Orban OPTIMOD-FM 8700, Руководство по эксплуатации

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OPTIMOD-FM DIGITAL OPERATION
3-15
ITU-R 412 Compliance for Analog FM Broadcasts
ITU-R 412 requires the “average multiplex power” to be limited to a standard value.
The 8700i contains a defeatable feedback multiplex power limiter that constantly
monitors the multiplex power according to ITU-R 412 standards. The power control-
ler automatically reduces the average modulation to ensure compliance. It allows
you to set the “texture” of the processing freely, using any preset. If a given pro-
cessing setting would otherwise exceed the multiplex power limit, the power con-
troller automatically reduces the drive to the peak limiting system. This action re-
tains the compression texture but reduces distortion while controlling multiplex
power.
The 8700i gives you control over the Multiplex Power Threshold (in the Input/output
Utilities screen). This allows you to compensate for overshoots in the signal path up-
stream from the 8700i, preventing excessive reduction of the multiplex power.
Power control is applied to all outputs, not just the composite output.
See ITU-R Multiplex Power Controller on page 3-83.
Two-Band Purist Processing
In addition to five-band processing, suitable for pop music and talk formats, the
8700i offers a very high-quality two-band algorithm. This is phase-linear and fea-
tures the same AGC as the five-band processor, followed by a two-band processor
with look-ahead limiting. Sophisticated multiband high frequency limiting and dis-
tortion-cancelled clipping complete the chain.
We believe that this is the ideal processing for classical music because it does not dy-
namically re-equalize high frequencies; the subtle HF limiter only acts to reduce high
frequency energy when it would otherwise cause overload because of the FM
preemphasis curve. We have heard four-band, allegedly “purist” processing that
caused dynamic HF lift. This created a strident, unnatural sound in strings and brass.
In contrast, the 8700i’s two-band phase-linear structure keeps the musical spectrum
coherent and natural.
The look-ahead limiter prevents speech from being audibly clipped and prevents
similar audible problems on instruments with rapidly declining overtone structures
like grand piano, classical guitar, and harp.
Digital Radio Processing
Only the phase rotator, highpass filter, AGC, Xponential Loudness processing, and
Multipath Mitigator are common between the FM analog and digital radio pro-
cessing chains. The processing chain splits into two paths after the AGC. Each path
contains a structurally identical but independently adjustable equalizer and multi-
band compressor. Each preset has an
FM

HD C
ONTROL
C
OUPLING
control that deter-
mines if audio controls affecting the HD equalizer and HD multiband compres-
sor/limiter will follow their counterparts in the FM analog processing chain or if the
HD and FM controls can be adjusted independently. (See
FM

HD Control Coupling
on page 3-81.)