Unexpected Delay Between The Program Feed And The On-Air Signal; Audible Distortion On-Air - Orban OPTIMOD-FM 8500S Operating Manual

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TROUBLESHOOTING

Unexpected Delay Between the Program Feed and the On-Air Signal

Audible Distortion On-Air

The 8500S normally controls peak modulation to an accuracy of ±2%. This accuracy
will be destroyed if the signal path following the 8500S has poor transient response.
Almost any link can cause problems. Even the FM exciter can have insufficient flat-
ness of response and phase-linearity (particularly at low frequencies) to disturb peak
levels. 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 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.
The diversity delay may have been accidentally applied to the output you are using
to drive your transmitter. See step (9.E) on page 2-29 and step (10.B) on page 2-30.
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 8500S'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
8500S'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 facility. This is possible because the A/D converter in the
8500S has higher dynamic range than older designs. Therefore, without
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