Orban OPTIMOD-FM 8700, Operating Manual

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OPTIMOD-FM DIGITAL INSTALLATION
2-25
The total gain reduction of the BS.1770 controller (see step 13 below) and
MPX power controller is split before and after the FM peak limiter such
that the first 3 dB of gain reduction occurs before the peak limiter.
Hence, you can achieve full
±
75 kHz deviation when the gain reduction is
between 0 and 3 dB. Any gain reduction greater than 3 dB is applied af-
ter the peak limiter, permitting you to achieve more of the “FM limiter
sound” with higher amounts of gain reduction. (Because some of the
gain control is applied after the peak limiter, the peak deviation may be
limited to less than
±
75 kHz.)
See the notes on the Multiplex Power Offset control on page 3- 48 . For
more information on the multiplex power controller, refer to ITU-R Mul-
tiplex Power Controller on page 3-83.
For “MX” presets or anytime the diversity delay is active, the 8700i com-
putes the multiplex power from the left and right outputs of the audio
processing because there is too much delay to use the output of the ste-
reo encoder. Under these circumstances, the 8700i cannot directly take
into account the setting of the
C
OMPOSITE
L
IMIT
D
RIVE
control.
13. Set up and activate the BS.1770 Safety Limiter in the analog radio processing chain
(optional)
The BS.1770 safety limiter and meter have been included to allow your Optimod
to comply with government regulations in countries that enforce BS.1770-based
loudness control (per EBU Recommendation R 128) in analog FM transmission.
Unless your country requires this, leave the BS.1770 Safety Limiter off.
The analog-chain BS.1770 safety limiter and meter are calibrated with reference
to 100% modulation with 50
µ
s preemphasis (
±
75 kHz deviation) per section 5.9
of EBU Tech 3344 (“Practical guidelines for distribution systems in accordance
with EBU R 128”).
The BS.1770 Safety Limiter is usually used in conjunction with the BS.412 Multi-
plex Power Limiter. The processing chain is configured as follows:
MPX Offset Control
→
BS.1770 controller (0 to 3dB GR)
→
Peak Limiter
→
BS.1770 controller (>3dB GR)
→
BS.412 Controller
The
MPX P
OWER
O
FFSET
control allows you to reduce the drive level into the
BS.1770/BS.412 chain to prevent excessive amounts of gain reduction. This ar-
rangement allows you to apply as much “FM processor sound” as you want to
the audio. Turn the
MPX P
OWER
O
FFSET
control up (closer to 0) to drive the peak
limiters harder.
We recommend using as little gain reduction as possible to prevent unnatural
sounding source-dependent loudness variations—a limitation of the BS.1770 al-
gorithm itself, which can over-indicate the loudness of material with substantial
dynamics compression by up to 3 dB compared to subjective impressions by lis-