Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 241

However, the MPEG-2 transport stream now to be sent to the satellite as
QPSK-modulated signal must first be provided with error protection before
being fed into the actual modulator. In DVB-S, two error protection
mechanisms are used, namely a Reed-Solomon block code which is cou-
pled with convolutional (trellis) coding. In the case of the Reed-Solomon
error protection, already known from the audio CD, the data are assembled
into packets of a certain length and these are provided with a special
checksum of a particular length. This checksum allows not only errors to
be detected but also a certain number of errors to be corrected. The number
of errors which can be corrected is a direct function of the length of the
checksum. In Reed-Solomon, the number of repairable errors always cor-
responds to exactly one half of the error protection bytes (checksum).

x 2 x (1.5code_rate)

x 204/188

Input data rate

= Output data rate
[2.17...(1.63)...1.36]

FEC1/
outer
coder

FEC2/
inner
coder

Inv. sync.

I

Basis
band
interf.

Reed
Solom.
enc.

Conv.
inter
leaver

TS in

Sync
invers.

Energy
disp.

Coded
data

Conv.
coder

Q

Synchronization

Code rate
1/2...(3/4)...7/8

same as DVBC

Fig. 14.5. Forward error correction (FEC) in DVB-S and DVB-T. DVB-S modula-
tor Part 1

It is possible then to always consider exactly one transport stream pa-
cket as one data block and to protect this block with Reed Solomon error
protection. An MPEG-2 transport stream packet has a length of 188 bytes.
In DVB-S, it is expanded by 16 bytes Reed Solomon forward error correc-
tion to form a data packet of 204 bytes length. This is called RS (204,188)
coding. At the receiving end, up to 8 errors can be corrected in this 204-
byte-long packet. The position of this/these error/s is not specified. If there
are more than 8 errors in a packet, this will still be reliably detected but it
is no longer possible to correct these errors. The transport stream packet is
then flagged as errored by means of the transport error indicator in the
transport stream header. This packet must then be discarded by the