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NTSC
16x9 Video Demystified
By Norm Kern |
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A
few universal truths:
1.
Standard NTSC video is 4 units wide and 3 units
high when viewed on a conventional TV screen.
2. Standard NTSC widescreen video is 16 units
wide and 9 units high when viewed on a widescreen
TV screen.
3. The digital NTSC video format pixel dimension
is 720 pixels wide by 480 pixels high. It is the
same number of pixels regardless of whether the
intended display is 4 x 3 or 16 x 9. If you do
the math, you will find that the pixel dimensions
are NEITHER 4 x 3 or 16 x 9 proportion. Native
NTSC pixel proportion is 6 x 4, the same proportion
as a 35 mm still picture in landscape format.
THEREFORE
When
viewed on a TV set, this means that digital NTSC
video is horizontally SCALED DOWN by 12.5% to
achieve a 4 x 3 proportion and SCALED UP by 18.6%
to achieve a 16 x 9 proportion. The effective
pixel dimensions are 640 x 480 and 854 x 480 respectively.
Thus, digital NTSC video is never displayed without
being horizontally scaled, regardless of whether
it is 4 x 3 or 16 x 9.
A
COMPUTER SCREEN IS NOT A TV SCREEN
Computer
displays are always derived from a digital grid
of pixels. The pixels on a computer screen are
always square. Video files that are created directly
for the computer screen or the internet all have
square pixels. That means the aspect ratio (e.g.
4 x 3, 16 x 9) is ALWAYS equal to the physical
pixel ratio. This is a direct contrast to NTSC
digital video, where we have seen that the aspect
ratio is NEVER equal to the pixel ratio.
Yes, DVD player programs display NTSC digital
video in its correct proportion on the computer
screen. That's because the player program knows
the characteristics of the video it is playing
and performs appropriate horizontal scaling of
the image before it is passed to the computer's
video card for display.
GOOD
NEWS / BAD NEWS FOR HD
All
of this pixel ratio vs aspect ratio stuff in NTSC
has been a source of confusion. The good news
is that the popular HD formats (720p and 1080i)
both have square pixels- the pixel ratios and
aspect ratios are all the same at 16 x 9.
The bad news is that the native resolution of
most of the acquisition formats (HDV, DVCPRO HD,
XDCAM HD, even HDCAM) are less than 1080 x 1920
and have to be scaled when dealing with them in
their native mode. That discussion will make NTSC
look simple by comparison! |
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