Orban OPTIMOD-FM 8600, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-73
room to compensate for data reduction-induced peak overshoots at the receiver,
which might otherwise cause clipping. In our experience, 2 dB is typically adequate.
Delay Difference between HD and FM Outputs
In order to make the receiver analog/digital crossfade free from comb filtering, the
time delays in the HD Radio’s FM and HD channels must have a fixed and predictable
offset, correctly implementing the HD Radio receiver’s “time diversity” processing.
This “diversity delay” is part of the HD Radio system specification. The availability of
the delay in the 8600 eliminates the need to use the delay line built into the HD Ra-
dio exciter. In turn, this allows you to connect any output receiving the analog-FM-
processed signal to the input of the analog FM exciter. This includes the 8600 com-
posite output, allowing you to use the 8600’s stereo encoder and composite limiter
during HD Radio broadcasts.
The MX presets have an input-to-output delay of approximately 265 ms, while the
8500-style presets typically have delays below 20 ms. To ensure a smooth transition
between presets of different types (like an 8500-style news preset and a music-
oriented MX preset), the following must be true:
•
The delay from the 8600’s input to its FM analog processed output must not
change when a preset is recalled (to prevent a audio segment from being re-
peated or dropped due to different time delays between presets). This criterion is
also relevant to FM analog transmissions without an accompanying HD channel
and therefore applies to the model 8600FM processor, which has no HD pro-
cessing.
•
The delay difference between the HD-processed output and the analog FM-
processed output must not change. This allows HD radio receivers to crossfade
smoothly between the digital and analog channels.
The 8600 will automatically obey these requirements if you do the following:
•
Set the 8600’s
D
IVERSITY
D
ELAY
T
RIM
control to 0.37 seconds or longer.
•
If you are using the analog left/right or an AES3 digital output to drive the FM
analog side of your transmission plant, assign that output’s
O
UT
S
OURCE
control
to
FM+D
ELAY
.
•
If you are using a composite output to drive the FM analog side of your transmis-
sion plant, set the stereo encoder’s
D
IVERSITY
D
ELAY
control to
I
N
. This control is
located in the
C
OMPOSITE
screen.
If you need the shortest possible input-to-output time delay (for example, if you are
driving talent headphones from the off-air feed), defeat the diversity delay on the
output driving the FM analog transmitter by setting the
O
UT
S
OURCE
to
FM
or
D
IVERSITY
D
ELAY
to
O
UT
as appropriate for the output you are using. In this case, the
delay difference between the analog and digital radio outputs will not stay constant
when you recall different presets. This is not important for these applications be-
cause one gives up proper diversity delay (around eight seconds) for the sake of driv-