Orban OPTIMOD-FM 8700, Operating Manual

Page: 295 / 390

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

s or 75

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 any of the Five-Band structures, this problem can be caused by ex-
cessive 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 set-
tings of the BAND 4 MIX and BAND 5 MIX controls (located in I
NTERMEDIATE
and
A
DVANCED
M
ODIFY
).
If you are driving an external stereo encoder with built-in preemphasis, you must set
the 8700i’s output to Flat in the System Setup / Output screen to prevent double
preemphasis, which will cause very shrill sound (and very poor peak modulation con-
trol).
You will always achieve better peak control by defeating the preemphasis and input
filters of an external stereo encoder, permitting the 8700i to perform these func-
tions without overshoot. Section 1 of this manual contains a detailed explanation 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 Multi-Band structure will automatically re-equalize such dull-
sounding program material to make its spectral balance more consistent with other
program material.
If the 8700i’s output is set to
F
LAT
in
I
NPUT
/O
UTPUT
> O
UTPUT
1
or
O
UTPUT
2
, there will
be no preemphasis 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 8700i can generate test tones itself. The 8700i 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 3-88.)