier
to extrapolate missing data, AND it is within the acceptable range of
impact to the user to extrapolate rather than guarantee bit-perfect. By
taking up less space for the ECC overhead, they can use more of the
raw capacity of the medium for storing the audio/video data.
Note that there are some schemes for backing up your hard drive
data to mini-DV tapes, etc. But they add extra ECC overhead to
the data stream before it ever reaches the DV tape, The extra ECC
makes up for the not-quite-bit-perfect performance of DV tape.
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