Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 375

lution of 100 ns steps is used for automatically measuring the feed dis-
tance. This time information is evaluated by the SFN adapter which auto-
matically corrects the delay from the playout center to the transmitter site
by means of a buffer store. It also requires information about the maximum
delay in the network. Given this information, which can either be input
manually at every transmitter site or is carried in the MIP packet, each
SFN adapter adjusts itself to this time. The MIP packet also contains a
pointer to the start of the next megaframe in numbers of TS packets. Using
this pointer information, each modulator is then able to start a megaframe
at the same time.

GPS

1pps
Puls

Pointer

Synchronization time stamp

... ...

MFP#0 MFP#1 MFP#2

MIP

MFP#0

MPEG2 TS Packet

Megaframe

Fig. 20.25. Megaframe structure at transport stream level

The length of a megaframe depends on the length of the guard interval
and on the bandwidth of the channel. The narrower the channel (8, 7 or 6
MHz), the longer the COFDM symbols since the subcarrier spacing be-
comes less. Every DVB-T modulator can now be synchronized by means
of the information contained in the MIP packet. The MIP packet can al-
ways be transmitted at a fixed position in the megaframe but this position
is also allowed to vary. Table 20.20. contains a list of the exact lengths of
one megaframe.

Table 20.20. Duration of a megaframe

Guard
interval

8 MHz
channel

7 MHz
channel

6 MHz
channel

1/32 0.502656 s 0.574464 s 0.670208 s

1/16 0.517888 s 0.598172 s 0.690517 s

1/8 0.548352 s 0.626688 s 0.731136 s

1/4 0.609280 s 0.696320 s 0.812373 s