Orban OPTIMOD-FM 8500S, Operating Manual

Page: 178 / 335

3-14
OPERATION ORBAN MODEL 8500S
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
Æ
HD C
ONTROL
C
OUPLING
is set to FM
Æ
HD. Earlier processing has often been adjusted to help com-
pensate 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
Æ
HD
mode, you can use the 8500S’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
Æ
HD C
ONTROL
C
OUPLING
set to I
NDEPENDENT
, there are several ap-
proaches 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 8500S’s HD / Digital Radio Processing (starting on page 3-64) for a
complete description of the HD setup and subjective adjustment controls.
BS.1770 Compliance for Digital Radio
The 8500S includes an ITU-R BS.1770 Loudness Meter and Safety Limiter in the digi-
tal radio processing path. The Safety Limiter should only be activated if the regula-
tory authority in your country requires constraining BS.1770 Integrated Loudness to
a specified threshold. See step 12 on page 2-35.
Because it only offers processing for analog radio, model 8500S/FM does
not include a BS.1770 Safety Limiter.
Input/Output Delay
The sophisticated look-ahead algorithms in the 8500S have one significant cost—the
input/output time delay is a minimum of approximately 13 ms and can be as high as
38 ms, depending on the setting of the
B
ASS
C
LP
M
ODE
control (found in A
DVANCED
M
ODIFY
> C
LIPPERS
)
. To make intelligent decisions about how to process, the 8500S
needs to look ahead at the next part of the program waveform. (Slowly changing
bass waveforms require particularly long look-ahead delays.) As digital on-air proc-