Audible Distortion On-Air; Audible Noise On Air - Orban Optimod-FM 8300 Operating Manual

5-2
TROUBLESHOOTING

Audible Distortion On-Air

Audible Noise on Air

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.
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 8300'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
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 peak input level must not exceed +27 dBu or the
8300's A/D converter will clip and distort.
Unlike earlier digital Optimods, there is no input peak level adjustment
for the A/D converter. Instead, we have provided adequate headroom for
virtually any plant. This is possible because the A/D converter in the 8300
has higher dynamic range than older designs. Therefore, without com-
promising the 8300's noise level, we could eliminate a control that was
frequently misadjusted.
If you are using the 8300's stereo enhancer (which most "pop music"-oriented pre-
sets do), then this can exaggerate multipath distortion in high multipath environ-
ments. You may want to reduce the setting of the stereo enhancer's R
trol. A similar problem can occur if you are using sum-and-difference processing in
the 8300's AGC. In this case, reduce the setting of the AGC's M
If you are using an external processor ahead of the 8300, be sure it is not clipping or
otherwise causing problems.
(See also "RFI, Hums, Clicks, or Buzzes" on page 5-1.)
Excessive compression will always exaggerate noise in the source material.
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