Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 334

As can also be seen from the block diagram, two MPEG-2 transport
stream inputs are possible which then provides for the so-called hierarchi-
cal modulation. However, hierarchical modulation is provided as an option
in DVB-T and has not as yet been put into practical use. Hierarchical
modulation was originally provided for transmitting the same TV pro-
grams with different data rate, different error correction and different qual-
ity in one DVB-T channel. The HP (high priority) path transmits a data
stream with a low data rate, i.e. a poorer picture quality due to higher com-
pression, but allows better error protection or a more robust type of modu-
lation (QPSK) to be used. The LP (low priority) path is used for transmit-
ting the MPEG-2 transport stream with a higher data rate, a lower error
protection and a type of modulation of higher order (16QAM, 64QAM).
At the receiving end, HP or LP can be selected in dependence on the con-
ditions of reception. Hierarchical modulation is intended to lessen the im-
pact, as it were, of the “fall off the cliff”. But it is also quite conceivable to
transmit two totally independent transport streams. Both HP and LP
branches contain the same channel coder as in DVB-S but, as already men-
tioned, this is an option in the DVB-T modulator, not the receiver where
this involves very little additional expenditure.

x 2 x (1.5code rate)

x 204/188

Data rate in

= Date rate out
[2.17...(1.63)...1.36]

FEC1/
outer
coder

FEC2/
inner
coder

Inv. sync.

Base
band
interf.

Conv.
inter
leaver

Coded
data
out

Reed
Solom.
enc.

TS in Ener

gy
disp.

Sync
invert.

Conv.
coder

Synchronization

Code rate
1/2...(3/4)...7/8

Same as in DVBC

Same as in DVBS

Fig. 20.2. Block diagram of the DVB-T modulator - part 2, FEC

Not every COFDM carrier in DVB-T is a payload carrier. There is also
a large number of pilot carriers and special carriers. These special carriers
are used for frequency synchronization, channel estimation and channel
correction, and for implementing a fast information channel. They are in-
serted into their locations in the DVB-T spectrum before the IFFT.