Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 374

to an MPEG-2 table (PSI/SI). For this purpose, the transport stream is di-
vided into sections, the lengths of which are selected to be approximately a
half second because they must correspond to a certain integral number of
transport stream packets fitting into a certain integral number of super-
frames. These sections are called megaframes.
A megaframe is composed of an integral number of superframes, as fol-
lows:

• 1 megaframe = 2 superframes in 8K mode

• 1 megaframe = 8 superframes in 2K mode

The 1 pps signals of the GPS satellites are also used for synchronizing
the timing of the DVB-T modulators. In the case of a single frequency
network, there is a professional GPS receiver outputting both a 10 MHz
reference signal and this 1 pps time signal at every transmitter site and at
the playout center (Fig. 20.24.) where the multiplexed stream is assembled.

1pps pulse Tx3, RF1

GPS

Tx1, RF1

MIP

GPS:
Global
Positioning
System

1pps pulse

Tx5, RF1

Playout
center

MPEG2 TS

MIP
inserter

Tx4, RF1

Tx2, RF1

Fig. 20.24. DVB-T distribution network with MIP insertion

At the multiplexer site there is a so-called MIP inserter which inserts
this special transport stream packet into one megaframe in each case,
which is why this packet is called the megaframe initializing packet (MIP).
The MIP has a special PID of 0x15 so that it can be identified and it con-
tains time reference and control information for the DVB-T modulators.
Among other things, it contains the time counting back to the time the last
1 pps pulse was received at the MIP inserter. This time stamp with a reso-