Orban OPTIMOD-FM 8600Si, Руководство по эксплуатации

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OPTIMOD-FM DIGITAL TROUBLESHOOTING
5-3
compromising the 8600Si’s noise level, we could eliminate a control that
was frequently misadjusted.
If you are using the 8600Si’s stereo enhancer (which most “pop music”-oriented pre-
sets do), then this can exaggerate multipath distortion in high multipath environ-
ments. You may want to reduce the setting of the stereo enhancer’s
R
ATIO
L
IMIT
con-
trol. A similar problem can occur if you are using sum-and-difference processing in
the 8600Si’s AGC. In this case, reduce the setting of the AGC’s
M
AX
D
ELTA
GR
controls.
If you are using an external processor ahead of the 8600Si, be sure it is not clipping
or otherwise causing problems.
Audible Noise on Air
(See also “RFI, Hums, Clicks, or Buzzes” on page 5-1.)
Excessive compression will always exaggerate noise in the source material.
The 8600Si has two systems that fight this problem. The compressor gate freezes the
gain of the AGC and compressor systems whenever the input noise drops below a
level set by the threshold control for the processing section in question, preventing
noise below this level from being further increased.
There are two independent compressor gate circuits in the 8600Si. The first affects
the AGC and the second affects the Multiband Compressor. Each has its own thresh-
old control. (See
M
B
G
ATE
on page 3-63.)
In the Multiband structure, dynamic single-ended noise reduction (see MB Down Ex-
pander on page 3-
64
) can be used to reduce the level of the noise below the level at
which it appears at the input.
If you are using the 8600Si's analog input, the overall noise performance of the sys-
tem is usually limited by the overload-to-noise ratio of the analog-to-digital con-
verter used by the 8600Si to digitize the input. (This ratio is better than 108 dB.) It is
important to drive the 8600Si with professional levels (more than 0 dBu reference
level) to achieve adequately low noise. (Clipping occurs at +27 dBu.)
The 8600Si's AES3 input is capable of receiving words of up to 24 bits. A 24-bit word
has a dynamic range of approximately 144 dB. The 8600Si's digital input will thus
never limit the unit's noise performance even with very high amounts of compres-
sion.
If an analog studio-to-transmitter link (STL) is used to pass unprocessed audio to the
8600Si, the STL's noise level can severely limit the overall noise performance of the
system because compression in the 8600Si can exaggerate the STL noise. For exam-
ple, the overload-to-noise ratio of a typical analog microwave STL may only be 70-75
dB. In this case, it is wise to use the Orban 8200ST Studio AGC to perform the AGC
function prior to the STL transmitter and to control the STL's peak modulation. This
will optimize the signal-to-noise ratio of the entire transmission system. An uncom-
pressed digital STL will perform much better than any analog STL. (See
Studio-
Transmitter Link, starting on page 1- Error! Bookmark not defined. .)