Orban OPTIMOD-FM 8600Si, Operating Manual

Page: 209 / 356

OPTIMOD-FM DIGITAL OPERATION
3-43
bandlimited to 53 kHz ( Figure 3-1 ), unlike conventional composite clip-
pers.
Composite Limit Drive sets the drive level, in dB, into the composite limiter.
This control has no effect on the 8600Si’s left and right analog or digital outputs.
The C
OMPOSITE
L
IMIT
D
RIVE
control is set to “0 dB” for most factory pre-
sets. At this setting, it removes a few tenths of a dB of residual overshoot
from the audio processing without affecting audio quality. We prefer to
use the audio-domain overshoot compensation to do most of the work
because it operates at a 256 kHz sample rate and is fully anti-aliased,
whereas the composite limiter will inevitably introduce aliasing around
38 kHz upon demodulation in the receiver. This is because it introduces
spectrum in the stereo subchannel area when it clips material in the 0 to
15 kHz area. The receiver will “see” this as stereo material and will de-
modulate it as if it were part of the stereo subchannel. Accordingly, har-
monics of L+R material will be frequency-shifted upon demodulation and
will no longer bear a harmonic relationship to the material that pro-
duced them.
If you want to use the composite limiter more heavily, one option is to
trade off composite limiting against left/right domain overshoot com-
pensation. To do this, back off the
OSC
OMP
D
R
(Overshoot Compensa-
tion) control and increase the C
OMPOSITE
L
IMIT
D
RIVE
control setting pro-
portionately.
Because of the improved distortion control in the MX presets, a very
small amount of composite limiting can significantly degrade the cleanli-
ness of the audio. If you competitive situation permits, we recommend
that you turn off the composite limiter, which will reduce the average
modulation by less than 3% compared to leaving it set to 0 dB, which is
where it is set in the MX factory presets.
Pilot Protect (Pilot Protection Filter) turns the 19 kHz notch filter on or off. It af-
fects the composite output only.
The 8600Si’s composite limiter always protects frequencies above 53 kHz.
SRS
57.088 kHz -72.881 dBVpk
0 Hz
FFT 1 Log Mag BMH
PkhAvg 20000
51.2 kHz 102.4 kHz
-100
dBVpk
0
dBVpk
10
dB/div
Figure 3-1: 0-100 kHz Baseband Spectrum
(Loud-Hot preset)
SRS
19 kHz -20.643 dBVpk
15.8 kHz
FFT 1 Log Mag BMH
PkhAvg 869
19 kHz 22.2 kHz
-100
dBVpk
0
dBVpk
10
dB/div
Figure 3-2: 19 kHz Pilot Notch Filter
Spectrum (Loud-Hot preset; detail)