Atec Rohde-Schwarz-SFQ Series - Manual (Page 7)

TV Test Transmitter R&S ® SFQ 7
For terrestrial transmissions, the signal is
made to pass through further FEC stages
because of the inherently unfavourable
propagation conditions: an inner bit inter-
leaver (at the antenna end) and a symbol
interleaver. Next, mapping is performed
according to the selected QPSK, 16QAM or
64QAM constellation. After insertion of the
pilot and TPS (transmission parameter sig-
nalling) carriers in the frame adapter, con-
version of the frequency domain to the time
domain is effected by an inverse fast Fou-
rier transform, to a 1705 (2K) or 6817 (8K)
carrier depending on the selected mode.
As a last step, the guard interval is inserted.
Prior to modulation, the spectrum has to be
limited by filtering. The roll-off factor
(root cosine) can be varied in for DVB-S,
DVB-DSNG and DVB-C.
ATSC/8VSB: coding and map-
ping for antenna
The I/Q coder for 8VSB of the TV Test
Transmitter R&S SFQ encodes the applied
transport stream for terrestrial transmis-
sion via antenna in line with standards
and processes it so that I and Q (inphase
and quadrature) signals are obtained.
With 8VSB, the R&S SFQ accepts MPEG
transport streams with a packet length of
188 bytes. The input interfaces are syn-
chronous parallel (TS parallel, SPI), asyn-
chronous serial (ASI) and serial
(SMPTE310M).
When using the TS parallel input, an
input data rate of 19.3926 Mbit/s
±10%
can be attained. Use of the optional input
interface yields a USEFUL DATA RATE in a
wide range of up to 19.3926 Mbit/s.
The R&S SFQ warns the user if the input
signal fails or if the USEFUL DATA RATE is
too high. Instead of the external transport
stream (DATA) being applied, an internal
data source can generate null transport
stream packets (NULL TS PACKET, NULL
PRBS PACKET). A SYNC PRBS is imple-
mented for bit error evaluation in receivers.
An unpacketed random sequence may
also be selected. With 8VSB the PRBS
sequence can be selected before (PRBS
BEFORE TRELLIS) or after the trellis coder
(PRBS AFTER TRELLIS). The PRBS
sequence is also available in packeted
form in the null transport stream packets
(NULL PRBS PACKET).
Generation of the standard frame is fol-
lowed by a randomizer which ensures that
energy is evenly distributed in the channel
(energy dispersal). The randomizer can be
disabled. Following energy dispersal, a
Reed-Solomon coder (208,188) is provided
for forward error correction (FEC). 20 parity
bytes are added to the unchanged
188 data bytes. Up to ten errors per seg-
ment can thus be corrected. A convolu-
tional interleaver changes the position of
the individual bytes so that consecutive
bytes are separated. Burst errors occurring
on the transmission path are split up by the
receiver into individual errors that can be
corrected by the Reed-Solomon decoder.
The interleaver can be disabled. A trellis
coder follows for further FEC. The segment
sync and the field sync pulses are inserted
after the interleaver or trellis coder. The
mapper assigns the relevant amplitude
steps to the symbols. The pilot used by the
receiver for synchronization is also added
in the mapper. The pilot amplitude can be
modified and switched off. Prior to modu-
lation, the spectrum must be limited by
appropriate filtering. The roll-off is perma-
nently set to 0.115 (root cosine).
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