Two-Band Purist Processing; Digital Radio Processing - Orban OPTIMOD-FM 8500S Operating Manual

OPTIMOD-FM DIGITAL
The 8500S gives you control over the Multiplex Power Threshold (in the In-
put/output Utilities screen). This allows you to compensate for overshoots in the sig-
nal path upstream from the 8500S, 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 3-72.

Two-Band Purist Processing

In addition to five-band processing, suitable for pop music and talk formats, the
8500S 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 pre-
emphasis 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 con-
trast, the 8500S's two-band phase-linear structure keeps the musical spectrum co-
herent 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, and AGC are common between the FM ana-
log and digital radio processing chains. The processing chain splits into two paths af-
ter the AGC. Each path contains a structurally identical but independently adjustable
equalizer and multiband compressor. Each preset has an FM HD C
control that determines if audio controls affecting the HD equalizer and HD multi-
band compressor/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 3-70.)
The peak limiter in the digital radio processing chain is a mastering-quality look-
ahead limiter. This limiter minimizes IM distortion in addition to minimizing har-
monic distortion. The resulting peak limiting is almost always undetectable when
used with reasonable amounts of gain reduction (i.e., frequently recurring gain re-
duction of 3-4 dB).
Certain unusual program material may cause infrequent instances of gain reduction
as high as 12 dB with the above settings. This occurs on isolated transients and is no
cause for concern unless it is frequent.
OPERATION
C
ONTROL OUPLING
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