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Architecturally, DBS is a simple concept.
As shown in the Figure, DBS operators receive analog TV reception from
the various networks at a single giant headend. The DirecTV headend,
for example, is in Castle Rock, Colorado. The analog programming is
encoded into Motion Picture Experts Group (MPEG) format for digital
retransmission. A control function regulates the amount of bandwidth
accorded to each MPEG stream and determines how the MPEG knobs
(control parameters), such as the length of a group of pictures, are
specified.
The settings of the knobs are closely
guarded secrets of the DBS operators. ESPN, for example, tends to
require more bandwidth than the Food Channel. ESPN has a lot more
motion; more importantly, it has a larger audience and greater
advertising revenue. How much more would ESPN pay for access than the Food
Channel? How much extra bandwidth is ESPN getting, and for how much?
What MPEG knobs should the carrier use, and what knobs does its
competition use? This is not public information.
ESPN, the Food Channel, and all other
channels are encoded into MPEG transport streams, multiplexed
together, and then converted to the uplink frequency.
The major North American geosynchronous
satellites for DBS so far are placed at longitudes 85 degrees west (Primestar),
101 degrees west (DirecTV), and 119 degrees west (Echostar). The
Primestar slot rests on the longitude that passes through the East
Coast of the United States, the DirecTV longitude bisects the center
of North America, and the Echostar longitude passes through the West
Coast. From these orbits, each satellite can broadcast over the
contiguous United States, southern Canada, and Mexico.
The satellite receives a signal and
remodulates it to the designated spectrum for DBS. DBS occupies 500
MHz in the 12.2 KU Band. The Ku band occupies the frequency range from
10.7 GHz to 12.75 GHz is called Ku-Band. DBS satellites are allowed by regulation to
broadcast at a higher power (120W) than the larger C-band satellite
dishes currently used by many households to enable reception on small
satellite dishes. This higher-powered transmission and smaller dish
distinguish DBS from other forms of satellite reception.
The DBS uses QPSK modulation to encode
digital data on the RF carriers. DirecTV encodes using MPEG-2 format
to enable a density of up to 720x480 pixels on the user's monitor. Primestar uses a proprietary video compression system developed by
General Instruments called DigiCipher-1. Echostar uses a transmission
system based on the European Digital Video Broadcast (DVB) standard.
DVB uses the MPEG-2 and standardizes control elements of the total
system, such as conditional access.
Although 720x480 is the maximum
resolution offered today, DBS is capable of higher pixel resolution.
In fact, DBS could be an early delivery vehicle for high-definition TV
(HDTV) programming.
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