Orban OPTIMOD-FM 8600Si, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-73
The look-ahead limiter includes a parametric high frequency shelving equalizer that
can be placed either before or after gain reduction. You can use it to equalize tex-
ture disparities between the FM and HD channels and to reduce codec artifacts at
high frequencies. You can also use the
HD B
AND
M
IX controls and/or HD compression
threshold to achieve this.
The HD output is designed to feed digital channels without pre-emphasis, which in-
clude almost all such channels. The only high-quality digital channels using pre-
emphasis of which we are aware are NICAM channels (which use J.17 pre-emphasis)
and some older CDs (which use EIAJ—50µs/15µs shelving pre-emphasis). If you use
the HD output to feed a digital channel with pre-emphasis, you must allow extra
headroom to compensate for the unpredictable peak level changes that the pre-
emphasis induces.
If the HD output is driving a channel without pre-emphasis but using a lossy codec,
you may want to allow headroom 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
To make the receiver analog/digital crossfade free from comb filtering, the time de-
lays 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 8600Si eliminates the need to use the delay line built into the HD
Radio 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 8600Si
composite output, allowing you to use the 8600Si’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 8600Si’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).
•
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 8600Si will automatically obey these requirements if you do the following:
•
Set the 8600Si’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