Some Video Arithmetic
Here are some numerical posers about
video that have crossed my screen recently.
"If the horizontal resolution of a digital camcorder
format is on the order of 500 lines, it would seem that the sort of image sensor used in a
number of models, with approximately 480,000 pixels, should be enough to capture this
amount of detail," writes Steve. "Correct my math if it is wrong," he says,
but "500 vertical x 500 horizontal pixels = 250,000 pixels. Isn't that plenty for 500
lines?"
It's easy to confuse pixels and lines of resolution. First,
horizontal resolution is the maximum number of distinct black lines with white spaces
between them a system can reproduce. That means two pixels for each line -- one black, one
white -- so 500 lines of resolution would require 1000 pixels.
But horizontal resolution is also defined as the number of
lines in an area as wide as the screen is high, rather than over the full width of the
screen. Since a standard 4:3 image is 1.333 times wider than it is high, the total number
of horizontal pixels required is 1333. Multiply that by the 480 lines in an NTSC signal
and you come up with a total pixel count of just under 640,000. That's what is necessary
to take full advantage of the digital camcorder system.
Barney says he has a 40" direct-view TV and hopes
someday to buy a widescreen flat display. "But I don't want to give up the height
that I'm used to," he says. "What screen size do I need to look for in a
widescreen HDTV set so that it can have the same height as my present screen?"
Screen sizes for widescreen televisions, with an aspect
ratio of 16:9, are measured in exactly the same way as for conventional 4:3 sets:
diagonally from one corner of the viewable area to the other. But because of the different
shape, a widescreen set will be considerably shallower and somewhat narrower than a 4:3
set with the same numerical rating.
I won't bore you with the arithmetic, but there are some
simple numbers you can use to compare sizes.
In my view, the greatest impression of size is the screen's
height: Letterboxed images on even large normal sets tend to look dinky. To find out what
the height of a widescreen set would translate into in ordinary terms, multiply by
approximately 80 percent. A 27" widescreen model would have a picture only as deep as
a 22" standard set.
Put another way, to get a widescreen as deep as that of a
27" 4:3 set, you'd need one with a diagonal measurement of about 33" (multiply
by 1.22). In the case of your 40" requirement, you will need a widescreen measuring
49 or 50 inches.
The image size of a 27" widescreen is about the same
as a letterboxed image on a 30" normal screen, assuming the same 16:9 aspect ratio.
Sirtaz asks, "When a widescreen-format DVD is being
played with a black area above and below, are there still 525 -- but thinner -- lines in
the viewing area, or is the picture now being displayed with fewer horizontal lines?"
When you watch a widescreen movie on a conventional NTSC
television, only those scanning lines in the middle of the screen are visible; the others
are still there, but receiving no signal. Thus, a widescreen image has significantly lower
vertical resolution than the equivalent fullscreen image. This can be quite drastic: if
the movie was shot in Panavision -- the most common anamorphic widescreen technique for
theaters -- only about half the scanning lines are used. None of this is true, of course,
with high-definition digital signals.
...Ian G. Masters
ian@mastersonaudio.com
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