Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 463

64QAM, error protection, guard interval, bandwidth). The usual data rate
is, however, approx. 15 Mbit/s in applications allowing portable reception
and approx. 22 Mbit/s in stationary applications with a roof antenna. A
DVB-T broadcast network is designed either for portable reception or for
roof antenna reception, i.e. if a roof antenna is used in a DVB-T network
designed for portable reception, this will not produce an increase in data
rate.
The MPEG-2 transport stream is the data signal supplied to the DVB
modulators. It consists of packets with a constant length of 188 bytes. The
MPEG-2 transport stream represents asynchronous transmission, i.e. the
individual contents to be transmitted are keyed into the payload area of the
transport stream packets purely randomly as required. The contents con-
tained in the transport stream can have completely different data rates
which do not need to be absolutely constant, either. The only rule relating
to data rates is that the aggregate data rate provided by the channel must
not be exceeded. And, naturally, the data rate of the MPEG-2 transport
stream must correspond absolutely to the input data rate of the DVB
modulators resulting from the modulation parameters.

Table 26.1. DAB and DVB comparison

Digital Audio
Broadcasting -
DAB

Digital Video Broadcasting -
DVB

Transfer mode synchronous asynchronous

Forward error
correction (FEC)

unequal equal

Modulation COFDM mit /4-

shift DQPSK

single carrier QPSK, 64QAM,
256QAM or COFDM with
QPSK, 16QAM, 64QAM

Transmission link terrestrical satellite, cable, terrestrical

In summary: DAB is a completely synchronous transmission system and
DVB is a completely asynchronous one. Remembering this will make it
easier to gain a better understanding of the characteristics of both systems
The error protection in DAB is unequal, meaning it can be selected to be
different for different contents, whereas it is equal for all contents to be
transmitted in DVB and, because of the asynchronous mode, could not
even be selected to be different since it is not known what content is being
transmitted when.
The DAB modulator demultiplexes the current content in the ETI signal
and takes it into consideration. The DVB modulator is not interested in the