Information Technology Reference
In-Depth Information
Video:
. . .
. . .
An Image
or Bitmap
A Pixel
Frame:
R G B
Pixel:
or
Y U V
Figure 2.2
Compositions of digital video
Each video frame is further sub-divided into a two-dimensional grid of pixels (Figure 2.2).
The numbers of horizontal and vertical pixels are defined by the resolution of the video. Each
pixel further sub-divides into a number of color channels. The common color models employed
in digital video are RGB, YUV, and YCrBr. In compressed video such as MPEG, the YCrBr
color model is employed to exploit different properties of the human visual system (e.g., the
human eye is more sensitive to intensity changes, i.e., the Y component, than color changes,
i.e., the Cr and Br components).
The following are some common video standards and their basic properties:
Common Interchange Format (CIF), (ITU-TS H.261):
352
×
288 for luminance (Y)
176
×
144 for chrominances (U, V)
36 Mbps.
Quarter-Common Interchange Format (QCIF):
Raw data rate
=
176
×
144 for luminance (Y)
176
×
144 for chrominances (U, V)
18 Mbps.
Digitizing NTSC Video Signal:
Raw data rate
=
Sampling rate: Y(13.5 Mhz), U (6.75 Mhz), V (6.75 Mhz)
Digitizing NTSC video signal
Raw data rate
=
(13.5
+
6.75
+
6,75)
×
8
=
216 Mbps
Raw pixel resolution
=
864
×
525 pixels (removing retrace, etc.)
Active video area
=
720
×
486 pixels
Sub-sampling (4:2:2) (reduce bit-rate by 33%)
Y (720
×
486), U (360
×
486), V (360
×
486)
8-bits per sample per signal channel
Net raw data rate after sub-sampling
=
168 Mbps.
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