Johannes Stezenbach wrote:
Well, since the LP stream is a DVB-S2 stream it could in theory
be either a SIS or a MIS stream (except that figure F.1
only shows a single input to the DVB-S2 outer & inner coding
block).
However, I think that MIS/stream_id filter is not used to
seperate the HP and LP stream.
Stream ID filter is described (somewhere that) IS is used for the
PLFrame to be demodulated.
The points that which i am a bit confused was this:
1) The presence of the S2 satellite delivery system descriptor is not
always, it appears in some cases and in some cases it doesn't.
2) Now, BBHEADER contains the info that is supposed to be there in the
S2 satellite delivery system descriptor
except for
(a) Backwards Compatibilty Indicator (This one is obvious since in NBC
mode, there is no Backward Compatibility)
(b) Scrambling sequence indicator
Whether it is in NBC or BC mode descrambling has to be done. For the
STB0899 it is fine , since index, n=0 is default, but in another case
the index can be possibly be changed, but in that case i don't
understand from where i can change the Scrambling Sequence Index in NBC-BS
Look again at figures F.1 and F2: The LP (DVB-S) stream
uses different framing that the HP (DVB-S2) stream. Although
in theory you could build a hardware which could receive
both streams at the same time, it doesn't make sense to
do so, since you need to seperate them later anyway.
The hierarchical modulator combines the HP and LP streams
in the following way: It takes two bits from the HP
stream to determine the quadrant in the constellation
diagram for the current symbol. Then it takes one
bit from the LP stream and modifies the phase a little.
What i understand is like this
DVB-S (QPSK)
|
|
x | x
|
--------|---------
|
x | x
. |
|
DVB-S2 (8PSK)
|
|
X | X
|
|
X | X
|
-----------------|-----------------------
|
X | X
|
|
X | X
|
|
After Hierarchial Modulation (DVB-S + DVB-S2)
|
|
X | X
|
x | x
X | X
|
-----------------|-----------------------
|
X | X
x | x
|
X | X
|
|
X = 8PSK
x = QPSK
So indeed, the envelope is modified.
From the specs,
The LP DVB-S2-compliant signal is BCH and LDPC encoded, with LDPC code
rates 1/4, 1/3, 1/2 or 3/5. Then the hierarchical mapper generates the
non-uniform 8PSK constellatio, the two HP DVB-S bits define a QPSK
constellation point, while the single bit from the DVB-S2 LDPC encoder
sets an additional rotation ±θ before transmission.
So, a (DVB-S) QPSK demodulator will see just the HP
stream (with a lot of phase noise). The DVB-S2 demodulator
in hierarchical BC mode (yes, the hw needs a dedicated mode)
will however use a 8PSK demodulator, modified to deal with
the non-uniform phase distribution, and will throw away the
two bits per symbol from the quadrant, and just use the
one bit offset-within-the-quadrant to extract the
DVB-S2 stream. If it wouldn't do this, the output of
the first demodulator stage would be a mix of a DVB-S
and DVB-S2 stream (with totally different frame formats),
which the next demodulator stages cannot make sense of.
Agreed. The relevant one follows it's own, even though the envelope is
modified.
These parts are fine.
Summary: Hierarchical BC mode and MIS mode are two totally
seperate concepts and have nothing to do with each other.
Just before Hierarchial modulation, the LP stream has DVB-S2 frames. So
MIS is in accordance to DVB-S2 itself.
What i understood was that Layered or Hierarchial modulations are just 2
ways to send two different framing methods to be sent simultaneously,
both done using slightly different methodologies.
But eventually in either case, there exists the case of DVB-S2 frames
and DVB-S frames for BC.
The only difference in NBC is that DVB-S frames, does not exist.
To receive the HP stream one needs a DVB-S demod, for
the LP stream one needs a DVB-S2 demod which supports
a "hierarchical LP stream" mode.
Agreed. DVB-S is indeed HP, which according to specs too say is to
maintain compatibility, and the LP stream is completely DVB-S2.
Whether it be in Layered modulation or Hierarchial modulations, the
concept is nearly the same
(1) Layered Modulations play with Satellite Beam power
(2) Hierarchial Modulations play with the envelope.
So in either case, DVB-S is indeed separate from DVB-S2
(I must admit, however, that I have difficulties to
find the "hierarchical LP stream" mode in the
STB0899 data sheet...)
I haven't seen either. Did search a lot in there, but did not find
anything related to hierarchy.
Manu
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