Digital Signal Processing Reference
In-Depth Information
In a single frequency network, all the individual transmitters must be
synchronized with one another. The program contribution is injected from
the playout center in which the MPEG-2 multiplexer is located, e.g. via
satellite, optical fiber or microwave link. It is clear that the MPEG-2 trans-
port streams are subject to different feed line delays due to different path
lengths. However, it is necessary that in each DVB-T modulator in an SFN
network the same transport stream packets are processed into COFDM
symbols. Every modulator must perform every operating step completely
synchronously with all the other modulators in the network. The same
packets, the same bits and the same bytes must all be processed at the same
time. Every DVB-T transmitter site must broadcast absolutely identical
COFDM symbols at exactly the same time.
The DVB-T modulation is structured in frames, one frame being com-
posed of 68 DVB-T COFDM symbols. Within a frame, the complete TPS
information is transmitted and the scattered pilots are scattered over the en-
tire DVB-T channel. Four such frames, in turn, make up one superframe.
Frame structure of DVB-T:
•
68 COFDM symbols = 1 frame
•
4 frames = 1 superframe
One superframe in DVB-T accommodates an integer number of MPEG-
2 transport stream packets, as follows:
Table 20.19.
Number of transport stream packets per superframe
Code-
rate
QPSK
2K
QPSK
8K
16QAM
2K
16QAM
8K
64QAM
2K
64QAM
8K
1/2
252
1008
504
2016
756
3024
2/3
336
1344
672
2688
1008
4032
3/4
378
1512
756
3024
1134
4536
5/6
420
1680
840
3360
1260
5040
7/8
441
1764
882
3528
1323
5292
In consequence, a superframe in a single frequency network must be
composed of absolutely identical transport stream packets and each modu-
lator in the SFN must generate and broadcast the superframe at the same
time.
These modulators must, therefore, be synchronized with one another
and, in addition, the differences in the feed line delays must be equalized
statically and dynamically. To achieve this, packets with time stamps are
inserted into the MPEG-2 transport stream in the playout center. These
packets are special transport stream packets which are configured similarly
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