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Re: [AV Media Matters] SVHS Correction & Preservation
I realise that the DV format is lossy and that is one of the reasons
have not yet committed. However, DV compression works differently to
MPEG and is far superior. My understanding of DV compression is that
does not result in a loss of detail, but a loss of some colour
information per four-pixel block.
With a good codec, DV does preserve full luma resolution even over
multiple generations. However, the 5:1 DCT compression (the same basic
compression used with MPEG formats) is lossy, and the tradeoffs you
make are full detail preservation at the expense of more mosquito noise
artifacts (as in the Apple codec) or fewer artifacts but at the expense
of high-frequency detail and less accuracy in multigeneration work
(i.e., Avid XpressDV).
The chroma subsampling used in PAL-format DV and DVCAM is 4:2:0, which
is fairly problematic (see
chroma-bug-4-2001.html for a good discussion and some nice graphics of
4:2:0 and interlacing). NTSC DV & DVCAM, as well as DVCPRO25 the world
over, use 4:1:1 (http://www.adamwilt.com/pix-sampling.html.
In either case, the luma and chroma detail capabilities of DV exceed
the detail you'll have on your SVHS masters, and the resolution loss
from the analog original nicely suppresses the finicky detail that
causes most of the compression conniptions (mosquito noise) in the
first place. DV's chroma bandwidth far exceeds that of SVHS so even
4:1:1 subsampling is not an issue. I've mastered SVHS and Hi8 to DV and
been well pleased with the results; from those media, a DV copy is
FireWire or SDTI copies of DV/DVCAM/DVCPRO25 are generationally
lossless as no decompression/recompression occurs. Likewise, file
copies of Quicktime, AVI, or DV Stream files is lossless.
The widespread acceptance of DV formats in both consumer and
professional applications means that there will be equipment and
software to deal with the format for quite a long time; the weight of
numbers alone makes long-term viability of DV format files more likely
than, say, Ampex DCT or Digital Betacam. DV's adoption as a
computer-based editing format means that bouncing DV files between
computer storage media (instead of relying on a format-specific tape
deck) will likely remain viable for at least the next 20 years.
Of course, your mileage may vary. :-)
Adam "I'm still writing my video files on clay tablets with a pointed
stick just to be on the safe side" Wilt
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