Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 387

A DVB-T test receiver (Fig. 21.2.) can be used for detecting all influ-
ences acting on the transmission link. A DVB-T test receiver basically dif-
fers from a set-top box in the analog signal processing being of a much
higher standard and the I/Q data and the channel estimation data being ac-
cessed by a signal processor (DSP). The DSP then calculates the constella-
tion diagram and the measurement values. In addition, the DVB-T signal
can be demodulated down to the MPEG-2 transport stream level.

IF1 IF2

MPEG2
TS

Anti
alias.
low
pass

RF/IF
down
conv./
tuner

A

RF

SAW
filter

Mixer

DVBT
dem.

X

D

Q

DSP

Noise
gen.

Display

Fig. 21.2. Block diagram of a DVB-T test receiver

21.1 Measuring the Bit Error Ratio

In DVB-T, as in DVB-S, there are 3 bit error ratios due to the inner and
outer error protection:

• Bit error ratio before Viterbi

• Bit error ratio before Reed Solomon

• Bit error ratio after Reed Solomon

The error ratio of greatest interest and providing the most information is
the pre-Viterbi bit error ratio. It can be determined by applying the post-
Viterbi data stream to another convolutional encoder of the same configu-
ration as that at the transmitter end. If the data stream before Viterbi is
compared with that after the convolutional encoder - taking into considera-
tion the delay of the convolutional encoder - the two are identical provided