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10
Overview
Technical
Technical
Overview
Uplink
The DSS System transports
digital data, video and audio to the customer's
home via
a high powered KU-band satellite.
The program provider sends its program material
to the uplink site where the signal is digitally encoded.
The "uplink" is the portion of
the signal transmitted
from the earth to the satellite.
The uplink site compresses the
video and audio, encrypts the video and formats the information into data "packets"
that are transmitted.
The signal is transmitted
to a satellite where it is relayed back to
the earth and decoded
by the customer's
receiver.
MPEG2 Compression
The video and audio signals are transmitted
as digital signals instead of conventional
analog.
The amount of data required to code all the video and audio information
would require a transfer rate well into the hundreds ofMbps
(Mega-bits per second).
This is too large and impractical
a data rate to be processed
in a cost effective way with
current hardware.
In order to minimize the data transfer rate, the data is compressed
using MPEG2 compression.
MPEG (Motion Pictures Expert Group) is an organiza-
tion who has developed
a specification
for transportation
of moving
images over
communication
data networks.
Fundamentally,
the system is based on the principle
that images contain a lot of redundancy
from one frame of video to another - the
background
stays the same for many frames at a time. Compression
is accomplished
by predicting motion that occurs from one frame of video to another and transmitting
motion
vectors
and background
information.
By coding
only the motion
and
background
difference instead of the entire frame of video information,
the effective
video data rate can be reduced from hundreds of Mbps to an average of 3 to 6 Mbps.
This data rate is dynamic
and will change
depending
on the amount of motion
occurring
in the video.
In addition to MPEG video compression,
MPEG audio compression
is also used to
reduce the audio data rate. Audio compression
is accomplished
by eliminating
soft
sounds that are near loud sounds in the fi'equency domain. The compressed audio data
rate can vary from 56 Kbs (Kilo-bits
per second) on mono signals to 384 Kbps on
stereo signals.
Data Encryption
To prevent unauthorized
signal reception,
the video signal is encrypted
(scrambled)
at the uplink site. A secure encryption
"algorithm"
or formula know as the Digital
Encryption
Standard (DES) is used to encode the video information.
The keys for
decoding the data are transmitted
in the data packets.
The customer's
Access Card
decrypts the keys which allows the receiver to decode the data. When a Access Card
is activated in a receiver for the first time, the serial number of the receiver is encoded
on the Access Card. This prevents the Access Card from activating
any other receiver
except the one in which it was initially authorized.
The receiver will not function
with
the Access
Card removed.
Data Packets
The program information is completely digital and is transmitted in data "packets."
This concept is very similar to data transferred by a computer over a modem. Five
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