UNAOHM
EP2500 (Rev. 0)
57
In fact, an increase depends on the Reed Solomon corrections carried out but they may be very low in
quantity (roughly one count every 30 minutes) even when the Channel BER is in the range of 1E-3.
12.6
QAM MEASUREMENTS
12.6.1
QAM CH BER Measurement
A QAM signal will rarely have no errors; if its quality is excellent it could have an error rate of 1 out of
10 million (1E-7), while greater than 1 out of 10,000 (1E-4) the quality (
of the signal, not of the
picture)
must be considered poor. In this case, the converter could be on the verge of not functioning
anymore even if the picture is perfect; a slight deterioration could be enough to completely block
reception, without any warning. The BER is indicated by the instrument in an exponential form; i.e., for
example, 1 out of 1000 becomes 1E-3, 2 out of 10000 becomes 2E-4, 7.5 out of 100000 becomes 7.5 E-
5, etc.
The instrument directly supplies the “Channel BER” (CH BER) or PRE REED SOLOMON BER
measurement which is the most significant parameter since:
1.
It is immediately measurable with all the signals present in the installation (from optimal to
mediocre).
2.
It supplies an indication of the noise margin from the reception threshold (obviously, if the
installation allows for an even minimal reception).
The diagram in Figure 28 supplies a comparison between the CH BER and the quality of the signal
present in the installation.
From the diagram we can easily calculate theEb/No (a parameter for signal quality) which is the ratio
between the energy per bit and the noise content in a 1 Hz bandwidth. Once the C/N ratio is known, the
following formula is applicable for the calculation:
Eb/No (in dB) = C/N - 10*log
[
(Rs*k)/Bw
]
where:
Eb = Energy per bit.
No = Noise energy in 1Hz band.
C/N = Carrier/noise ratio measured in dB.
Rs = Symbol rate in M symb/sec.
Bw = Signal band.
K = 4 (QAM = 16), 5 (QAM = 32), 6 (QAM = 64), 7 (QAM = 128), 8 (QAM = 256)
Example:
For DVB-C, QAM 64 with BW=7MHz - Rs=6.8 M S/s obtains:
Eb/No=C/N - 10*log
[
(6.8*6)/7
]
Eb/No=C/N-7.6
