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ENGLISH
TV & SAT Level Meter MTD 120
User Manual
5.5.- COFDM modulation
Before to measure the BER it is necessary to define some parameters concerning the digital signal, which are described next. To see its
present value or to modify it, being in the BER measuring screen, press the ENTER [5] key, a multiple-choice menu showing the functions
relative to the BER measurement will appear. The parameters that must be defined by the user are the following:
1) Carriers
It defines the number of modulation carriers between 2k and 8k. To modify its value, place the marker over the Carriers field by pressing
keys
[6] and then press ENTER [5] key: a menu will appear on the screen. With keys
[6] select the desired value for the
Carriers parameter and finally press ENTER [5] key again to validate.
2) Guard interval
The Guard Interval parameter corresponds to the time between symbols, its purpose is to permit a correct detection in multi-path situa-
tions. This parameter is defined according to the symbol length: 1/4, 1/8, 1/16, 1/32. To modify its value, press keys
[6] and place
the marker over the Guard Interval field and then press ENTER [5] key: a menu with the available values will appear. Then pressing keys
[6] select the desired value and finally press ENTER [5] key to validate.
3) Attenuator
It permits to select attenuation between 0 and 30 dB. It is advisable to activate the 30 dB attenuator under that measurement conditions
where the signal level is near to the maximum input level (approximately starting from 20 dB under the maximum level) and it is possible
that the tuner becomes saturated. Under no-saturation conditions, when increasing the attenuation value the BER measure must to main-
tain or to increase (insufficient signal level) but never to decrease.
This configuration menu shows, besides the user definable COFDM signal parameters, the value of the rest of COFDM signal parameters
detected automatically:
Code Rate
Also known as Viterbi ratio, defines the ratio between the data bits number and the total number of bits transmitted (the
difference corresponds to the number of control bits for the error detection and recovery).
Modulations
Carriers modulation. It also defines the system noise immunity. (QPSK, 16-QAM and 64-QAM).
Spectral Inv.
Defines the state of the spectral inversion.
Hierarchy
The DVB-T norm contemplates the possibility to make a DTT transmission with hierarchical levels, it is to say a simul-
taneous transmission of the same program with different image qualities and noise protection levels, in order the
receiver decodes the signal in the best conditions.
Once you have defined the COFDM signal parameters, it will be possible to measure the BER. When the BER measuring mode is selec-
ted, the monitor will show a picture like the following:
Figure 30.- COFDM signals BER measuring screen.
Two measures are shown:
1) CSI: Channel status information
2) BER after Viterbi
The CSI measure (Channel Status Information) is a qualitative measure about channel state, between 0 and 100%. The optimum value
corresponds to 0%. This measure permits to look for the best situation even in those measuring conditions where the measured BER is best
than the minimum readout (in this way, in the example of the previous figure, the measured BER is lower that the minimum readout, 1.0x10
-7
,
but the CSI measurement, 27%, still can be improved).
Next it is shown the BER after Viterbi measurement both in numeric and graphic bar format.
In a reception system of terrestrial digital signal, after the COFDM decoder two error correction methods are applied. Obviously, each time
we apply an error corrector to the digital signal, the error rate changes, therefore if we measure the error rate at the output of the COFDM
demodulator, at the output of the Viterbi decoder, and at the output of the Reed-Solomon decoder, we obtain nothing more than different error
rates. This is the reason because of the QPSK-COFDM option provide the BER after Viterbi and the number of Wrong packets received
after Reed- Solomon.
COFDM
1.0E-7
CSI:
27%
0 25 50 75 100
BER after Viterbi:
-7 -6 -5 QEF -3 -2 -1
Chan: 67
W.P. 0 in 00:22:59
MPEG-2
DVB-T
