Orban OPTIMOD-FM 8700, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-79
bandwidth to 15 kHz improves subjective quality. This can occur because
the codec then uses all of its bits to encode information in the most sub-
jectively important part of the ear’s bandwidth. If the codec employs
Spectral Band Replication® technology (as does the iBiquity HD Codec),
then you can set the bandwidth at 20 kHz without quality penalty, alt-
hough the difference between 15 and 20 kHz is unlikely to be audible
following the encode / decode cycle.
HD Polarity sets the polarity of the output of the HD processing to
P
OSITIVE
or
N
EGATIVE
. The switch allows you to match the polarity (sometimes informally called
“phase”) of the audio through the analog FM and HD transmission channels, regard-
less of how your facility is configured. It is important to match polarity to avoid a
momentary decrease in loudness during analog/digital receiver crossfades.
This control is best adjusted by observing an HD radio. Using a variable RF attenua-
tor, vary the RF level feeding the radio to force crossfades. If you have correctly ad-
justed the 8700i’s
A
NALOG
C
HANNEL
D
ELAY
to time-align the analog and digital chan-
nels, the incorrect setting of the
HD P
OLARITY
control should be clearly audible as a
momentary loudness decrease during the crossfades.
The
HD P
OLARITY
control can make it easier to match the analog and HD channel de-
lay in an HD Radio installation. Temporarily set this control to cause a null, sum the
HD and FM channels, and adjust the
HD D
ELAY
control to achieve the deepest null.
This works best if the HD and FM analog loudnesses are matched; use the
HD L
IMIT
D
R
control to do this.
If your facility has two FM analog exciters. one of which inverts polarity and one of
which does not, you can use the 8700i’s
FM P
OLARITY
control to compensate when
switching between transmitters. (See step 14 on page 2-37.) This function can also
be controlled via the 8700i’s clock-based automation (see page 3- 42) and by its API
(see step 13 on page 2-55).
Digital Output
You will normally use the AES2 digital output to drive the digital transmitter. How-
ever, you can use any output (
A
NALOG
L/R, AES1 , and AES2 ). The digital outputs
have the following controls, located in the I
NPUT
/ O
UTPUT
screen.
Out Level (“Digital Output 1” and “Digital Output 2”) sets the digital-channel out-
put level with respect to digital full scale. It is normally set at 0 dBFS, which uses all
the headroom available in the HD transmission channel. To match the loudness of
the analog and digital channels at the receiver, use the
L
IMITER
D
RIVE
control. This
minimizes the amount of peak limiting necessary to match the loudness of the ana-
log and digital channels at the receiver.
When the BS.1770 Safety Limiter is active, it changes the way the O
UTPUT
L
EVEL
con-
trol operates. See step 10 on page 2-32.
Samp Rate (“Sample Rate”) sets the output sample rate of the digital-channel out-
put to 32, 44.1, 48, 88.2, or 96 kHz.
The 8700i’s fundamental sample rate is always 64 kHz, but the internal
sample rate converter sets the rate at the 8700i’s digital output. This ad-