Orban OPTIMOD-FM 8600Si, Operating Manual

Page: 291 / 356

OPTIMOD-FM DIGITAL TROUBLESHOOTING
5-5
Amplitude modulation of the carrier that is synchronous with the program (“syn-
chronous AM”) can cause program-related crosstalk into subcarriers. Synchronous
AM should be better than 35 dB below 100% modulation as measured on a syn-
chronous AM detector with standard FM de-emphasis (50
μ
s or 75
μ
s).
The subcarrier receiver itself must receive a multipath-free signal, and must have a
wide and symmetrical IF passband and a linear, low-distortion FM demodulator to
prevent program-related crosstalk into subcarriers.
Shrill, Harsh Sound
If you are using the Five-Band structure, this problem can be caused by excessive HF
boost in the HF Equalizer and HF Enhancer. It could also be caused by an excessively
high setting of the
BAND 4 THRESH
control, or by excessively high settings of the
BAND 4 MIX
and
BAND 5 MIX
controls (located in Intermediate and Advanced Mod-
ify).
If you are driving an external stereo encoder with built-in pre-emphasis, you must
set the 8600Si’s output to Flat in the System Setup > Output screen to prevent dou-
ble pre-emphasis, which will cause very shrill sound (and very poor peak modulation
control).
You will always achieve better peak control by defeating the pre-emphasis and in-
put filters of an external stereo encoder, permitting the 8600Si to perform these
functions without overshoot. Section 1 of this manual contains a detailed explana-
tion of these, and other, system design considerations.
Dull Sound
If you are using the Two-Band structure, dull-sounding source material will sound
dull on the air. The Five-Band structure will automatically re-equalize such dull-
sounding program material to make its spectral balance more consistent with other
program material.
If the 8600Si’s output is set to Flat in System Setup > Output, there will be no pre-
emphasis unless it is supplied somewhere else in the system. This will cause very dull
sound.
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 8600Si can generate test tones itself. The 8600Si 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 3-85.)