Final Thesis

 
3.1.1 Source Coding and multiplex 
The DVB-S system is based on the coding of the image and sound by MPEG-2. The frame’s structure of the 
transport by MPEG (TS) consists in a packages with a fixed length, that allows to bring a great number of 
video services, audio and data in the same plot. The total package has a length of 188 bytes, including 1 
byte of synchronism, 3 bytes of headline and 184 bytes of data. Protection against errors is not included, it 
will be added in a later process. 
Fig.4: MPEG-2 transport MUX packet 
 
Antenna
Data
A/D
A/D
MPEG-2 
Audio/Video
Compressor
Multiplexation 
and
interleaver
Sync. byte
Inversion
Dispersion
External
Encoder
Reed Solomon
RS(204,188)
Interleaver
(i=12 bytes)
Clock and sync generator
Speed Control
Convolution 
Encoder
Base band
shapping
QPSK modulator
Mixer
Amplifier
Video
Audio
I
Q
Code's Speed Control
187 Bytes 
Sync 
1 byte 

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TAMK University of Applied Sciences 
Escola Politécnica Superior de Castelldefels 
Marcos Tejedor Alonso 
16 (44) 
3.1.2 Channel encoding and modulation 
The purpose of the channel encoding and modulation is to adapt the signal in base band to the 
characteristics of the satellite's channel. The services DTH (Direct To Home) are particularly affected by the 
limitations of power , and in such a way, the protection against the noise and the interferences is necessary, 
as well as an efficient use of the spectrum. Due to this uses, this system requires a modulation QPSK 
associate to a powerful correction block of errors based on the concatenation of convolution codes and the 
Reed Salomon code. 
Once is obtained the transport block, these bits are used in a random way to facilitate the recovery of the 
clock signal in the receiver. Later the packages are coded with the Reed-Solomon's code RS(204,188), 
which adds 16 bytes of redundancy to each package, providing a capacity of correction for 8 random errors. 
To increase the capacity of correction is used a convoluted interleaving based on the approach of Forney.
Moreover, it is coded again with a convoluted flexible code depending on the requirements of the service. 
Finally, the coded bits are changed using a code Gray in the QPSK constellation and filtrate in the base band 
to generate an spectrum with the form of cosine enhanced with a factor of roll-off around 0,35.