Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 461

26.1 Comparing DAB and DVB
In a comparison of DAB and DVB, the basic characteristics of both meth-
ods will first be compared, pointing out properties and differences. In prin-
ciple, it is possible to transmit data synchronously or asynchronously (Fig.
16.1.). In synchronous transmission, the data rate is constant for each data
channel and the time slots of the individual data channels are fixed. In
asynchronous transmission, the data rate of the individual data channels
can be constant or it can vary. The time slots have no fixed allocation.
They are allocated as required and their order in the individual channels
can thus be completely random. Examples of synchronous data transmis-
sion are PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous
Digital Hierarchy) and DAB (Digital Audio Broadcasting). Examples of
asynchronous data transmission are ATM (Asynchronous Transfer Mode)
and the MPEG-2 transport stream/Digital Video Broadcasting (DVB).
DAB is a completely synchronous system, a completely synchronous
data stream being produced right back in the playout center, i.e. at the
point where the DAB multiplex signal is generated. The data rates of the
individual contents are constant and are always a multiple of 8 kbit/s. The
time slots in which the contents from the individual sources are transmitted
are permanently allocated and vary only when there is a complete change
in the multiplex, i.e. in the composition of the data stream. The data signal
coming from the multiplexer which is supplied to the DAB modulator and
transmitter is called ETI (Ensemble Transport Interface) (Fig. 26.2.). The
multiplexed data stream, or multiplex, itself is called ensemble. The ETI
signal uses E1 transmission paths known from telecommunication which
have a physical data rate of 2.048 Mbit/s. E1 would correspond to 30
ISDN channels and 2 signalling channels of 64 kbit/s each, also called
G.703 and G.704 interface. Physically, these are PDH interfaces but DAB
uses a different protocol. Although the physical data rate is 2048 kbit/s, the
actual net data rate of the DAB signal transported across it is between (0.8)
1.2 ... 1.73 Mbit/s. The ETI signal is transmitted either without error pro-
tection, or with a Reed Solomon error protection code which, however, is
removed again at the input of the DAB modulator. The error protection of
the DAB system itself is added only in the DAB modulator although this is
often wrongly shown to be different in various references. The modulation
method used in DAB is COFDM (Coded Orthogonal Frequency Division
Multiplex) and the subcarriers are /4-shift DQPSK modulated. After the
error protection has been added the gross data rate of the DAB signal is 2.4
Mbit/s. A special feature of DAB consists in that the different contents can
be error protected to a different degree (unequal FEC).