Optimod-AM 9400 V1.2 Operating Manual - Orban

OPTIMOD-AM DIGITAL TROUBLESHOOTING 5-5
fully realigned, the only cure for any remaining excessive out-of-band radiation is to
reduce the setting of the 9400's lowpass filter until the transmission is within specification.
These transmitters are prime candidates for replacement with a modern
solid-state transmitter, which will reduce AC power costs and also sound much better
on-air.
Negative overmodulation that causes carrier pinch-off will also cause the bandwidth
to increase rapidly. Older transmitters may respond better to negative modulation
slightly below 100%, as their distortion can rise rapidly as they approach 100%
modulation.
System Will Not Pass Line-Up Tones at 100% Modulation
This is normal. Sine waves have a very low peak-to-average ratio by comparison to
program material. The processing thus automatically reduces their peak level to
bring their average level closer to program material, promoting a more consistent
and well-balanced sound quality.
The 9400 can generate test tones itself. The 9400 can also be put into Bypass mode
(locally or by remote control) to enable it to pass externally generated tones at any
desired level. (See Test Modes on page 3-54.)
System Will Not Pass Emergency Alert System (“EAS” USA Standard) Tones
at the Legally Required Modulation Level
See System Will Not Pass Line-Up Tones at 100% Modulation (directly above) for an
explanation. These tones should be injected into the transmitter after the 9400, or
the 9400 should be temporarily switched to BYPASS to pass the tones.
System Receiving 9400’s Digital Output Will Not Lock
Be sure that the 9400’s output sample rate is set match the sample rate that the
driven system expects. Be sure that the 9400’s output mode (AES3 or SPDIF) is set to
match the standard expected by the driven system.
L–R (Stereo Difference Channel) Will Not Null with Monophonic Input
This problem is often caused by relative phase shifts between the left and right
channels prior to the 9400’s input. This stresses codecs, which can misinterpret these
phase shifts as intentional stereo imaging effects and try to encode them, wasting
bits. It is wise to minimize any left/right phase shifts in your audio facility prior to
the 9400.
To minimize L–R energy in the signal, it may be wise to turn off the 9400’s stereo
enhancer and to operate the digital radio chain’s five-band compressor with 100%
coupling, which prevents the processing from adding L–R energy to the signal. The
only possible exception to this rule is when Band 5 is used as a de-esser. See Excessive
Sibilance (“ess” sounds) in the Digital Radio Channel immediately below.
AM Analog and Digital Channels Have Unequal Loudness
Adjust the HD LIMITER DRIVE control in the on-air HD preset to match the loudness of
the two channels. Do not reduce the loudness of the digital channel by turning
down its associated output level control. Using the HD LIMITER DRIVE control to re-