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[AV Media Matters] LOSSY as Loss-less - hmmm
Adam brings up some very important points, and frankly if he hadn't I
would have feel honour bound to jump in just a bit.
I have become increasingly distressed over time to find that MANY people
seem to think that making a copy of a digital file - no matter what type
of copy, no matter how it is made, is a lossless procedure - i.e. there
is no difference mathematically (or visually) between copies. This is an
incorrect assumption. IF and I repeat IF copies are made in an
particular way it is possible for them to be identical, but often they
are not identical. Very often.... and I am very concerned that many
people think that making a copy of a DV Tape (for instance) into an
MPEG2 8mbit DVD will result in a copy that is IDENTICAL. This is not the
To discuss this in full depth would take more time and space then I have
at the moment - but suffice it to say that there are many places where
files - even though they are "digital" can and do change - particularly
when transcoding to a different compression format or algorithm set.
Codecs offer varying levels of quality - most of us know that well, but
there are also the more suble issues of error correction and concealment
and what it can and does do when playing back from tape. In addition to
the obvious manipulations of bit rate, there are changes in color space
- and as Adam points out the sampling rate and levels of color space...
i.e. a 4:2:2-10 bit file is NOT the same as an 4:2:0-8 bit one - and if
you go from one to another there has to be some things that change. For
many of us on this list this is obvious - but to MANY other people - it
is not so obvious, and as far as they think - if it digital then all
copies are identical - period end of discussion. This is very scary.
I am very concerned about projects that go through several different
codecs of totally different types. For example an original show shot on
DVCAM, edited in MJPEG, and burned into a MPEG2 DVD. While all of these
file formats certainly are digital - one cannot assume that in any way
what the picture looks like goimg into this process and what it comes
out as will be anything close to identical. Will it look good - maybe.
Is it different - definately. Is it good enough for what is needed for a
project - usually. Is it identical - absoulutely not.
> > 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
> > it
> > 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
> "visually lossless".
> 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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