Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 245

passed through completely transparently and are not mixed with the pseu-
do random bit sequence.

Transmission
link

MPEG2
TS

MPEG2
TS

DVB
mod.

DVB
demod.

RS

RS

204 byte

184 byte
payload

4 byte
header

16 byte
RS FEC

188 byte

Fig. 14.9. Reed-Solomon coding

The next stage contains the outer coder (Fig. 14.5. and 14.9.), the Reed-
Solomon forward error correction. At this point, 16 bytes of error protec-
tion are added to the data packets which are still 188 bytes long but are
now energy-dispersed. The packets now have a length of 204 bytes which
makes it possible to correct up to 8 errors at the receiving end. If there are
more errors, the error protection fails and the packet is flagged as errored
by the demodulator by the transport error indicator in the transport stream
header being set to 'one'.
Frequently, however, burst errors occur during a transmission. If this re-
sults in more than 8 errors in a packet protected by Reed-Solomon coding,
the block error protection will fail. The data are, therefore, interleaved, i.e.
distributed over a certain period of time in a further operating step.
Any burst errors present are then broken up in the de-interleaving (Fig.
14.10.) at the receiving end and are distributed over a number of transport
stream packets. It is then easier to correct these burst errors, which have
now become single errors, and no additional data overhead is required.
In DVB-S, the interleaving is done in a so-called Forney interleaver
(Fig. 14.11.) which is composed of two rotating switches and a number of
shift registers. This ensures that the data are scrambled, and thus distrib-