Orban OPTIMOD-FM 8500S Operating Manual page 229

OPTIMOD-FM DIGITAL
C to I
OUPLING NDEPENDENT
fore you set the FM>HD C
To control excessive brightness in the HD processing when operating the HD and FM
processing independently:
•
Use little or no high frequency boost in the HD equalization section.
•
Set Band 4>5 coupling to 100%.
•
Set the band 5 compression threshold to match the codec that the 8500S's HD
output is driving. Adjust the threshold until you find a good compromise be-
tween 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 Band 4 compression threshold.
An advanced-design look-ahead limiter controls the peak level of the HD output.
The look-ahead limiter (which receives the output of the HD multiband compres-
sor/limiter) is optimized to make the most of the limited bit-rate codec used in the
HD Radio system's digital channel. By eschewing any clipping, the HD output pre-
vents the codec from wasting precious bits encoding clipping distortion products,
instead allowing the codec to use its entire bit budget to encode the desired pro-
gram material.
The 8500S/HD's look-ahead limiter implements "true peak" control by oversampling
the HD peak limiter's sidechain at 256 kHz. This allows the 8500S to prevent clipping
in a playback device's analog signal path by predicting and controlling the analog
peak level following the playback device's reconstruction filter to an accuracy of bet-
ter than 0.4 dB. For typical program material, accuracy is 0.2 dB
Thanks to true peak control, sample rate conversion, unless it removes high fre-
quency program energy or introduces group delay distortion, cannot cause sample
peaks to increase more than 0.4 dB. For example, sample rate con-version from 48
kHz to 44.1 kHz is highly unlikely to cause sample peak clipping of the 44.1 kHz au-
dio data.
The look-ahead limiter includes a parametric high frequency shelving equalizer that
can be placed either before or after gain reduction. You can use it to equalize tex-
ture disparities between the FM and HD channels and to reduce codec artifacts at
high frequencies. You can also use the HD B
threshold to achieve this.
The HD output is designed to feed digital channels without pre-emphasis, which in-
clude almost all such channels. The only high-quality digital channels using pre-
emphasis of which we are aware are NICAM channels (which use J.17 pre-emphasis)
and some older CDs (which use EIAJ—50µs/15µs shelving pre-emphasis). If you use
the HD output to feed a digital channel with pre-emphasis, you must allow extra
headroom to compensate for the unpredictable peak level changes that the pre-
emphasis induces.
, this will restore any edits you made to the HD controls be-
C back to C
ONTROL OUPLING
AND
.
OUPLED
controls and/or HD compression
M
IX
3-65
OPERATION