Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 462

MPEG-2, and thus DVB, is a completely asynchronous system.
The MPEG-2 transport stream is a baseband signal forming the input
signal to a DVB modulator. The MPEG-2 transport stream is generated in
the playout center by encoding and multiplexing the individual programs
(services) and is then supplied to the modulator via various transmission
paths (Fig. 26.3.). In the DVB modulator, it must be decided how, i.e. by
which transmission path, the MPEG-2 transport stream is to be emitted:
terrestrial (DVB-T), by cable (DVB-C) or by satellite (DVB-S). Naturally,
the transmission rates and modulation methods differ for the individual
transmission methods. In DVB-T, COFDM is used in conjunction with
QPSK, 16QAM or 64QAM. In DVB-C, it is either 64QAM or 256QAM
depending on the type of cable link (coaxial cable or optical fibre). In
DVB-S, the modulation method of choice has been QPSK because of the
poor signal/noise ratio in the channel.

DVB
mod.

Playout

MPEG2
transport
stream
approx. 13…40 Mbit/s

RF RF

COFDM @ DVBT (terrestrical)
64QAM/256QAM @ DVBC (cable)
QPSK @ DVBS (satellite)

Fig. 26.3. DVB transmission link

In DVB, all contents transmitted carry the same degree of error protec-
tion (equal FEC).
As a rule, the data rate in DVB-S is about 38 Mbit/s. It only depends on
the symbol rate selected and on the code rate, i.e. the error protection. Us-
ing QPSK 2 bits/symbol can be transmitted. The symbol rate is mostly
27.5 Msymbols/s. If 3/4 is selected as code rate, the resultant data rate is
38.01 Mbit/s.
If, e.g., 64QAM (coax networks) is selected in DVB-C, and a symbol
rate of 6.9 Msymbols/s, the resultant net data rate is 38.15 Mbit/s.
In DVB-T, the possible data rate is between 4 and about 32 Mbit/s de-
pending on operating mode (type of modulation - QPSK, 16QAM,