L64780 LSI Logic Corporation, L64780 Datasheet - Page 12

• Manufacturer Part Number: L64780
• Description: Dvb-t Cofdm Demodulator Technical Manual 2/00
• Manufacturer: LSI Logic Corporation

The concatenated coding used in the DVB-T/ETSI specification is
identical to that in the DVB-S specification for satellite transmissions. The
FEC decoder can take the same form in receivers for satellite as well as
terrestrial transmissions. Thus, the L64780 chip provides all the
necessary demodulation functions except for FEC decoding, which is
done by a separate chip, such as the L64724 or the L64705. The L64780
I/O format and pinouts simplify its connection to the L64724/L64705.
The DVB-T/ETSI specification incorporates many modes, providing a
wide range of capacity/performance trade-off options. The L64780
demodulates all these modes.
Terrestrial transmission paths are prone to multipath, which can result in
“ghosting” on analog television pictures. With conventional methods of
digital transmission, multipath causes inter-symbol interference. This
becomes increasingly problematic as the bit rate increases.
The DVB-T/ETSI specification uses a special form of modulation that
uses Coded Orthogonal Frequency Division Multiplexing, which is
well-suited to channels with significant multipath. It can tolerate signals
with long delay and high relative amplitude. Consequently, the L64780
can accommodate natural multipath (from terrain, buildings, etc.) and it
can be used in a Single Frequency Network (SFN). In an SFN, many
transmitters operate on the same frequency with the same modulation
(used in Digital Audio Broadcasting, which also uses COFDM).
In the frequency domain, multipath can be seen as channel frequency
selectivity. COFDM applies concatenated FEC coding, then distributes
the coded data over many carriers (1705 or 6817 in this case, depending
on the mode). At the receiver, the frequency selectivity of the channel
causes some carriers to be degraded or suppressed. However, the
receiver can determine how much each carrier is affected by noise, then
pass this information to the inner-code Viterbi decoder by means of
soft-decision bits. This allows the Viterbi decoder to decode the data
more efficiently. A second decoder, for the Reed-Solomon FEC,
completes the process.
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Introduction