Orban OPTIMOD-FM 8600, Operating Manual

Page: 301 / 397

OPTIMOD-FM DIGITAL TROUBLESHOOTING
5-3
tem (using
S
YSTEM
S
ETUP
). If your peak program level exceeds the peak level you
have specified on setup, the 8600's A/D converter will clip and distort. (See page 2-
27). The 8600’s
I
NPUT
L
EVEL
meters will clearly indicate any such clipping.
If you are using the 8600’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 8600’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 8600, 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 8600 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 8600. The first affects the
AGC and the second affects the Multiband Compressor. Each has its own threshold
control. (See
MB G
ATE
T
HRESH
on page 3-62.)
In the Multiband structure, dynamic single-ended noise reduction (see
MB D
OWN
E
XPANDER
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 8600's analog input, the overall noise performance of the system
is usually limited by the overload-to-noise ratio of the analog-to-digital converter
used by the 8600 to digitize the input. (This ratio is better than 108 dB.) It is im-
portant to drive the 8600 with professional levels (more than 0 dBu reference level)
to achieve adequately low noise. (Clipping occurs at the level set by the
C
LIP
L
EVEL
control located in
I
NPUT
/O
UTPUT
> I
NPUT
.)
The 8600'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 8600's digital input will thus nev-
er limit the unit's noise performance even with very high amounts of compression.
If an analog studio-to-transmitter link (STL) is used to pass unprocessed audio to the
8600, the STL's noise level can severely limit the overall noise performance of the
system because compression in the 8600 can exaggerate the STL noise. For example,
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 an Orban studio level control device like the 8200ST Stu-
dio 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 uncompressed digital STL will perform much better than any
analog STL. (See
Studio-Transmitter Link, starting on page 1-15.)