If the trellis coder makes an error, the outer code will correct it. In DVB, Viterbi convolutional coding may be used. Figure 8.8 shows that following interleave, the data are fed to a shift register. The contents of the shift register produce two outputs that represent different parity checks on the input data so that bit errors can be corrected. 8.7 Transmitting digits Figure 8.9 shows the elements of an ATSC digital transmitter.
Service information describing the transmission is added to the transport stream.
This stream is then randomized prior to routing to an outer R-S error correction coder that adds redundancy to the data. A convolutional interleave process then reorders the data so that adjacent data in the transport stream are not adjacent in the transmission. An inner trellis coder is then used to produce a multi-level signal for the VSB modulator. Figure 8.10 shows a DVB-T transmitter. Service information is added as before, followed by the randomizing stage for energy dispersal.