"NTSC Video is a 4:3 aspect ratio, interlaced analog signal with 525 total
lines of resolution."
The definition of NTSC Video has several parts. The first we'll discuss
are resolution and aspect ratio.
Resolution
Let's begin with resolution. Resolution is the amount of individual elements
that make up an image. (See Figure 1).
Figure 1: Resolution
Any image that has been recreated, whether it's been printed, produced by a
computer or produced from film, is actually a collection of very small dots that,
when seen through the human eye, form an image.
Resolution is a common term in our tech-savvy, every day lives. We encounter it
when referring to printers, digital cameras, and HDTV, among other things. And
not surprisingly, the more dots there are, the sharper the image will be, so a
higher resolution is always going to create a clearer, sharper, more detailed
image than the same image displayed at a lower resolution.
When the term is related to NTSC video, it is only stated as the number of
horizontal lines (counted vertically) that make up an image. NTSC video is
defined as 525 total lines of resolution, of which 480 lines are actually
displayed by your television or monitor. Since you only see 480 lines, we'll
use this number from here on out when describing NTSC video.
A CRT actually draws these lines one-by-one from top to bottom across your
television. These lines are referred to as "horizontal scan lines" (Figure 2)
and represent the vertical resolution of NTSC video (vertical because we count
the lines vertically).
Figure 2. Horizontal Scan Lines
One might ask, "So what is the vertical resolution of NTSC video when counted
horizontally (horizontal scan lines)?" The answer depends on the video source
and can get complicated. For now, we'll keep it simple. We'll talk about the
different sources of NTSC video and their horizontal resolutions in a later section.
Aspect Ratio
An aspect ratio is defined as the width of an image in comparison to its
height. NTSC video is displayed in a 4:3 aspect ratio, meaning that the
width is 1.333 times wider than it is tall. How do we know this? It's simple;
we use this equation: 4/3 = 1.333. (See Figure 3).
Figure 3. 4:3 Aspect Ratio
The material in Figure 3 could be from any source. It could be from a projector
shining on a wall, the image from your old television, or from a new, large
rear projection television. Our point is that all NTSC video material will
always be 1.333 times wider than it is tall.
At this point we'll discuss other terms related to aspect ratio. For example,
"widescreen" is a generic term used to refer to any image that is wider than
the standard NTSC video image of 1.33:1.
When television was first invented, movies were all shot in a 4:3 aspect ratio.
Naturally, television adopted the same practice. When television became a
fixture in most American households, movie studios saw it as a major threat
to their industry. They scrambled to find something to differentiate their
movies from TV and ultimately came up with films that boasted wider screen
images, allowing audiences to feel as if there were in the middle of the
on-screen action.
From the 1950's on, all movies were offered in an aspect ratio of 1.78:1 or
greater, or in other words, a screen width of 1.78 times its height, also
known as widescreen. (See Figure 4).
Figure 4. Widescreen image
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