OPTIMOD-FM TROUBLESHOOTING
5-3
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 ref-
erence receiver can falsely indicate peak program modulation higher than that actually
being transmitted if the monitor overshoots at high and low frequencies. Many commer-
cial monitors have this problem, but most of these problem units can be modified to indi-
cate peak levels accurately.
Orban uses the Belar “Wizard” series of DSP-based monitors internally for
testing, because these units do not have this difficulty.
Audible Distortion On-Air
Make sure that the problem can be observed on more than one receiver and at several
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 8400's audio inputs is clean. Heavy processing can
exaggerate even slightly distorted material, pushing it over the edge into unacceptability.
The subjective adjustments available to the user have enough range to cause audible dis-
tortion at their extreme settings. There are many controls that can cause distortion, in-
cluding
Multiband Clipping, Final Clip Drive, and Composite Clip Drive 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 some-
times cause audible distortion with certain atypical program material.
If you are using analog inputs, the headroom of the unit's analog-to-digital (A/D) con-
verter must be correctly matched to the peak audio levels expected in your system (using
System Setup). If your peak program level exceeds the peak level you have specified on
setup, the 8400's A/D converter will clip and distort. (See page 2-30).
If you are using the 8400’s stereo enhancer (which most
“pop music”-oriented presets
do), then this can exaggerate multipath distortion in high multipath environments. You
may want to reduce the setting of the stereo enhancer’s
Ratio Limit control. A similar
problem can occur if you are using sum-and-difference processing in the 8400’s AGC. In
this case, reduce the setting of the AGC’s
MaxDeltaGR controls.
If you are using an external processor ahead of the 8400, be sure it is not clipping or oth-
erwise causing problems.
Audible Noise on Air
(See also “RFI, Hums, Clicks, or Buzzes” on page 5-2.)
Excessive compression will always exaggerate noise in the source material.
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