Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 494

In a single-frequency network, all individual transmitters must operate
synchronised with one another. The contributions are supplied by the play-
out center in which the DAB multiplexer is located, e.g. via satellite, opti-
cal fibre or microwave link. It is obvious that due to different path lengths
the ETI signals fed in will carry different delays.
However, in each DAB modulator in a single-frequency network the
same data packets must be processed to form COFDM symbols. Each
modulator must perform all operating steps in complete synchronism with
all other modulators in the network. The same packets, the same bits and
the same bytes must be processed at the same time. At each DAB transmit-
ter site, absolutely identical COFDM symbols must be radiated at the same
time.
The DAB modulation is organized in frames.
To carry out delay compensation in the DAB SFN, Time Stamps (TIST)
derived from the GPS signal are added to the ETI signal in the multiplexer.
At the end of an ETI frame the TIST is transmitted which is derived by
the DAB ensemble multiplexer by GPS reception and is keyed into the ETI
signal. It specifies the time back to the reception of the last GPS Fig.
26.30. 1pps signal. The time information in the TIST is then compared in
the modulator with the GPS signal also received at the transmitter site and
used for performing a controlled ETI signal delay.
26.8 DAB Data Broadcasting
In the following section, the possibility of data broadcasting in DAB will
be briefly discussed. In DAB data broadcasting (Fig. 26.31.), a distinction
is made between the MOT (Multimedia Object Transfer ) standard as de-
fined in the [ETS301234] Standard, and the IP transmission via DAB. In
both cases, a DAB subchannel is operated in packet mode, i.e. the data
packets to be transmitted are divided into short constant-length packets.
Each of these packets has a packet ID in the header section by means of
which the transmitted content can be identified.
In the Multimedia Object Transfer (MOT) according to [ETS301234], a
distinction is made between file transmission, a slide show and the
“Broadcast Web Page” operation. In file transmission, only files are fed
out cyclically. A slide show can be configured with respect to its display
speed. It is possible to transmit JPEG or GIF files.
In the “Broadcast Web Page”, a directory of HTML pages is cyclically
transmitted and a starting page can be defined. The resolution corresponds
to ¼ VGA.