MPEG applications - The MPEG - page 222

Figure 7.12: Multipath reception can place notches in the channel spectrum. This will require equalization at the
receiver.
Although the use of transform-based receivers appears complex, when it is considered that such an approach
simultaneously allows effective equalizatation the complexity is not significantly higher than that of a conventional
receiver which needs a separate spectral analysis system just for equalization purposes. The only drawback of
OFDM is that the transmitter must be highly linear to prevent intermodulation between the carriers. This is readily
achieved in terrestrial transmitters by derating the transmitter so that it runs at a lower power than it would in analog
service.
Figure 7.13: Error-correcting strategy of digital television broadcasting systems.
This is not practicable in satellite transmitters which are optimized for efficiency, so OFDM is not really suitable for
satellite use.
Broadcast data suffer from both random and burst errors and the error- correction strategies of digital television
broadcasting have to reflect that. Figure 7.13 shows a typical system in which inner and outer codes are employed.
The Reed-Solomon codes are universally used for burst- correcting outer codes, along with an interleave which
will be convolutional rather than the block-based interleave used in recording media. The inner codes will not be R-
S, as more suitable codes exist for the statistical conditions prevalent in broadcasting. DVB uses a parity based
variable-rate system in which the amount of redundancy can be adjusted according to reception conditions.
Figure 7.14 shows a block diagram of a DVB-T (terrestrial) transmitter. Incoming transport stream packets of 188
bytes each are first subject to R-S outer coding. This adds 16 bytes of redundancy to each packet, resulting in 204
bytes. Outer coding is followed by interleaving. The interleave mechanism is shown in Figure 7.15 . Outer code
blocks are commutated on a byte basis into twelve parallel channels. Each channel contains a different amount of
delay, typically achieved by a ring-buffer RAM. The delays are integer multiples of 17 bytes, designed to skew the
data by one outer block (12 × 17 = 204). Following the delays, a commutator reassembles interleaved outer blocks.
These have 204 bytes as before, but the effect of the interleave is that adjacent bytes in the input are 17 bytes
apart in the output. Each output block contains data from twelve input blocks making the data resistant to burst
errors.