Pagina 136 - TELE-satellite-1207

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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 06-07-08/2012
Hide the SD inside the HD
Jacek Pawlowski
If you examine satellite charts, you
will see that DVB-S2 transponders
usually use 8PSK or QPSK modula-
tion. However these two modulations
are not the only possibilities the DVB-
S2 standard specifies. Except for
the four basic modulation schemes:
QPSK, 8PSK, 16APSK, 32APSK, the
norm gives you other interesting op-
tions. In another Feature story in
TELE-satellite we report on the Vari-
able & Adaptive Coding and Modula-
tion (VCM/ACM). In this article, we
would like to discuss yet another pos-
sibility. It is called H-8PSK and it is a
form of a hierarchical modulation.
In the classical DVB-S, one tran-
sponder is able to transmit just one
transport stream. Of course, there
can be a number of TV and radio pro-
grams multiplexed in one transport
stream. However, everything in this
transport stream is transmitted with
one modulation and one forward er-
ror correction (FEC) setting, for ex-
ample: QPSK 3/4.
H-8PSK is a method to transmit
two transport streams in one trans-
mission channel and thus by one
transponder. These two transport
streams are modulated in different
ways onto the radio frequency car-
rier and may have quite different
FEC values. The transport streams
are designated as the high priority
(HP) stream and the low priority (LP)
stream. H-8PSK is a rather unique
concept in digital TV as it provides
backward compatibility what is not
so common in digital world. Namely,
every DVB-S receiver should be able
to receive a HP stream. Only the re-
ception of the LP stream requires a
H8-PSK compatible DVB-S2 receiver.
How does it work?
Let’s start with a reminder of a
classical QPSK modulation. Figure
1 shows an unmodulated RF carrier
and in Figure 2 this carrier is QPSK
modulated with all possible combina-
tions of a symbol: 00, 10, 11 and 01.
Satellite Transmission Technology
A symbol in QPSK is simply 2 suc-
cessive bits in a bit stream. So we
have just four possibilities – as men-
tioned above. For each symbol value,
one specific phase shift is assigned.
These are respectively: 45, 135, 225
and 315 degrees. Figure 3 shows
the phase shift corresponding to the
modulation shown in the example in
Figure 2.
Your receiver converts the received
phase shift and converts each phase
shift to a pair of a bit. So the output
of a QPSK demodulator supplied with
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Bit stream after QPSK demodulation
QPSK phase shift [deg]
QPSK modulated carrier
Unmodulated carrier