Orban OPTIMOD-FM 8600Si, Operating Manual

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OPTIMOD-FM DIGITAL INSTALLATION
2-37
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.
While the MPX power controller only produces gain reduction after the clippers,
the action of the BS.1770 controller is split. The first 3 dB of gain reduction oc-
curs before the clippers so that 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 after the clippers, permitting you to achieve more of the “FM limiter
sound” with higher amounts of gain reduction. (Because some of the gain con-
trol is applied after the peak limiter, the peak deviation may be limited to less
than
±
75 kHz.)
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-
teners
1
. We suggest using a maximum of 3 dB gain reduction in the BS.1770 safe-
ty limiter.
A) In PC Remote, navigate to the
I
NPUT
/
OUTPUT
> U
TILITIES tab.
B) Turn the
1770 S
AFETY
L
IMITER control
O
N
.
C) Set the
1770 L
OUDNESS
T
HRESH
control to the target loudness specified by your
country’s governing authority. EBU B 128 specifies –23 LUFS.
The BS.1770 Loudness Meter is calibrated in relative units (LU) so that it
indicates “0” when the loudness is equal to the target loudness set by
1
See Begnert, Fabian; Ekman, Håkan; Berg, Jan, “Difference between the EBU R-128
Meter Recommendation and Human Subjective Loudness Perception,” AES Conven-
tion Paper 8489, 131
st
AES Convention, (October 2011). This paper states, “These
loudness-equalized signals gave rise to a perceived maximum loudness difference of
2.8 dB.”