Orban OPTIMOD-FM 5518, Operating Manual

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5-2
TROUBLESHOOTING ORBAN MODEL 5518
Poor Peak Modulation Control
When set up correctly and with its overshoot limiters (left/right and composite) en-
abled, the 5518 normally controls peak modulation to an accuracy of ±2%. This ac-
curacy will be destroyed if the signal path following the 5518 has poor transient re-
sponse. Almost any link can cause problems. Even the FM exciter can have insuffi-
cient flatness of response and phase-linearity (particularly at low frequencies) to dis-
turb peak levels. Section 1 of this manual contains a complete discussion of the vari-
ous 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 pro-
gram material. The amount of overshoot will depend on data rate — the higher the
rate, the lower the overshoot. However, when such a path is used between your au-
dio processor and the 5518, the 5518’s left/right overshoot limiter should be able to
remove most of the overshoot, while the 5518’s composite limiter should be able to
remove the overshoot that remains.
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 are using SSB/VSB mode, you must activate the 5518’s composite limiter to
control modulation. See
SSB Stereo Encoder Operation on page 3-6.
Unexpected Delay Between the Program Feed and the On-Air Signal
The diversity delay may have been accidentally applied to the output you are using
to drive your transmitter. See step (5.F) on page 2-24 and step (6.B) on page 2-25.
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 output of the audio processor driving the 5518 is clean. Heavy proc-
essing can exaggerate even slightly distorted material, pushing it over the edge into
unacceptability.
If you are using analog inputs, the peak input level must not 27 dBu or the
5518's A/D converter will clip and distort.
Unlike earlier digital 5518s, there is no input peak level adjustment for
the A/D converter. Instead, we have provided adequate headroom for
virtually any facility. This is possible because the A/D converter in the
5518 has higher dynamic range than older designs. Therefore, without
compromising the 5518’s noise level, we could eliminate a control that
was frequently misadjusted.