20/11/2014
Satellite TV Solution
Satellite television is television delivered by the means of
communications satellite and received by a satellite dish
and set-top box. In many areas of the world it provides a
wide range of channels and services, often to areas that are
not serviced by terrestrial or cable providers.
Technology
Satellites used for television signals are generally in either
naturally highly elliptical (with inclination of +/-63.4 degrees
and orbital period of about 12 hours, also known as Molniya
orbit) or geostationary orbit 37,000 km (22,300 miles) above
the earth’s equator.
Satellite television, like other communications relayed by
satellite, starts with a transmitting antenna located at an
uplink facility. Uplink satellite dishes are very large, as much
as 9 to 12 meters (30 to 40 feet) in diameter. The increased
diameter results in more accurate aiming and increased
signal strength at the satellite. The uplink dish is pointed
toward a specific satellite and the uplinked signals are
transmitted within a specific frequency range, so as to be
received by one of the transponders tuned to that frequency
range aboard that satellite. The transponder 'retransmits'
the signals back to Earth but at a different frequency band
(a process known as translation, used to avoid interference
with the uplink signal), typically in the C-band (4–8 GHz) or
K u -band (12–18 GHz) or both. The leg of the signal path
from the satellite to the receiving Earth station is called the
downlink.
A typical satellite has up to 32 transponders for Ku-band
and up to 24 for a C-band only satellite, or more for hybrid
satellites. Typical transponders each have a bandwidth
between 27 MHz and 50 MHz. Each geo-stationary C-band
satellite needs to be spaced 2 degrees from the next satellite
(to avoid interference). For K u the spacing can be 1 degree.
This means that there is an upper limit of 360/2 = 180
geostationary C-band satellites and 360/1 = 360
geostationary K u -band satellites. C-band transmission is
susceptible to terrestrial interference while K u -band
transmission is affected by rain (as water is an excellent
absorber of microwaves at this particular frequency).
The downlinked satellite signal, quite weak after traveling
the great distance (see inverse-square law), is collected by a
parabolic receiving dish, which reflects the weak signal to
the dish’s focal point. Mounted on brackets at the dish's
focal point is a device called a feedhorn. This feedhorn is
essentially the flared front-end of a section of waveguide
that gathers the signals at or near the focal point and
'conducts' them to a probe or pickup connected to a low-
noise block downconverter or LNB. The LNB amplifies the
relatively weak signals, filters the block of frequencies in
which the satellite TV signals are transmitted, and converts
the block of frequencies to a lower frequency range in the L-
band range. The evolution of LNBs was one of necessity
and invention.
The original C-Band satellite TV systems used a Low Noise
Amplifier connected to the feedhorn at the focal point of the
dish. The amplified signal was then fed via very expensive
50 Ohm impedance coaxial cable to an indoor receiver or in
other designs fed to a downconverter (a mixer and a voltage
tuned oscillator with some filter circuitry) for
downconversion to an intermediate frequency. The channel
selection was controlled, typically by a voltage tuned
oscillator with the tuning voltage being fed via a separate
cable to the headend. But this simple design evolved.
Designs for microstrip based converters for Amateur Radio
frequencies were adapted for the 4 GHz C-Band. Central to
these designs was concept of block downconversion of a
range of frequencies to a lower, and technologically more
easily handled block of frequencies (intermediate
frequency).
The advantages of using an LNB are that cheaper cable
could be used to connect the indoor receiver with the
satellite TV dish and LNB, and that the technology for
handling the signal at L-Band and UHF was far cheaper than
that for handling the signal at C-Band frequencies. The shift
to cheaper technology from the 50 Ohm impedance cable
and N-Connectors of the early C-Band systems to the
cheaper 75 Ohm technology and F-Connectors allowed the
early satellite TV receivers to use, what were in reality,
modified UHF TV tuners which selected the satellite
television channel for down conversion to another lower
intermediate frequency centered on 70 MHz where it was
demodulated. This shift allowed the satellite television DTH
industry to change from being a largely hobbyist one where
receivers were built in low numbers and complete systems
were expensive (costing thousands of Dollars) to a far more
commercial one of mass production.
Direct broadcast satellite dishes are fitted with an LNBF,
which integrates the feedhorn with the LNB.
The satellite receiver demodulates and converts the signals
to the desired form (outputs for television, audio, data, etc.).
Sometimes, the receiver includes the capability to
unscramble or decrypt; the receiver is then called an
Integrated receiver/decoder or IRD. The cable connecting
the receiver to the LNBF or LNB must be of the low loss type
RG-6, quad shield RG-6 or RG-11, etc. It cannot be standard
RG-59.
Standards
Analog television distributed via satellite is usually sent
scrambled or unscrambled in NTSC, PAL, or SECAM
television broadcast standards. The analog signal is
frequency modulated and is converted from an FM signal to
what is referred to as baseband. This baseband comprises
the video signal and the audio subcarrier(s). The audio
subcarrier is further demodulated to provide a raw audio
signal.
If the signal is a digitized television signal or multiplex of
signals, it is typically QPSK.
In general, digital television, including that transmitted via
satellites, are generally based on open standards such as
MPEG and DVB-S /S2 .
DVB-S is the original Digital Video Broadcasting forward
error coding and modulation standard for satellite television
and dates from 1994, in its first release, while development
lasted from 1993, to 1997. The first application was
commercially available in France via Canal+, enabling
digitally broadcast, satellite-delivered television to the
public.
It is used via satellites serving every continent of the world.
DVB-S is used in both MCPC and SCPC modes for
broadcast network feeds, as well as for direct broadcast
satellite services like Sky Digital (UK) via Astra in Europe,
Dish Network and Globecast in the U.S. and Bell TV in
Canada.
Digital Video Broadcasting - Satellite - Second Generation
( DVB-S2 ) is an enhanced specification to replace the DVB-S
standard, developed in 2003 and ratified by ETSI (EN
302307) in March 2005. The development of DVB-S2
coincided with the introduction of HDTV and H.264 (MPEG-4
AVC) video codecs.
The system allows transmission of one or more MPEG-2 or
MPEG-4 audio/video streams , using QPSK or 8PSK or
16/32APSK (amplitude and phase shift keying) modulation
with concatenated encoding.
DVB-S2 is based on the DVB-S standard which is used for
satellite broadcasting, and the DVB-DSNG standard, which
is used by mobile units for sending external footage back to
television stations. Two new key features which were added
to DVB-S are :
VCM (Variable Coding and Modulation) which optimizes the
transmission parameters for various users.
Changing encoding parameters in real time (ACM, Adaptive
Coding and Modulation)
The authors claim that the DVB-S2 performance gain over
DVB-S is around 30%.
When the contribution of improvements in video
compression is added, an (MPEG-4 AVC) HDTV service can
now be delivered in the same capacity that supported an
early DVB-S-MPEG-2 SDTV service only a decade before.
While the actual DVB-S standard only specifies physical link
characteristics and framing, the overlaid transport stream
delivered by DVB-S is mandated as MPEG-2, known as
MPEG-TS.
The conditional access encryption/scrambling methods
include BISS, Conax, Digicipher, Irdeto, Nagravision,
PowerVu, Viaccess, Videocipher, and VideoGuard. Many
conditional access systems have been compromised.
