Orban OPTIMOD-FM 8700, Руководство по эксплуатации

Страница: 245 / 390

OPTIMOD-FM DIGITAL OPERATION
3-75
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 8700i’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 8700i’s look-ahead limiter implements “true peak” control by oversampling the
HD peak limiter’s sidechain at 256 kHz. This allows the 8700i 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 better
than 0.5 dB. For typical program material, accuracy is 0.2 dB
Thanks to true peak control, sample rate conversion, unless it removes high frequen-
cy program energy or introduces group delay distortion, cannot cause sample peaks
to increase more than 0.5 dB. For example, sample rate con-version from 48 kHz to
44.1 kHz is highly unlikely to cause sample peak clipping in the 44.1 kHz audio 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
AND
M
IX
controls and/or HD compression
threshold to achieve this.
The HD output is designed to feed digital channels without preemphasis, which in-
clude almost all such channels. The only high-quality digital channels using
preemphasis of which we are aware are NICAM channels (which use J.17 preempha-
sis) and some older CDs (which use EIAJ—50µs/15µs shelving preemphasis). If you use
the HD output to feed a digital channel with preemphasis, you must allow extra
headroom to compensate for the unpredictable peak level changes that the
preemphasis induces.
If the HD output is driving a channel without preemphasis, it will control peak levels
with an uncertainty of approximately 1 dB. However, you may want to allow head-