Orban OPTIMOD-FM 8700, Руководство по эксплуатации

Страница: 246 / 390

3-76
OPERATION ORBAN MODEL 8700i
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
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 8700i 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 8700i com-
posite output, allowing you to use the 8700i’s stereo encoder and composite limiter
during HD Radio broadcasts.
To avoid introducing artifacts in the analog FM path, its delay is held constant and
HD delay is varied if the change does not exceed ±1.3 seconds. Larger adjustments
change the analog FM delay and re-center the HD delay within the ±1.3 second
window. The ±1.3 second window is sufficient for use with hardware that constantly
measures the FM/HD delay off-air and automatically adjusts the Optimod’s delay to
compensate for delay drift in the transmission path. When no output emits the
FM+DELAY source and the delay in the composite output is OFF, the extra 1.3 sec-
ond delay is removed from both the FM and HD paths.
The MX presets have an input-to-output delay of approximately 265 ms, while the
8500-style presets typically have delays below 20 ms. The Multipath Mitigator adds
146 ms of additional delay, and can be bypassed in situations like outside broadcasts
where talent needs to monitor off-air. The defeatable subharmonic synthesizer adds
67.5 ms of delay when active.
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 8700i’s input to its FM analog processed output must not
change when a preset is recalled (to prevent an 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 8700i will automatically obey these requirements if you do the following:

Set the 8700i’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
.