Digital Signal Processing Reference
In-Depth Information
stream structure. These time stamps are needed for resetting processes in
the energy dispersal block at the transmitting and receiving end. This, in
turn means that both the modulator or transmitter and the demodulator or
receiver receives this sequence of eight packets of the sync byte inversion
transparently in the transport stream and uses them to control certain proc-
essing steps. It may happen that relatively long sequences of zeroes or ones
occur purely accidentally in a data signal. However, these are unwanted
since they do not contain any clock information or cause discrete spectral
lines over a particular period. To eliminate them, virtually every digital
transmission method applies energy dispersal before the actual modulation.
Initializing bits
1
0
0
1
0
1
0
1
0
0
0
0
0
0
0
123456789 0 1 2 3 4 5
=1
&
=1
Randomized
data out /
transparent
sync out
Enable/disable
randomizing
MPEG2 data In
Fig. 14.8.
Energy dispersal stage (randomizer)
To achieve energy dispersal, a pseudo random binary sequence (PRBS)
(Fig. 14.8.) is first generated which, however, is restarted time and again in
a defined way. In DVB-S, the starting and resetting takes place whenever
a sync byte is inverted.
The data stream is then mixed with the pseudo random binary sequence
(PRBS) by means of an Exclusive OR operation which breaks up long se-
quences of ones or zeroes. If this energy-dispersed data stream is mixed
again with the same pseudo random binary sequence at the receiving end,
the dispersal is cancelled again.
The receiving end, therefore, contains the identical circuit, consisting of
a 15-stage shift register with feedback which is loaded in a defined way
with a start word whenever an inverted sync byte occurs. This means that
the two shift registers at the transmitting end and at the receiving end are
operating completely synchronously and are synchronized by the sequence
of 8 packets of the sync byte inversion block. This synchronization only
becomes possible because the sync bytes and the inverted sync bytes are
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