Digital Terrestrial TV to North American ATSC Standard - Digital Video and Audio Broadcasting Technology - page 438

The Reed-Solomon encoder RS(188,208) is followed by a data inter-
leaver, which changes the time sequence of the data, i.e. it scrambles the
data. At the receiver end, the de-interleaver restores the original time se-
quence of the data. With interleaving, even long burst errors can be cor-
rected as they are distributed over several frames and can thus be handled
more easily by the Reed-Solomon decoder. Interleaving is followed by a
second error correction in the form of a trellis encoder. The trellis encoder
can be compared to the convolutional encoder used in DVB-S and DVB-T.
The ATSC system employs a trellis encoder (Fig. 23.15.) with two sig-
nal paths. From the incoming bit stream, one bit is taken to a precoder with
a code rate of 1, and the second bit to a trellis encoder with a code rate of
1/2. This yields an overall code rate of 2/3. The three data streams gener-
ated by the precoder and the trellis encoder are fed to a symbol mapper,
which outputs the 8-level VSB baseband signal. The counterpart of the
trellis encoder at the receiver end is the Viterbi decoder.

Symbol rate = 10.76 MS/s

Data
segment
sync

Data
segment
sync

Data + FEC
828 symbols, 207 byte

+7

+5

+3

+1
1

3

5

7

Level
before
pilot
addition

4 symbols
(372 ns)

4 symbols

Data segment
832 symbols
208 byte (77.3 s)

Fig. 23.16. 8VSB data segment

The Viterbi decoder corrects bit errors by retracing the path through the
trellis diagram that has with the highest probability been followed through
the encoder (see also the chapter on DVB-S). Parallel to the FEC block, a
sync generator is provided in the 8VSB modulator. This generator pro-
duces, at defined intervals, special sync patterns that are transmitted in-