Orban OPTIMOD-FM 8500S Operating Manual page 79

OPTIMOD-FM DIGITAL
the 8500S (including the transmitter), instead set the multiplex power
threshold so that it equals the amount of peak overshoot (in dB) in the
transmission system. If you do not do this, the 8500S's ITU-R BS412 con-
troller will set the average multiplex power too low.
The easiest way to measure system overshoot is to turn the multiplex
power controller off temporarily. Then set the 8500S's output level (using
its built-in 100Hz reference tone) so that the transmitter produces ±75
kHz deviation. Finally, play program material with lots of high frequency
energy and bass transients (like bright rock music with heavy kick drum)
and observe the peak deviation produced by the program material. The
overshoot is the amount (in dB) by which the deviation with program
material exceeds ±75 kHz deviation.
See the notes on the M
Multiplex Power Controller on page 3-72.
2. Set up modulation reduction to compensate for subcarriers, if needed.
In the United States, F.C.C. Rules permit you to add 0.5% modulation for every
1% increase in subcarrier injection. For example, if your subcarrier injection to-
tals 20%, you can set the total modulation to 110% (±82.5 kHz deviation). The
8500S has the ability to reduce audio modulation to compensate for subcarriers.
The advantage of using the modulation reduction function is that the pilot injec-
tion stays constant when the audio modulation is reduced. However, using the
modulation reduction function is slightly inconvenient because it requires pro-
gramming and activating at least one 8500S GPI input. If you have the same sub-
carrier injection at all times, a more convenient alternative is to set the desired
modulation level by using the
injection control until the injection equals 9% modulation.
If you wish to use the modulation reduction function anyway:
A) Navigate to Setup > Next > M
B) Hold down the appropriate M
the amount of modulation reduction produced by the M
M . R
OD EDUCTION
You can program these to be activated via any rear-panel GPI input or by
the 8500S's clock-based automation. (See Remote Control Interface Pro-
gramming starting on page 2-42 and Automation starting on page 2-36.)
When both modulation reduction functions are active, the modulation
reduction is the sum of their settings.
To comply with FCC Rules, set the modulation reduction to one-half the
injection of the associated subcarrier. For example, if your subcarrier in-
jection totals 20% from two 10% subcarriers, set M
TO "5%" and M
modulation to 90% (100% – 5% – 5%). When you add back the 20%
modulation due to the subcarriers, you get the required 110% total
modulation.
P O
PX OWER FFSET
C
OMPOSITE
R
ODULATN
. R
OD EDUCTION
2 functions.
R
ODULATION EDUCTION
control on page 3-43 and ITU-R
control(s). Then turn up the pilot
L
EVEL
.
EDUCTN
button and turn the knob to set
OD
ODULATION
2 to 5%. This will reduce your audio
INSTALLATION
. R 1 and
EDUCTION
R 1
EDUCTION
2-23