UNAOHM
52 EP2500 (Rev. 0)
Figure 25 Simplified block diagram of an IRD DVB-C (QAM) demodulating chain
Figure 26 Simplified block diagram of an IRD DVB-T (OFDM) demodulating chain
Note the following nomenclature for a better understanding of acronyms, notations and abbreviations:
Channel BER = pre Viterbi BER = BER before Viterbi
Post Viterbi BER = BER after Viterbi = BER before Reed Solomon
RC (Reed Solomon Corrected) = Errors corrected by the Reed Solomon stage
RU (Reed Solomon Uncorrected ) = Errors not corrected by the Reed Solomon stage
12.4 QPSK MEASUREMENTS
12.4.1 QPSK CH BER & POST-VITERBI BER Measurement
As far as the QPSK signal is concerned, there are two measuring points: one just before the
Convolutional Error Corrector ( or Viterbi) and, in that case, the measurement is called Channel BER
(or pre Viterbi BER), or right after the first correcting stage and, in that case, called post Viterbi BER
(or pre Reed Solomon BER, the name of the second correcting stage).
Since the two measurements are theoretically equated but yield different results, it is necessary to
understand how to “weight” them, what their limitations are in order to carry out a top quality
installation.
Tuner
Serializer
Digitizer
RF Signal
QAM coded
I
Q
Adaptation
equalizer
Reed Solomon
error corrector
Ch-BER (pre R. Solomon BER)
Data output
MPEG coded
RU
DVB-C (QAM) Reception chain
TUNER
I
Q
A/D Converter
Decoder
DVB-T (OFDM) Reception chain
Viterbi Corrector Reed Solomon
Corrector
Data output MPEG
coded
Post Viterbi BER Ch-BER
RU
RF Signal
OFDM coded
Constellation
Code Rate
No. of carriers
Guard
Automatic search
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