Video Basics - Television Video, Digital Video,
Resolutions, Framerates, and Aspect Ratios
Television Video
Obviously, if you're reading this, you know at least something about
video. You know that it's basically a bunch of pictures changing really
fast to give the illusion that something's moving. I won't bore you
all with a history lesson about the origins of motion pictures, but
first we need to understand the origins of what we today consider "Video".
In the beginning, there was Analog. And it was good. And the Goverment
came down upon the masses and said unto them, "Hark, man has created
a great technology called television. For the good of the consumer we
must standardize this miraculous medium." Thus NTSC was
born.
NTSC stands for National Television Standards
Comittee, which was the body that created the standard which
was used in US televisions almost exclusively until a few years ago
(there is now also ATSC, or more commonly known as HDTV/DTV). NTSC defined
several basic things about how a television received and displayed data.
First off, NTSC said how wide a carrier signal to use to store a television
signal, as well as other things such as number of scanlines (the
vertical resolution, or the number of horizontal lines on the screen).
NTSC also defined how often the television screen was to be redrawn.
They set it at 29.97 Frames per Second. Don't ask why - it's
a long story (and a pretty stupid one). For most purposes people just
say 30 frames per second, but in actuality it's 29.97. Please note this
doesn't take into account interlacing and fields, topics
which will be discussed shortly. NTSC also defined the basic
resolution of television video (although that's not entirely accurate,
as analog video doesn't really have any fixed resolution, but for the
purpose of this guide we will say it has such a thing). NTSC defines
the resolution of TV at 720 pixels wide and 525 scanlines.
Yes, you heard me, 525.
"But what about 480 scanlines? That's what I've always heard!
My video capture card captures in 720x480 and it's the entire picture!"
Well that's not entirely accurate. There are 525 actually scanlines,
but you can't see the top and bottom chunks - they are called "overscan"
because they aren't visible on the screen. So for all intensive purposes
we can say that the theoretically visual portion of an NTSC picture
is 720x480. Now that's not entirely accurate either, because
most Televisions block out the edges and even more of the top and bottom.
But when viewing video on a computer screen (which doesn't have this
issue since your monitor resolution is probably higher than 720x480)
you can see all of those pixels (but you still don't see the extra overscan
most of the time). For those interested, that extra overscan area of
the video is where Macrovision copy-protection is stored.
For those of you in Europe or some other parts of the world, you may
use PAL or SECAM. Please accept my condolences. =) PAL/SECAM
uses a pratcial resolution of 720x576 (625 scanlines counting overscan)
and has 25 Frames per Second. So while you may get a higher resolution
picture, it's not updated as often. For the purpose of these guides,
I will be focusing on NTSC video because it's what the
majority of the readers of this article probably use.
Digital Video
Unlike conventional Analog video - Digital Video is stored/transmitted
very differently. Analog video essentially turns the picture into a
waveform and transmits it in terms of an electrical/radio wave (OK that's
a GROSS simplification but it's enough for the purposes of this article).
Digital Video on the other hand, stores the video in 0's and 1's, storing
a color value for each pixel of each frame. There are several ways to
store color data, the two main ones being RGB and YUV.
The differences between these two are explained in the next section.
Framerates
As discussed previously, the framerate of a video is the number of
times the entire screen is refreshed with a new picture. In NTSC land
this is 29.97, in PAL land it's 25, in conventional film it's 24, and
in IMax film it's 48.
Aspect Ratios
Now some of you may be wondering why a TV displays video in 720x480,
while your monitor displays pictures at 640x480, 800x600, etc. The reason
is because your monitor has square pixels. A TV does not. Televisions
have pixels which are taller than they are wide. This means that you
need more pixels in the horizontal direction to make up for it.
What does this mean to you? Well, it means that when you view pure
NTSC video on your monitor, people look a little fatter than normal.
TV video is stored in 3:2 aspect ratio (for every three horizontal
pixels, there are two vertical pixels), while monitors store picture
data in 4:3 aspect ratio. Now - don't be fooled, TVs still display
video in 4:3, it's just not stored that way. A lot of capture cards
will crop off the 16 pixels on the horizontal edges and capture in 704x480
or 352x480 (various reasons for this exist, one beign that 360 is not
a multiple of 16, one of the magic powers of two like 4, 16,
256, 65536, etc).
Film aspect ratios include 1.66:1 (used mostly for animated features
like Disney movies), 1.85:1 or 1.78:1 (commonly known as 16:9 or Academy
ratio), and 2.35:1 (called cinescope, or scope for short, which
is usually used in action movies).
ErMaC (September 2002)
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