Orban OPTIMOD-FM 8600, Operating Manual

Page: 51 / 397

OPTIMOD-FM DIGITAL INTRODUCTION
1-23
In FM modulation (FM / VHF radio and television broadcast, microwave or analog
satellite links), it is the maximum-permitted RF carrier frequency deviation. In AM
modulation, it is negative carrier pinch-off. In analog telephone / post / PTT trans-
mission, it is the level above which serious crosstalk into other channels occurs, or
the level at which the amplifiers in the channel overload. In digital, it is the largest
possible digital word.
For metering, the transmission engineer uses an oscilloscope, absolute peak-sensing
meter, calibrated peak-sensing LED indicator, or a modulation meter. A modulation
meter usually has two components—a semi-peak reading meter (like a PPM) and a
peak-indicating light, which is calibrated to turn on whenever the instantaneous
peak modulation exceeds the overmodulation threshold.
Line-Up Facilities
Metering of Levels and Subjective Loudness
The meters on the 8600 show left/right input and output levels and composite mod-
ulation. Left and right input level is shown on a VU-type scale
(
0 to –40 dB), while
the metering indicates absolute instantaneous peak (much faster than a standard
PPM or VU meter). The input meter is scaled so that 0 dB on the scale corresponds
+27 dBu, which is the absolute maximum peak level that the 8600 can accept. If you
are using the AES3 digital input, a full-scale digital word corresponds to the 0 dB
point on the 8600’s input meter.
Left/right Output Level
Left and right output level is shown on a VU-type scale. The metering indicates abso-
lute instantaneous peak (much faster than a standard PPM or VU meter). The meter
is scaled so that 0 dB is calibrated to the highest left and right peak modulation lev-
el, before de-emphasis, that the processing will produce, under any program, pro-
cessing, or setup condition (except when the processing is switched to
B
YPASS
). The
Figure 1-3: VU Meter Indication Compared to Peak Level vs. Time
0
-5
-10
-15
-20
-25
-30
-36
1
2