Orban OPTIMOD-FM 8600, Operating Manual

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5-2
TROUBLESHOOTING ORBAN MODEL 8600
Because of its 64 kHz base sample rate, the output bandwidth of the 8600’s FM
channel processing is slightly higher (17 kHz) than that of Orban processors using 32
kHz base sample rate. The signal path to the transmitter must be essentially flat to
17 kHz to take advantage of the 8600’s excellent peak control at its digital and ana-
log left/right outputs. Using a link with 32 kHz sample rate to pass the 8600’s output
will increase overshoot by as much as 1.5 dB compared to links with 44.1 kHz or
higher sample rate that have essentially flat audio response to 20 kHz. If you have a
32 kHz STL, we recommend placing the 8600 at the transmitter and using the STL it
to pass the unprocessed audio to the 8600’s input(s). This will allow you to use the
8600’s composite output to drive the transmitter. Thanks to the 8600’s “Half-Cosine
Interpolation” composite limiter, the composite output has exceptionally low over-
shoot.
Digital STLs using lossy compression algorithms (including MPEG1 Layer 2, MPEG1
Layer 3, Dolby AC2, and APT-X) will overshoot severely (up to 3 dB) on some pro-
gram material. The amount of overshoot will depend on data rate—the higher the
rate, the lower the overshoot.
Even if the transmission system is operating properly, the FM modulation monitor or
reference receiver can falsely indicate peak program modulation higher than that
actually being transmitted if the monitor overshoots at high and low frequencies.
Many commercial monitors have this problem, but most of these problem units can
be modified to indicate peak levels accurately.
Orban uses the Belar “Wizard” series of DSP-based monitors internally for testing,
because these units do not have this difficulty.
If you have inadvertently activated the ITU412 multiplex power controller (see step
12 on page 2-24) then the average modulation can be very low and peak modula-
tion can be held below 100%, depending on program material.
Audible Distortion On-Air
Make sure that the problem can be observed on more than one receiver and at sev-
eral locations. Multipath distortion at the monitoring site can be mistaken for real
distortion (and will also cause falsely high modulation readings).
Verify that the source material at the 8600's audio inputs is clean. Heavy processing
can exaggerate even slightly distorted material, pushing it over the edge into unac-
ceptability.
The subjective adjustments available to the user have enough range to cause audi-
ble distortion at their extreme settings. There are many controls that can cause dis-
tortion, including
M
ULTIBAND
C
LIPPING
,
F
INAL
C
LIP
D
RIVE
, and
C
OMPOSITE
L
IMIT
D
RIVE
Setting the
LESS-MORE
control beyond “9” will cause audible distortion of some
program material with all but the Classical and Protect presets. Further, the “Loud”
family of presets can sometimes cause audible distortion with certain program mate-
rial; this is the price to be paid for “competitive” loudness as it is defined in certain
markets.
If you are using analog inputs, the headroom of the unit's analog-to-digital (A/D)
converter must be correctly matched to the peak audio levels expected in your sys-