[opendtv] Re: Mediaflo Technology
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 11 Nov 2006 21:20:30 -0500
Mark Schubin wrote:
> ... It's pretty scary ...
And hard to visualize but I'd like to see it. If you'll email it to me
I'll host it for awhile and post a link here tomorrow with my reply,
assuming there are no prohibitive copyright restrictions.
I'm not familiar with viewer CSF functions.
- Tom
Tom Barry wrote:
I guess another way of looking at it is the delivery resolution should
be approaching (with diminishing returns) what we CAN see. But the
display resolution should be safely into the zone of what we CAN'T
see, letting our visual limitations furnish that free filtering for
the tiny square pixels on fixed pixel displays. This again goes back
to my liking for a 720p delivery but 1080p displays, at least when
bandwidth is expensive.
That's not bad (1440p or more would be even better) for a situation in
which the visual-acuity limitation is 720p. But visual-acuity
limitations involve both resolution and contrast.
I'd love to attach a slide I created involving a contrast-resolution
sine grating (contrast increasing towards the top, resolution increasing
towards the right) with two curves superimposed on it, but I think
Freelists won't allow attachments, so I'll describe it.
Near the bottom is a lopsided smile-shaped curve representing the
contrast-sensitivity function (CSF) of a particular observer. The
trough of the smile (maximum sensitivity, i.e., the ability to perceive
minimum contrast) is at around three cycles per degree. The necessary
contrast increases to the left (less resolution) and even more sharply
to the right (greater resolution). Everything under that curve is
invisible to an observer with that particular CSF .
Starting at the top left is a curve like the right side of a
bell-shape. It's a modulation-transfer-function (MTF) curve. It ends
up at the bottom right, but the exact shape of that curve is determined
by such factors as the number of samples in the display. Anything above
that curve simply isn't reproduced by the system.
The two curves intersect. Where the visual system needs the most
contrast, the television system provides the least (i.e., where the CSF
curve is highest, the MTF curve is lowest). There is a wedge at the
left side of the grating, which is both visible (above the CSF curve)
and reproduced (below the MTF curve). EVERYTHING we see from a
particular television system must fit within that wedge. It's pretty
scary. The first time I showed it to a top engineer in our field, he
argued that it couldn't be, but eventually he accepted it.
We have no control over a viewer's CSF. The only way to increase the
size of the wedge is to increase the MTF. More samples is one way to do
that.
TTFN,
Mark
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--
Tom Barry trbarry@xxxxxxxxxxx
Find my resume and video filters at www.trbarry.com
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