Metz Slim TV - Manual (Page 90)

90
41. Basic differences between analog terrestrial
reception and DVB-T
Information for our dealers and interes-
ted customers.
In order to understand how the beha-
viour of DVB-T receivers differs from that
of the analog terrestrial receivers used
until now, two things are important: digi-
tal transmission and the simultaneous
broadcasting network.
Digital transmission
A DVB transmitter does not transmit only
a single programme, but a set of pro-
grammes – normally six to eight. The
information for each programme – the
picture, the sound and teletext – is con-
verted into a digital format, i.e. into
numerical values which are grouped
together in packets. The packets of the
various programmes are then transmit-
ted sequentially in a so-called transport
stream.
Further information which permits the
receiver to detect and correct any errors
which occur during transmission is
added to this transport stream.
Example:
If the information 0 or 1 is to be trans-
mitted, the 0 could be encoded as 000
and the 1 as 111. If one of the three
digits numbers is falsified during trans-
mission, i.e. the receiver receives, for
example, 010 instead of 000, the recei-
ver can decide that the presence of at
least two zeros indicates that the inten-
ded information value was 0 and, cor-
respondingly, that at least two ones indi-
cates the information 1.
If two of the numbers in a block are
changed, the receiver can detect that an
error has occurred, but any attempt to
correct this would have the wrong result.
Example:
If the information 0 is encoded as 000
and is received as 101, the receiver
would assume that this should have been
111 and will then decode it, incorrectly,
as 1.
To sum up:
several programmes are transmitted by
a single transmitter in the form of digital
packets. Additional information is trans-
mitted to permit complete correction of a
certain number of transmission errors.
Simultaneous broadcasting
network
For DVB-T, the programmes are not
transmitted on a modulated carrier fre-
quency; instead, one channel with a
width of 8 MHz contains 8,192 individu-
al carrier frequencies.
Some of the signals reaching the receiver
come directly from the transmitter, others
are signals which have been reflected
from obstacles in the vicinity and some
are even those transmitted by other
transmitters. Normally, these signals
reinforce each other. In extreme cases,
however, if they are out of phase and
have the same amplitude, the signals
can cancel each other out: