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Posted by Frank on 11/05/07 10:38
On Mon, 05 Nov 2007 04:17:33 GMT, in 'rec.video.production',
in article <Re: Canon HV-20 wins 2007 award as "Best HDV Camcorder">,
"My Name Is Nobody" <nobody@msn.com> wrote:
>OK so, please explain why an HDV signal is referred to as both 1440x1080 and
>1920x1080? I seem to have missed some distinction.
As explained to you by several others, 1080i HDV (and HDCAM and XDCAM
HD as well) is stored (written to magnetic tape in HDV and HDCAM or
burned to optical disc in XDCAM HD) in an anamorphically squeezed,
non-square pixel, 1440 by 1080 form. At display time, that 1440 by
1080 frame is expanded to 1920 by 1080. That's the distinction.
In the case of 1080i HDV, the 1440 by 1080 frame size was chosen so
that the video data rate could be set the same as DV25 (approximately
25 Mbps), thus allowing the use of existing DV tape cassette transport
mechanisms. This allowed the cost to be kept down, and allowed
products to come to market sooner.
Storing full-raster 1920 by 1080 frames at a data rate of only 25 Mbps
would have resulted in the need to compress the video so much that it
would have visually been unacceptable. As it is, 1080i HDV video
compression is right on the edge.
The difference between a full-raster, square pixel, 1920 by 1080 frame
(2,073,600 pixels) and an anamorphically squeezed, non-square pixel,
1440 by 1080 frame (1,555,200 pixels) is a 25 percent data reduction.
It was easier to keep to the 25 Mbps data rate with this 25 percent
reduction (having 25 percent fewer pixels to compress).
Storing 1440 by 1080 frames, rather than full-raster 1920 by 1080
frames, results in a quality loss (in terms of horizontal resolution),
but that's the way it is. Besides, it had already been an accepted way
of life with the HDCAM format, so a precedent had been set.
If you want full raster (square pixel) HDV, then you must use JVC's
720p flavor of HDV, where a full 1280 pixel by 720 pixel frame size is
stored.
The AVCHD format is different. The format supports both 1920 by 1080
and 1440 by 1080. Some camcorders implement one and some implement the
other (and some implement both depending upon the recording mode
setting you choose), so read the specs carefully if this of concern to
you.
Also, if you purchase a 1920 by 1080 AVCHD camcorder and plan to edit
your footage, be certain that the NLE you plan to use supports 1920 by
1080 files. Some of the original programs that claimed to support
AVCHD editing only supported the 1440 by 1080 flavor and not the 1920
by 1080 flavor. This wasn't a problem in the beginning, when all 1080i
AVCHD camcorders were storing 1440 by 1080 frames, but now that
full-raster 1920 by 1080 products are available, it's a consideration.
Non-square pixel digital video formats, whether over-sampled or
under-sampled, have been with us for some time now. The two most well
known examples are NTSC DV, over-sampled and stored to tape as 720 by
480 frames but displayed at 640 by 480, and PAL DV, which is stored to
tape as anamorphically squeezed 720 by 576 frames but displayed at 768
by 576.
--
Frank, Independent Consultant, New York, NY
[Please remove 'nojunkmail.' from address to reply via e-mail.]
Read Frank's thoughts on HDV at http://www.humanvalues.net/hdv/
(also covers AVCHD and XDCAM EX).
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