TANDBERG Alteia Instruction Manual page 89

6.4.9 Terrestrial Receiver M2/PTR/9xx
High BER Reading
This may be due to Signal Degradation being enabled in Menu #4 (Input),
Page 8. Disable it and note the difference.
Signal Degradation Mode
The Signal Degradation Mode is a test function designed to enable the
extension of the Post-Viterbi Bit Error Rate (BER) measurement capability
of the OFDM Receiver. Normally the measurement of BER is limited to a
figure of 10
increasing the measuring time. Thus it would require an unacceptably long
measurement time to measure lower BERs using the current range of
Transport Stream bit-rates.
In practice the range of BER measurement capability means that the Quasi
Error Free (QEF) threshold (the level just above the OFDM failure level),
equating to a BER of approximately 2 x 10
carrier-to-noise ratio (C/N) of approximately 20 dBs. Above this C/N, BER
measurements can usefully be made for about 4 dBs to a C/N of
approximately 24 dBs. To measure BER at higher levels of C/N would take
too long for normal system parameter monitoring purposes.
An apparent reduction in the transmitter output C/N is caused by
non-linear operation of the transmitter, whereby inter-modulation products
are generated which appear like an increase in the noise floor. This effect
is known as Equivalent Noise Degradation (END). It is this type of
performance degradation that the Signal Degradation mode is primarily
aimed at measuring.
The Signal Degradation mode extends the range of measurement by
approximately a further 5 dBs, such that BER can be measured up to a C/N
of approximately 28 dBs. Normally it is anticipated that this would be used
as a demodulator test function where it would not be in the broadcast
signal path. In this mode it would be used to monitor the performance of
the transmitter and left switched on. It would not normally be used in an
off-air reception mode of operation because the performance of the system
would be significantly degraded. It could be switched on occasionally to
give some indication of field strength if the Receiver is not indicating any
Post-Viterbi BER.
It should be noted that the effect of the signal degradation mode varies
with input signal level. As measured at different signal levels and
comparing the degradation of the BER caused by (1) the signal
degradation mode and (2) varying the C/N to achieve the same change in
BER as signal degradation mode, the equivalent effect of the signal
degradation mode to changes in C/N, varies with level of the received
signal. Therefore it may be necessary to adjust the input level to achieve
the optimum measurement criteria.
In the Alteia, seven signal degradation modes are provided, whilst in the
'0' position the signal degradation mode is switched off. Mode 1 provides a
setting about mid-range. Modes 2 to 7 bracket mode 1 and range from the
minimum degradation of 2 to the maximum of 7. If mode 7 is selected
near the MPEG failure threshold, the Alteia may experience difficulty
re-locking after the loss of input signal. In this case mode 0 should be
reselected to acquire the signal and once locked mode 7 can be reselected.
Instruction Manual: Alteia and Alteia plus Professional Receivers and Decoders
ST.TM.E9200.8
7
errors. Measurement of lower error rates is dependent upon
Preventive Maintenance and Fault-finding
-4
, corresponds to a
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