Orban OPTIMOD-FM 8700, Руководство по эксплуатации

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3-16
OPERATION ORBAN MODEL 8700i
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.
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 C
ONTROL
C
OUPLING
is set to FM

HD. Earlier processing has often been adjusted to help com-
pensate for the inevitable high frequency loss caused by preemphasis 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 8700i’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 iBiquity HD Radio system.
With the FM

HD C
ONTROL
C
OUPLING
set to I
NDEPENDENT
, there are several approach-
es to minimizing brightness and conditioning the signal to work well at low bitrates.

Use little or no high frequency boost in the HD equalization and band mix sec-
tions.

Set the HD BAND 4>5 COUPLING to 100%.

Set the HD B5 THRESH to match the codec and its bitrate. Adjust the threshold
until you find a good compromise between presence and high frequency codec
artifacts. We find the range from -6.0 to +6.0 dB to be useful.

Use a moderate Band 5 attack time. 25 ms works well.

If necessary, lower the
HD B4 THRESH
.
See About the 8700i’s HD / Digital Radio Processing (starting on page 3-74) for a
complete description of the HD setup and subjective adjustment controls.
BS.1770 Compliance for Digital Radio
The 8700i includes an ITU-R BS.1770 Loudness Meter and Safety Limiter in the digital
radio processing path. The Safety Limiter should only be activated if the regulatory
authority in your country requires constraining BS.1770 Integrated Loudness to a
specified threshold. See step 10 on page 2-32.