Digital TV HD System Ilustrated - pág.22

control of each individual broadcaster.  This is known as «Convolutional Error
Correction» or Viterbi correction.
 

 
FEC modifies the bit stream in a controlled fashion before
transmission and also sends information on how it was modified.  Based on this
information, the receiver will expect a specific bit sequence and can correct a
bit stream if the sequence is different to that expected.
The FEC is quoted as a fraction eg. 3/4, which means that,
for every 3 bits transmitted, one parity bit will be added, so the final number
of bits will be 4, not 3.  The highest FEC would be 1/2 ie. one parity bit for
each symbol bit.  This would make the system very rugged (ie. more immune to
interference) but at the expense of greatly increased bandwidth requirements. 
The terrestrial TV channels use a FEC of 2/3 or 3/4.  Most satellite
broadcasters currently use a FEC of 3/4 but BSkyB use 2/3, the additional error
correction allowing for the use of smaller satellite dishes.
It is necessary to specify the symbol rate (the speed at
which the data is being transmitted) and the FEC before downloading channels,
so that system can correctly distinguish between the error correction bits and
the actual data being loaded in.
To summarize, the more error correction that is applied, the
more rugged the signal becomes (at the expense of signal bandwidth, of course),
and this in turn affects the required dish size.
The procedures for measuring BER are outlined in part nine.

8. Digital Bandwidths