Digital Signal Processing Reference
In-Depth Information
An MIP can also be used for transmitting additional information such as
the DVB-T transmission parameters which makes it possible to control and
configure the entire DVB-T SFN from one center. For example, it can be
used for changing the type of modulation, the code rate, the guard interval
length etc. However, although this is possible, it may not be supported by
every DVB-T modulator.
If the transmission of the MIP packets stops for some reason or if the in-
formation in the MIP packets is corrupted, the single frequency network
will lose synchronization. If a DVB-T transmitter detects that it has
dropped lock or that it has not received a GPS signal for some time and the
1 pps reference and the 10 MHz reference have, therefore, drifted, it has to
go off air or it will only be a source of noise in the single frequency net-
work. Reliable reception is then only possible with directional reception
close to the transmitter. For this reason, the MIPs in the transport stream
arriving at the transmitter are often monitored using an MPEG-2 test de-
coder (see Fig. 20.26.).
MIP content
188 bytes
4 byte
TS header
184 byte
payload
13 bit packet identifier = PID
1 byte sync byte = 0x47
0x15
Fig. 20.26.
Megaframe initializing packet
Fig. 20.27. (MIP = Megaframe Initializing Packet) clearly shows that
the multiplexed MPEG-2 stream is now carrying a further table-like
packet, namely the MIP packet, containing the synchronization time
stamp, the pointer and the maximum delay. It also contains the transmis-
sion parameters. It can also be seen that every transmitter in the link-up
Search WWH ::
Custom Search