Introduction to Video Storage
Digital video is made up of many "frames" of video, where each frame is
basically a photograph or image, displayed in rapid succession.
In addition, there is usually one or more tracks of audio that is
synchronized with the video, and possibly subtitle tracks as
well. There are a nearly innumerable different number of ways to
format it, store it, and transmit it. Here we will break it down
into the different aspects and components and cover some of the basics
behind how it works.
Aspect Ratio and Resolution
Video typically comes in two different aspect ratios: 4:3 (the same
as typical photographs), and 16:9 (or 17:9). 4:3 was a common
standard for
television, but it is being replaced with HD TV which uses the 16:9
aspect ratio. The wider 16:9 ratio is generally considered better
for humans to watch. The reason for this is that our eyes are
towards the side of our head, and this gives us a much wider field of
view to the sides than it does up and down. The wider aspect
ratio does a better job at filling that natural field of view.
Here are some of the common resolutions used for video and some of the
names that go with them.
16:9 formats - HD TV and Internet Video
1920x1080 - 1080p, 1080i, hd1080
1280x720 - 720p, hd720
1024x576 - 576p, WSVGA
854:480 - 480p, hd480
640:360 - 360p
463:240 - 240p
4:3 formats - old TV and Internet Video
768:576 - 576i, PAL
640:480 - VGA, 480i, NTSC
320:240 - QVGA
17:9 formats - specifically for movies in theaters and film editing
4096x2160 - 4k
2048x1080 - 2k
http://en.wikipedia.org/wiki/Aspect_ratio_(image)
http://es.wikipedia.org/wiki/Relación_de_aspecto
Interlacing
You may notice that many of the numeric representations for the formats
have a p or i after the number of lines of resolution. This
indicates that the video is Progressive scan or Interlaced scan.
Old, analog televisions used interlaced scan to cut in half the amount
of data they needed to send. This is commonly called interlacing
and it works by alternating which lines are broadcast. In one
frame of video all the odd numbered lines would be broadcast, then in
the next frame of video, all the even numbered lines would be
broadcast. If this interlacing of frames happens fast enough it
tricks the human eye into thinking that they are full frames.
With one single exception (1080i), all the modern video formats are
progressive scan. The 1080i format was really just used as a
marketing gimmick to tell people that they are getting 1080 quality
video on their TVs without having to processes full 1080p video.
In reality, 720p video will look just as good if not better than 1080i
video, especially for high-motion action scenes. When creating
videos, interlacing should only be used if the video is primarily
targeting a analog broadcast television. Digital video should
always be progressive scan (it helps it compress better).
http://en.wikipedia.org/wiki/Interlaced_video
http://es.wikipedia.org/wiki/Exploración_entrelazada
Frame Rates
The number of frames shown per second, also known as the "Frame Rate"
is another variable when creating video. Typically in this
course we will be
using 30fps (Frames Per Second), unless the source material is
something different.
The faster the frame rate is, the more smoothly it will seem that the
video is. For a somewhat extreme example of the difference in
framerates, watch these two examples of the same clip at 30fps
and 5fps.
Here are some common frame rates in use, in frames per second:
23.976 - NTSC film (24/1.001)
24 - Typical for film
25 - PAL television/film
29.97 - NTSC television (30/1.001)
30 - Digital Video
48 - High Speed Film
60 - Digital Video
http://en.wikipedia.org/wiki/Frame_rate
http://es.wikipedia.org/wiki/Imágenes_por_segundo
Pixel Formats
There are a lot of different formats used to store the pixels (it
can also be referred to as the color space) and it gets very
complicated, so we're not going to go too far into it. A quick
overview, when we worked on the photo unit, we assumed that everything
was represented in terms of its RGB values (8 bits each, for a total of
24 bits). That format is also common in video, however there are
many more. Most are defined in terms of YUV, this defines a
Luminance (brightness and darkness) channel and two different chroma
(color) channels, this can roughly re-produce all the colors of the 24
bit RGB. However, when the human eye sees an image, it is mostly
looking at the lightness and darkness (luminance) of it, and though the
color is important it doesn't take a large amount of color to make it
seem fully colored. Because the color information isn't as
important to the eye, some of these formats take out some of that
information to save space. Common formats you will see are
YUV4:2:2 (16 bits), YUV4:1:1 (12 bits), and YUV4:2:0 (12 bits).
Don't worry about what the numbers mean, just be aware that they are
out there.
http://en.wikipedia.org/wiki/YUV
http://es.wikipedia.org/wiki/YUV