Orban OPTIMOD-FM 8700, Operating Manual

Page: 71 / 390

OPTIMOD-FM DIGITAL INSTALLATION
2-9

At the output of the AGC before the analog-FM and HD processing paths split,
allowing a single ratings encoder to be used for both the analog FM and HD
channels while still receiving level-controlled audio. Set the
AES 2 S
OURCE
con-
trol to AGC>R
ATINGS
in PC Remote or AGC>RE on the Optimod.

Between your Optimod’s left/right audio processing and stereo encoder, main-
taining consistently high audio drive levels into the ratings encoder while allow-
ing your Optimod’s composite limiter to be used. Set the
AES 2 S
OURCE
control
to
L
IMIT
>R
ATINGS
in PC Remote or
LIM>RE
on the Optimod.
In Rating Encoder mode, if a valid AES3 signal is not detected at its SYNC input, your
Optimod will automatically bypass the loop-through. While this will prevent dead
air, it means that ratings encoding will not be applied to your broadcast.
Because of the importance of proper ratings encoder operation, your Optimod will
flash a warning on its screen and on PC Remote if it enters the fallback mode. This
will help detect potential failures in the ratings encoder hardware.
Composite Output and Subcarrier Inputs
There are two analog composite outputs. These carry the encoded stereo signal, the
stereo pilot tone, and any subcarriers that may have been applied to the 8700i’s ana-
log subcarrier inputs (on the rear panel). They do not carry SCAs that have been ap-
plied to the digital composite SCA inputs.
Each output’s level is independently adjustable from –13.7 dBu to +10.6 dBu.
The output impedance of composite output #1 and composite output #2 can be set
to 0

or 75

via jumpers J2 and J3 respectively (located on the I/O Board). As
shipped, the link is on pins 3 and 4, yielding 0

impedance. To reset a given output
to 75

, place the link on pins 1 and 2 of its associated jumper. (See the schematic on
page 6-55 and the parts locator diagram on page 6-51.)
Each output can drive up to 75

in parallel with 0.047

F before perfor-
mance deteriorates significantly (see Figure 2-3on page 2-10).
Connect the 8700i’s composite output to the exciter input with up to 100 feet
(30.5m) of RG-58 / U or RG-59 / U coaxial cable terminated in BNC connectors.
Longer runs of coax may increase problems with noise, hum, and RF
pickup at the exciter. In general, the least troublesome installations place
the 8700i close to the exciter and limit the length of the composite cable
to less than 6 feet (1.8m).
We do not recommend terminating the exciter input by 50

or 75

un-
less this is unavoidable. The frequencies in the stereo baseband are low
by comparison to RF and video and the characteristic impedance of coax-
ial cable is not constant at very low frequencies. Therefore, the transmis-
sion system will usually have more accurate amplitude and phase re-
sponse (and thus, better stereo separation) if the coax is driven by a very
low impedance source and is terminated by greater than 1k

at the ex-