Orban OPTIMOD-FM 5700i, Руководство по эксплуатации

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3-14
OPERATION ORBAN MODEL 5700i
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 5700i’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 5700i’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 5700i 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 12 on page 2-35.
Because it only offers processing for analog radio, model 5700iFM does
not include a BS.1770 Safety Limiter.
Input/Output Delay
The sophisticated look-ahead algorithms in the 5700i 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 5700i
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-