Audible Noise On Air - Orban OPTIMOD-FM 5500 Operation Manual

OPTIMOD-FM DIGITAL
compromising the 5500's noise level, we could eliminate a control that
was frequently misadjusted.
If you are using the 5500'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
trol. A similar problem can occur if you are using sum-and-difference processing in
the 5500's AGC. In this case, reduce the setting of the AGC's M
If you are using an external processor ahead of the 5500, be sure it is not clipping or
otherwise causing problems.
If the 5500 is in stand-alone stereo encoder mode, you may be overdriving its
left/right protection limiter and/or composite limiter. It is unwise to do more than 3
dB of gain reduction in the protection limiter or to set the C
control higher than 1.0
2-22.

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 5500 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 5500. The first affects the
AGC and the second affects the Multiband Compressor. Each has its own threshold
control. (See M
G
B
In the Multiband structure, dynamic single-ended noise reduction (see D
on page 3-49) 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 5500'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 5500 to digitize the input. (This ratio is better than 108 dB.) It is
important to drive the 5500 with professional levels (more than 0 dBu reference
level) to achieve adequately low noise. (Clipping occurs at +27 dBu.)
The 5500'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 5500'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
5500, the STL's noise level can severely limit the overall noise performance of the
system because compression in the 5500 can exaggerate the STL noise. For example,
the overload-to-noise ratio of a typical analog microwave STL may only be 70-75 dB.
. See Setup for Stand-Alone Stereo Encoder Mode on page
DB
on page 3-48.)
ATE
TROUBLESHOOTING
con-
L
ATIO IMIT
GR controls.
D
AX ELTA
L D
OMPOSITE IMIT RIVE
T
WNEXP HR
5-3