Orban OPTIMOD 5750 Operating Manual page 54

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Orban 5750 Technical Manual Operation
The 5750 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 upstream from the 5750, preventing excessive reduction of the
multiplex power.
Power control is applied to all outputs, not just the composite output.
Two-Band Purist Processing
In addition to five-band processing, suitable for pop music and talk formats, the 5750 offers a very high-quality two-
band algorithm. This is phase-linear and features the same AGC as the five-band processor, followed by a two-band
processor with look-ahead limiting. Sophisticated multiband high frequency limiting and distortion-cancelled
clipping complete the chain.
We believe that this is the ideal processing for classical music because it does not dynamically 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 contrast, the 5750'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, and Multipath Mitigator are common between the FM analog and digital
radio processing chains. The processing chain splits into two paths after the AGC. Each path contains a structurally
identical but independently adjustable equalizer and multiband compressor. Each preset has an FM->HD CONTROL
COUPLING control that determines if audio controls affecting the HD equalizer and HD multiband compressor/limiter
will follow their counterparts in the FM analog processing chain or if the HD and FM controls can be adjusted
independently.
The peak limiter in the digital radio processing chain is a mastering-quality lookahead limiter. This limiter minimizes
IM distortion in addition to minimizing harmonic distortion. The resulting peak limiting is almost always undetectable
when used with reasonable amounts of gain reduction (i.e., frequently recurring gain reduction 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.
Except for the fact that its input has been de-emphasized, the HD look-ahead limiter receives the same processing
as the FM peak limiting section if the FM->HD CONTROL COUPLING is set to FM->HD. Earlier processing has often
been adjusted to help compensate for the inevitable high frequency loss caused by pre-emphasis limiting in the FM
peak limiter. Therefore, the HD output can be excessively bright without further adjustment.
In FM->HD mode, you can use the 5750's parametric high frequency shelving filter to supply a high frequency rolloff
that tames excessive brightness in the HD output. Simultaneously, this HF rolloff may reduce high frequency artifacts
in the relatively low bite-rate codec used in the Xperi HD Radio system.