Sony DTV-01, Manual

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9
Introduction to MPEG-2 Compression
MPEG stands for Motion Picture Experts Group, named after the com-
mittee that developed the standard.
Why Compress?
MPEG defines a compression scheme for Video which evolved from the
need to transmit digital video on existing communication channels with
limited bandwidth. In addition to the bandwidth issue there is an obvious
storage one as well.
Fastest Communication Channels Typical Bandwidth
ISDN Line 144Kbps
T1 Line 1.5 Mbps
T3 Line 45 Mbps
With a whopping 4.7 gigabytes of data capacity, DVD-Video would seem
to have more than enough room for motion pictures. Unfortunately, digital
video has an incredibly voracious appetite for storage.
Raw or uncompressed Digital Video requires an enormous 252 Mega-
bits/sec of bandwidth and approx 31 Mbytes per second of storage.
720x480 (Res.) X 8bits/sample X 30frames/sec X 3 Components
To understand this figure, we need to understand video in its purest form.
Component Video
In its purest form, Video is made up of 4 components.
-
Luminance or Y which defines the brightness level and
- Color which is made up of 3 components called R-Y, B-Y and G-Y.
As it works out, we can mathematically calculate the 4 th component (G-
Y) from the others. Therefore, we only require 3 components for Video
(Y, R-Y and B-Y).
Y = (R-Y) + (B-Y) + (G-Y)
Each sample we take of the video is represented by an 8 bit digital word
which translates to 2
8 or 256 different levels of each component. There-
fore, each pixel is made up of 3 components which can represent up to
2
8
(Y) X 2
8
(R-Y) X 2
8
(B-Y) = 2
24
(commonly known as 24 bit color)or 16
million colors.
R-Y B-Y Y
One PIXEL
The resolution or picture detail we require also plays an important role in
our bandwidth and storage requirement. DVD uses a 720 Horizontal by
480 Vertical resolution or 720 pixels across times 480 rows or lines.
720
480
In summary we need to:
-
sample 3 components (Y, R-Y and B-Y) each of which is composed
of 720 x 480 (350,000) pixels.
- Represent each one by an 8 bit word (3 X 350,000 X 8 =
8,400,000bits). Therefore, each frame is made up of 8,400,000
bits.
- Finally, we would need to display at 30 frames per second (30 X
8,400,000 =252,000,000 bits/sec or 252 Mbits/sec Bandwidth.
To calculate the storage we simply divide the Bandwidth in bits by 8 bits
per byte and we get 31.5Mbytes/sec Storage requirement.
As illustrated, the requirement is enormous which paves the way for com-
pression and MPEG.