Optimod-AM 9400 V1.2 Operating Manual - Orban

OPTIMOD-AM DIGITAL TROUBLESHOOTING 5-1
Section 5
Troubleshooting
Problems and Potential Solutions
Always verify that the problem is not the source material being fed to the 9400, or
in other parts of the system.
RFI, Hum, Clicks, or Buzzes
A grounding problem is likely. Review the information on grounding on page 2-11.
The 9400 has been designed with very substantial RFI suppression on its analog and
digital input and output ports, and on the AC line input. It will usually operate adjacent
to high-powered transmitters without difficulty. In the most unusual circumstances,
it may be necessary to reposition the unit to reduce RF interference, and/or
to reposition its input and output cables to reduce RF pickup on their shields.
The AES3 inputs and output are transformer-coupled and have very good resistance
to RFI. If you have RFI problems and are using analog connections on either the input
or output, using digital connections will almost certainly eliminate the RFI.
Poor Peak Modulation Control
The 9400 ordinarily controls peak modulation to an accuracy of ±2%. This accuracy
will be destroyed if the signal path (including the STL and transmitter) following the
9400 has poor transient response. Almost any link can cause problems. The transmitter
itself is particularly likely to cause problems, especially if it is plate-modulated.
Section 1 of this manual contains a complete discussion of the various things that
can go wrong.
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 program
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 AM 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.